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Prepulse inhibition: Stimulus parameters and its relationship to visual backward masking

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Prepulse inhibition: Stimulus parameters and its relationship to visual backward masking

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Content PREPULSE INHIBITION: STIMULUS PARAMETERS AND ITS
RELATIONSHIP TO VISUAL BACKWARD MASKING
Copyright 2002
by
Jonathan Kajzovar Wynn
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(Psychology)
August 2002
Jonathan Kajzovar Wynn
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UMI Number: 3094387
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UNIVERSITY OF SOUTHERN CALIFORNIA
THE GRADUATE SCHOOL
UNIVERSITY PARK
LOS ANGELES, CALIFORNIA 90089-1695
This dissertation, written by
Jonathan- Wynn- -
under the direction o f h 1S dissertation committee, and
approved by all its members, has been presented to and
accepted by the Director o f Graduate and Professional
Programs, in partial fulfillment o f the requirements fo r the
degree o f
DOCTOR OF PHILOSOPHY
Director
P Q te—Augu s t - 6 , 2002
Dissertation Committee
VCcd.
Chair
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ii
Acknowledgements
There are many people whom I’d like to thank for supporting me over
the years. First are my fellow students and undergraduate assistants: Serkan
Oray, Veronica Stuart, Gary Thorne, Anthony Rissling, Kelle Leber, Susan
Kesser, James Dye, and most notably Anna Marie Medina. Next are those
who’ve provided technical and professional assistance: Michael Green, Erin
Hazlett-Oakes, Diane Filion, Rand Wilcox, Laura Baker, Roger Dow, and Bill
Williams.
Many thanks are also given to all my friends and family who’ve
supported me for the last six years: Mom, Dad, Mark, Josh, Jess, Drew, Jeff,
Aaron and Karla, and Kirk and Denise. Thank you all very much.
Last, I would like to thank my committee members: Mitch Earleywine,
Richard John, David Walsh, William McComas, Anne Schell, and Mike
Dawson. Special thanks go to Mike and my co-advisor Anne. Your support,
guidance, and friendship over the years has been invaluable. I can’t thank
you enough for your help in guiding me through the years.
This research was supported by grant MFI46433 from the National
Institutes of Mental Health to Michael Dawson.
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iii
Table of Contents
Acknowledgments ii
List of Tables v
List of Figures vi
Abstract vii
Preface ix
Chapter 1: Introduction and Overview 1
Chapter 2: Theories of Sensorimotor Gating 4
Chapter 3: Prepulse Inhibition 7
Chapter 4: Backward Masking 14
Chapter 5: Schizophrenia and Information Processing Deficits 18
Chapter 6: Converging Evidence Across Paradigms 27
Chapter 7: Summary and General Hypotheses 29
Chapter 8: Experiment 1 32
Methods 33
Results 40
Discussion 50
Chapter 9: Experiment 2 57
Methods 58
Results 64
Discussion 77
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iv
Chapter 10: General Discussion
Summary of Experimental Findings 81
Implications for the Study of Prepulse Inhibition 84
Implications for Sensorimotor Gating 87
References 96
Appendix A: Experiment 1 Informed Consent 104
Appendix B: Experiment 1 Instructions 108
Appendix C: Experiment 1 Data 110
Appendix D: Experiment 2 Informed Consent 114
Appendix E: Experiment 2 Instructions 118
Appendix F: Experiment 2 Data 119
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V
List of Tables
Table 1. Physical intensities for auditory prepulses (in dB(A)) and 42
visual prepulses (in ft./cd2 ).
Table 2. Means (standard errors) for the three dependent 66
variables at each SOA.
Table 3. Summary of the results of the HLM analysis for 72
Experiment 2.
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vi
List of Figures
Figure 1. Demonstration of a prepulse inhibition paradigm. 8
Figure 2. Demonstration of a discrete and continuous PPI 10
paradigm.
Figure 3. Mean percent change PPI scores, averaged over to-be- 44
attended and to-be-ignored prepulses, as a function of group,
modality, and lead interval.
Figure 4. Mean percent change PPI scores, averaged over lead 46
interval, as a function of group, modality, and attention.
Figure 5. Mean skin conductance responding to prepulses 49
presented alone as a function of group and attention.
Figure 6. Examples of visual stimuli used in Experiment 2. Top: 61
target alone; Middle: target + prepulse; Bottom: target + startle
probe.
Figure 7. Visual and auditory PPI and percent correct hits in the 67
backward masking task as a function of SOA.
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vii
Abstract
Two experiments are presented which clarify the nature of
sensorimotor gating. In the first experiment, prepulse inhibition (PPI) was
used to assess sensorimotor gating and how stimulus parameters and
directed attention can affect PPI. Very brief, transient changes in the
environment initiate PPI (referred to as a “transient advantage”) and attention
can enhance the amount of PPI seen. What is unclear is whether the
transient advantage is seen with stimuli in different modalities, as the
transient advantage has only been found using auditory prepulses, and how
attention is able to enhance PPI.
In Experiment 1, auditory and visual prepulses that were either
attended or ignored were presented to groups either as discrete (transient) or
continuous (sustained) prepulses. It was found that attention acts to increase
PPI in part by reducing the facilitatory effects on startle by the continuous
portion of a prepulse; auditory and visual PPI were positively correlated,
providing evidence of a single sensorimotor gating mechanism; and the
transient advantage extended to the visual modality.
Experiment 2 determined whether sensorimotor gating was related to
recovery from backward masking effects. It was hypothesized that as the
interval between the target and mask increases, one begins to detect the
target without interruption by the mask because sensorimotor gating begins
to block out the effects of the mask. A hierarchical linear regression revealed
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that PPI was related to escape from backward masking effects independently
of their common relationship to stimulus onset asynchrony. Additionally,
auditory and visual PPI were positively correlated. These two findings
provide further support for a single sensorimotor gating mechanism that is
evident in a backward masking paradigm.
The results further our knowledge of normative cognitive functioning
and also are able to elucidate the nature of the cognitive deficits exhibited by
schizophrenia patients. More specifically, the evidence from the two
experiments reported implies that the deficits schizophrenia patients exhibit
in PPI and backward masking paradigms are most likely due to a deficit in
their sensorimotor gating mechanism.
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Preface
In order to alleviate any possible confusion, I would like to define the
terms stimulus onset asynchrony (SOA) and interstimulus interval (ISI) here.
SOA refers to the delay in the onset of one stimulus (S1) to the onset of
another stimulus (S2). ISI refers to the delay in the offset of S1 to the onset
of S2.
In the review of the literature I refer to whichever method was used by
the study being cited and am not casually jumping back and forth between
the two definitions. However, in the two experiments conducted herein, I use
the term and method SOA consistently. Similarly, in terms of prepulse
inhibition, the term lead interval should also be treated in the same manner
as SOA; that is, onset to onset of stimuli.
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Chapter 1
Introduction and Overview
Paradigms thought to index a sensorimotor gating mechanism have
been used as tools to study inhibitory processes in attention. These
paradigms typically use paired “S1-S2" stimuli that occur within less than one
second of each other. These paired-stimulus paradigms have proven
invaluable in gaining a better understanding of how the brain organizes and
analyzes information. Moreover, these paradigms have been used to gain an
understanding of the cognitive deficits exhibited by schizophrenia patients.
However, it is unclear whether these processes reflect a common inhibitory
mechanism or whether there are distinct inhibitory mechanisms seen only
within the context of one particular paradigm or another. The experiments
reported in this paper attempted to gain a better understanding of how the
sensorimotor gating mechanism is triggered and modified and to show that
there is, in fact, a single mechanism which is operating in two different
paradigms: prepulse inhibition and backward masking.
The brain is constantly barraged by multiple types of stimuli that it
must continually organize and interpret. In order to accomplish this task, the
b rain m u s t m a k e d ec is io n s re g a rd in g w h ich stim uli a t th e e a rlie s t,
preattentive stage of processing should be passed on for higher processing,
and which stimuli do not require further processing. In other words, the brain
must selectively attend to certain stimuli and selectively inhibit certain stimuli.
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If either, or both, of these processes cease to function properly, the brain
could become overwhelmed with the amount of information it receives and
subsequently be unable to make intelligible sense out of that information.
Over the years several physiological and behavioral paradigms have
been developed that are thought to tap into these selective attention and
inhibition mechanisms of information processing. These paradigms
specifically examine how information is processed when two stimuli are
presented very closely in time. Two of these paradigms, prepulse inhibition
and backward masking, are thought to reflect basic information processing
mechanisms that are able to show how the brain attends to specific stimuli
and how the brain blocks out competing or non-relevant stimuli.
When breakdowns in inhibitory mechanisms occur, severe cognitive
impairments can ensue. One group of people afflicted with impaired inhibitory
mechanisms and thus attentional and information processing deficits are
schizophrenia patients. It has long been known that schizophrenia patients
exhibit a deficit of some sort which “floods” them with information (e.g.,
Bleuler 1911/1950; Kraepelin, 1913/1919). It is not surprising, then, that
schizophrenia patients show deficits in the two paradigms mentioned above.
However, it is unclear whether the deficits exhibited by schizophrenics are
specific to the paradigm being used or to a general failure to gate out
competing stimuli from interrupting ongoing processing.
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Two experiments will be reported in this paper. The first attempted to
gain a better understanding of how prepulse inhibition is initiated and
influenced by stimulus modality. The first experiment also investigated how
attention modifies PPI. Two measures, the startle eyeblink and skin
conductance responding, were used to assess the influences of stimulus
length, modality, and attentional set on prepulse inhibition. The second study
attempted to show for the first time a direct relationship between
sensorimotor gating, as assessed by prepulse inhibition, and backward
masking. If there is a direct relationship indicated by the results, a strong
case would be made for tying the two phenomena to a single sensorimotor
gating mechanism.
This paper will review in the following chapters the theory of
sensorimotor gating. Next will be separate reviews of prepulse inhibition and
backward masking. Following will be a review of the literature showing the
dysfunctions exhibited by schizophrenia patients in both of these phenomena
and how these dysfunctions might point to a common sensorimotor gating
deficit. Finally, the rationale and hypotheses of the two experiments will be
reviewed.
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Chapter 2
Theories of Sensorimotor Gatina
Neurons distributed from the cochlear nucleus to the primary auditory
cortex have been distinguished that differentially respond to the transient
portion of auditory stimuli or to the sustained portion of auditory stimuli (see
Musiek & Lamb, 1992, and Pickles, 1988, for reviews). Therefore, auditory
prepulses containing an onset and a sustained portion can activate both
short-time constant transient-sensitive neurons and long-time constant
sustained sensitive neurons. As Graham and Murray (1977) noted, “Gersuni
(1971) proposed that one function of the short-time system was rapid
conduction to higher centers of the information that environmental change
had been detected, while the more slowly acting long time system allowed for
finer analysis of stimulus information” (p. 114). Therefore, transient neurons
act as stimulus detectors while sustained neurons act as stimulus analyzers
or “discriminators” (Berg, 1985). Similar systems for responding to transient
or sustained portions of stimuli have been identified in the visual and tactile
modalities (Breitmeyer & Ganz, 1976; Gescheider, Hoffman, Harrison,
Travis, & Bolanowski, 1994; Schwartz & Loop, 1984).
Graham (1992) hypothesized that the onset of a stimulus initiates an
automatic process called the transient-detecting response (TDR). The TDR
rapidly conveys information that a transient change in the environment has
been detected, but not necessarily recognized or discriminated. Graham
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(1992) argued that the TDR initiates a gating response that attenuates
processing of subsequent stimuli from interfering with previous stimulus
processing. Naatanen (1992) further describes the TDR as a system
sensitive only to onsets and offsets of stimulus energy and able to detect the
change in energy but not able to process the qualitative aspects of the
energy.
Graham and Murray (1977) hypothesized that acoustic elicitation of
startle and PPI are controlled by the activation of short-time constant
neurons. Graham (1992) conceptualized two different processing “filters” that
are activated when low-intensity stimuli are presented: a low-pass filter that
elicits an orienting response (OR) and a high-pass filter that elicits a TDR.
The low-pass filter has the characteristic of responding to prolonged stimuli,
whereas the high-pass filter responds to brief stimuli. Low-pass filtering is
then conceivably responsible for processing continuous stimuli whereas high-
pass filtering is responsible for processing discrete stimuli, such as a
transient change in the environment. If a stimulus has characteristics that
would activate both filters, such as the transient onset of a tone with a
sustained output, the filters can sum algebraically.
Graham (1992) argues that the TDR is the mechanism for gating
subsequent stimuli or attenuating the effects of high intensity stimuli, such as
a startle burst. Furthermore, the OR, which is elicited by a stimulus that is
novel or unexpected and which facilitates sensory input and processing
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(Graham, 1975), can lead to startle facilitation at short lead intervals (e.g.,
Blumenthal & Levey, 1989).
Graham (1975) suggested the existence of an automatic preattentive
sensorimotor gating mechanism which protects processing of stimuli from
being interrupted by other stimuli. Graham formulated this as a negative
feedback loop which reduces distraction and protects preattentive stimulus
processing. Graham (1992) further refined this gating theory by defining
gating as a mechanism that separates the perception of one stimulus and
another following closely and prevents perceptual flooding. Researchers
have characterized the gating mechanism as a filtering process (e.g., Geyer
& Braff, 1987) which Graham (1975) maintained protects the processing of
the prepulse. Thus, greater PPI would be indicative of more thorough
processing of the prepulse.
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Chapter 3
Prepulse Inhibition
When a weak stimulus precedes a startling noise, the startle eyeblink
elicited by that startling noise will be reduced (Figure 1), even though the
startle eyeblink is an automatic reflex mediated at the brainstem level
(Graham, 1975; Hoffman & Ison, 1980). More specifically, if the interval
(referred to as the lead interval) between the onset of the prepulse and the
startle stimulus is relatively short (between 30 and 500 ms), the startle
eyeblink is inhibited in a phenomenon known as “prepulse inhibition” (PPI)
(see reviews by Blumenthal, 1999, and Filion, Dawson, & Schell, 1998).
Prepulse inhibition is thought to index a sensorimotor gating mechanism or a
protection of processing mechanism (see Filion et al., 1998 for a review).
Considerable evidence suggests that PPI is an automatic process in
that it requires only midbrain and lower brain structures (Leitner & Cohen,
1985), occurs in decorticate animals (Ison, O’Connor, Bowen, & Bocirnea,
1991), occurs in sleeping human adults (Silverstein, Graham, & Calloway,
1980) or while one is actively engaged in another task such as reading
(Reiter & Ison, 1977), and does not habituate due to repeated presentations
of the prepulse (Blumenthal, 1997; Lipp, Arnold, Siddle, & Dawson, 1994;
Schell, Wynn, Dawson, Sinaii, & Niebala, Experiment 1, 2000). PPI can be
elicited by a wide range of prepulses, acoustic, visual or tactile. The
maximum PPI in general is obtained at approximately 120 ms for acoustic
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baseline startle
blink
prepulse inhibition
EMG
startle alone prepulse + startle
Stimuli Events
Figure 1. Demonstration of a prepulse inhibition paradigm.
00
prepulses, 150 ms or later for visual prepulses, and between 150-250 ms for
tactile prepulses (see Blumenthal, 1999).
Several studies have supported the hypothesis that the transient
system makes the greatest contribution to PPI (Blumenthal & Levey, 1989;
Graham & Murray, 1977; Lane, Ornitz, & Guthrie, 1991; Wynn et al., 2000).
In these studies, the amount of PPI following discrete prepulses (brief,
nonstartling stimuli that terminate before the startle stimulus is presented;
Figure 2, top) was compared to the amount of PPI generated by continuous
prepulses (longer, nonstartling stimuli that continue up to and sometimes
beyond the onset of the startle stimulus; Figure 2, bottom) and results
showed that discrete prepulses produced larger amounts of PPI than
continuous prepulses, indicating that the transient portion is responsible for
gating. On the other hand, the continuous portion of a prepulse leads to
startle facilitation, partially canceling out the initial amounts of startle
inhibition produced by the transient change of the prepulse.
As pointed out above, Graham and Murray (1997) hypothesized that
the TDR is elicited by transient stimuli and the OR by prolonged stimuli. One
study which has measured both skin conductance orienting responses and
PPI has provided evidence supporting this hypothesis. Wynn, Dawson, and
Schell (2000) showed that transient, discrete auditory prepulses produced
significantly greater PPI than sustained, continuous prepulses, whereas the
continuous prepulses generated larger skin conductance responses (SCRs)
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Discrete Prepulse Paradigm
40 ms startle
190 me 500
120 ms
20 ms discrete prepulse Tim e (ms)
1000
Continuous Prepulse Paradigm
40 ms startle
500
Tim e (ms)
1000
120 ms
120 ms continuous prepulse
Figure 2. Demonstration of a discrete and continuous PPI paradigm.
o
11
than discrete prepulses, consistent with Graham’s (1992) hypothesis of
separate mechanisms for the TDR and the OR.
Although PPI is thought to be automatic, several studies have shown
that directing attention towards a prepulse enhances PPI relative to a to-be-
ignored prepulse or some other baseline PPI measure. Filion, Dawson, and
Schell (1993; 1994) used a selective attention paradigm involving a tone-
counting task. Subjects were presented 800 or 1200 Hz tones and were
instructed to attend to one and ignore the other. These tones were either 5 s
(standard) or 7 s (longer than usual) in length and served as prepulses.
Subjects were instructed to count the number of longer than usual tones of
one pitch and simply ignore the other tones. Using these to-be-attended and
to-be-ignored tones as prepulses, greater PPI was seen at a 120 ms lead
interval following the to-be-attended tone prepulse than following the to-be-
ignored prepulse. However, equal amounts of PPI were seen for the to-be-
attended and to-be-ignored tones at 60 ms lead intervals. Overall, PPI was
greatest at the 120 ms lead interval. The interpretation was that only
automatic processes are in effect at 60 ms lead intervals but controlled
attentional processes, which take time to develop, are influential at 120 ms
and dissipate after that (see Dawson, Schell, Swerdlow, & Filion, 1997, for a
review). These results have been replicated in many similar studies using
this attention-to-prepulse paradigm or a variation of it (Dawson, Hazlett,
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Filion, Nuechterlein, & Schell, 1993; Jennings, Filion, Schell, & Dawson,
1996; Schell, Dawson, Hazlett, & Filion, 1995; Schell et al., 2000).
However, all of these studies used continuous prepulses to assess the
effects of attention on PPI. Attentional modulation of PPI has also been seen
in other studies that used prepulses that were transient in nature. Hackley
and Graham (1987, Experiment 2) demonstrated greater inhibition of startle,
elicited by airpuffs to the eye, at a lead interval of 63 ms when discrete
auditory prepulses (19 or 40 ms in duration) were presented monaurally and
the subjects were directed to attend to which ear the prepulse would be
presented. DelPezzo and Hoffman (1980) used 50 ms visual prepulses
(spots of light near, but above, detection threshold) presented 150 ms before
a glabellar tap and found that focusing attention on the light produced more
PPI than not focusing attention on the light.
Hazlett, Dawson, Schell, and Nuechterlein (2001) measured PPI in a
continuous performance task (CPT). The stimuli for the CPT served as the
prepulses and were 50 ms visual presentations of numbers. Subjects were
instructed to press a button when a 3 (an attended target) was followed by a
7. Startle probes were presented either 120 or 1200 ms after the prepulses.
Attended prepulses were assessed by measuring PPI after the 3 (the target)
and ignored prepulses were assessed by measuring PPI after a non-target
number. The results of this study showed significantly greater PPI following
target (attended) stimuli compared to non-target (ignored) stimuli. This would
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13
indicate that PPI elicited by discrete visual stimuli can be enhanced by
directed attention.
It is apparent from the above summarized studies that the
phenomenon of prepulse inhibition is ubiquitous. PPI is seen regardless of
the modality of the prepulse or the startle, across subject populations
(including non-human animals), and across experimental design and
laboratories. It is generally maximal at lead intervals around 120-150 ms. PPI
is believed to be an automatic mechanism, elicited by a transient change in
the environment, that gates out subsequent stimuli which could detract from
the processing of immediately preceding stimuli. However, much recent
evidence points to the fact that this automatic process is influenced by
stimulus parameters (e.g., Braff et al., 2001; Wynn et al., 2000) and can be
enhanced or modified by controlled attentional processing.
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Chapter 4
Backward Masking
Visual backward masking refers to the phenomenon in which the
processing of a visual stimulus (a target) is interrupted by the presentation of
another visual stimulus (a mask) which follows the target very quickly. As it is
the stimulus that follows the presentation of the target that interrupts
processing of the target, it is said that the mask works backward in time to
interrupt that processing. This phenomenon is a theoretically fascinating
effect and, furthermore, its use in the study of clinical disorders, particularly
schizophrenia, has been very beneficial in gaining a better understanding of
how visual information is processed and how breakdowns in that processing
are evidenced in psychopathological populations (e.g., Green, Nuechterlein,
& Mintz, 1994a,b).
Visual backward masking can occur using four different techniques,
each distinguishable by the nature of the stimuli used, the time elapsed from
presentation of the target to the mask, and the effects of these variables on
masking functions. These four methods are reviewed in detail elsewhere
(e.g., Breitmeyer & Ogmen, 2001). Experiment 2 presented in this
dissertation employed structural masking, in which the mask, which shares
similar structural features of the target (e.g., straight lines such as those
found in the letter “T”), spatially overlaps the target (e.g., Turvey, 1973).
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Structural masking can occur due to integration of the target and the
mask or due to interruption of processing of the target by the presentation of
the mask. In integration, the target and the mask form a confusing
composite. Integration is strongest when the mask is of high energy and
overlaps the target spatially and when the interstimulus interval (ISI; offset of
the first stimulus to onset of the second stimulus) is shortest. Integration then
results in a confusing image due to the target and mask being presented in
quick succession and overlapping spatially (Breitmeyer & Ganz, 1976).
Interruption occurs when the mask interrupts a later stage of processing of
the target, rather than the mask and target fusing to form a confusing image.
In normal controls, interruption interferes with visual processing between 20
and 70 ms; at ISIs below 20 ms the effect is negligible and the effect
decreases after approximately 70 ms, when recovery from backward
masking begins. A nonmonotonic U-shaped curve is a result of backward
masking by interruption (Breitmeyer & Ganz, 1976).
In the visual system, neurons that respond either to the transient
nature or to the sustained nature of a visual presentation are present. Much
of the work detailing backward masking in terms of transient and sustained
neurons was reviewed by Breitmeyer and Ganz (1976). Briefly, transient cells
in the visual system are responsive to stimuli that have low spatial frequency
(e.g., blurry), are sensitive to the onset, offset, and location of a stimulus, and
have a short response latency. Sustained cells, however, are responsive to
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stimuli with a high spatial frequency (e.g., focused), and to stimuli that are
continuous, and have a longer response latency. Therefore, transient cells
are thought to detect the onset or offset of visual stimuli in order to locate
those stimuli, and sustained cells are thought to detect the finer details of the
visual stimulus in order to identify that stimulus (Breitmeyer & Ganz, 1976). It
can further be assumed that once the transient neurons detect a target,
sustained neurons begin to operate in order to more finely identify that target.
However, the transient portion of the mask can interfere with the sustained
processing of the target, thus interrupting target identification (Breitmeyer &
Ganz, 1976). Breitmeyer and Ganz (1976) have proposed that transient
stimulation resulting from the presentation of the mask laterally inhibits the
processing by sustained channels of the target, thus interrupting processing
of the target. If the temporal separation of the target and the mask is either
too short or too long for the activation of the transient channels by the
presentation of the mask to affect the sustained processing of the target,
masking effects are decreased or nonexistent.
Backward masking is thought to reflect interference by the mask with
processing of the target that is already in progress (Green et al., 1994a, b).
Green et al. (1994a, b), in studying backward masking deficits (that is, the
requirement of more time to recover from backward masking) in
schizophrenia, go further in describing the functional nature of backward
masking. They suggest that “backward masking might be viewed as a special
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condition of the general phenomenon of sensory gating” (p. 949) and that the
backward masking deficits exhibited by schizophrenia patients may be due to
the failure to gate out the disruptive effects of the mask. This raises the very
interesting possibility that recovery from backward masking effects may be
due to an active sensorimotor gating mechanism, not merely the passive
passage of time so that the transient onset of the mask no longer interferes
with sustained processing of the target.
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Chapter 5
Schizophrenia and Information Processing Deficits
Schizophrenia has long been associated with deficits in attention and
information processing (Bleuler, 1911/1950; Kraepelin, 1913/1919). The work
of McGhie and Chapman (1961) has most explicitly shown these
impairments by relating first hand accounts from patients themselves, a few
of which are noted below:
“My concentration is very poor. I jump from one thing to another. If I
am talking to someone they only need to cross their legs or scratch
their heads and I am distracted and forget what I was saying. I think I
could concentrate better with my eyes shut. ”
“I can’t move if I am distracted by too much noise. ”
“Things are coming in too fast. I lose my grip of it and get lost. I am
attending to everything at once and as a result I do not really attend to
anything. ”
“My trouble is that I’ve got too many thoughts. ”
“Everything seems to go through me. I just can’t shut things out. ”
These quotes were selected to represent the varieties of flooding that
schizophrenia patients report and that in turn affect attention and thought.
McGhie and Chapman (1961) believe that there is an initial heightening of
sensory input into the auditory and visual channels, which might lead the
patient to attend involuntarily to features which are normally in the
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19
background. That is, there is a breakdown in the inhibitory mechanisms
related to attention which would then cause flooding of the consciousness
“with an undifferentiated mass of incoming sensory data, transmitted from the
environment via the sense organs” (p. 112).
Where this breakdown occurs in the train of automatic or controlled
attentional processes is unclear. Nuechterlein and Dawson (1984) believe
that the variety of experiments which have shown deficits in information
processing in at-risk, chronic, symptomatic, and relatively remitted
schizophrenia patients fail to point to a single deficient process but rather to a
variety of processes. Callaway and Naghdi (1982) on the other hand believe
that schizophrenia patients have normal or above normal automatic
processing whereas they have a deficit in later controlled processing. By
examining how PPI and backward masking are both dysfunctional in
schizophrenia patients, one might begin to see how the two point to a single
dysfunctional sensorimotor gating mechanism.
Prepulse Inhibition and Schizophrenia
Braff, Stone, Callaway, Geyer, Glick, and Bali (1978) were among the
first to examine PPI in both normal controls and schizophrenia patients.
Using continuous prepulses that lasted the length of the lead interval, and
lead intervals of 30, 60, 120, 240, 500, and 2000 ms, 20 normal controls and
12 schizophrenia patients (tested on average 3 weeks after hospitalization)
were tested. It was found that for normal controls maximal and significant
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20
blink inhibition occurred at 120 ms. With schizophrenia patients inhibition at
120 ms was not statistically significant (although, by visual examination of the
graphs provided, it appears that inhibition for patients was also maximal at
120 ms). Braff et al. interpret their results as indicating that in schizophrenia
patients there is a loss of inhibition which “may lead to faulty information
intake and disordered early processing due partly to the distracting effects of
the motor components of the blink” (p. 343, Braff et al., 1978).
In a later study, Braff, Grillon, and Geyer (1992) examined PPI in
schizophrenia patients and normal controls using acoustic prepulses and
acoustic and tactile startle-eliciting stimuli. This study used inpatient
schizophrenics (average duration of illness was 12. 5 years) and normal
controls. PPI was examined at three lead intervals: 30, 60, and 120 ms for
the acoustic startle and at 120 ms for the tactile startle. It was shown that the
normal controls exhibited significantly greater PPI across all lead intervals
using either tactile or acoustic startle. They interpreted their results to support
and expand the hypothesis that schizophrenia patients have an impairment
in sensorimotor gating which may be quite global and more general as their
impaired PPI results occurred across both the acoustic and tactile modalities.
The two previous studies examined PPI in a passive setting, where
the subjects were not given any instructions regarding the prepulses.
Dawson et al. (1993) wanted to determine the effects of attentional
manipulation of the prepulse in normal controls and schizophrenia patients.
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21
They further wanted to examine patients who were in a relatively remitted
state and who were more recent- in the onset of their illness, as opposed to
the two previous studies. Using the same paradigm as Filion et al. (1993;
1994), both patients and normal controls received to-be-attended and to-be-
ignored prepulses at lead intervals of 60, 120, and 240 ms. As in the Filion et
al. and other studies with normal individuals, normal controls exhibited
significantly greater PPI to the to-be-attended prepulses at 120 ms than to
the to-be-ignored prepulses. Schizophrenia patients, on the other hand,
failed to exhibit greater PPI to the to-be-attended versus the to-be-ignored
prepulse; however, the patients exhibited equal amounts of inhibition to the
to-be-ignored prepulse as the normal controls at 120 ms, and equal amounts
as the controls to both prepulses at 60 ms.
Dawson et al. (1993) hypothesized that the increased amount of PPI
to the attended prepulse as one goes from a 60 ms to 120 ms lead interval is
due to the call for or allocation of controlled attentional processes. They
hypothesize that at the 60 ms lead interval, only preattentive processes are
in effect in order to detect and evaluate the prepulse. With regards to the
results for the schizophrenia patients, they hypothesized that these patients
have normal amounts of preattentive processing of the prepulse, as
evidenced by normal amounts of PPI compared to normal controls at 60 ms
to both prepulses and at 120 ms to the ignored prepulse, but are impaired in
allocating additional, controlled attentional processes in order to enhance
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22
processing of the attended stimulus at a lead interval of 120 ms, as
evidenced by the lack of differential PPI to the to-be-ignored and to-be-
attended prepulses.
Therefore, it appears that in relatively asymptomatic and recent-onset
schizophrenia patients, their automatic sensorimotor gating processes
remain intact but their ability to enhance this gating process is abnormal,
contrary to the results of Braff et al. (1978; 1992). Regardless of whether the
impairment is solely in automatic processing or is in attentional regulation of
an automatic process, it is evident that schizophrenia patients have some
impairment in PPI, reflecting a dysfunctional sensorimotor gating mechanism.
Backward Masking and Schizophrenia
In light of the early information processing deficits hypothesized in
schizophrenia patients, researchers made the logical choice to study
backward masking within this population. Numerous studies in the past 25-30
years have attempted to determine the nature of this visual processing deficit
in schizophrenia.
Saccuzzo, Hirt, and Spencer (1974) studied schizophrenia patients
and controls on a backward masking paradigm. Critical viewing periods of
the target were determined individually to where each subject had an 80%
detection rate (schizophrenia patients had a significantly higher viewing time
than controls). There was a no-mask control condition and five ISI conditions:
50, 100, 150, 200, and 300 ms. Using a correct detection amount as the
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23
dependent variable, they found significant group differences, ISI differences,
but most importantly a significant group x ISI interaction. Normal controls
showed decreasing masking effects as the ISI increased from 100 to 150 ms,
leveling off after that point. Schizophrenia patients showed an increase in
correct responses from 50 to 150 ms but did not begin to reach their no-mask
control level until approximately 300 ms. Saccuzzo et al. interpreted these
results as showing a possible disturbance in early information processing
mechanisms. They hypothesized that since patients only began to show
escape from masking at 300 ms, patients were perceiving a mix of the target
and mask which degraded their performance over a longer duration of time.
Studying ten relatively asymptomatic, young schizophrenia patients
who were on antipsychotic medication, Miller, Saccuzzo, and Braff (1979)
studied backward masking effects using targets and masks of equal energy
with durations of 2 and 4 ms, with SOAs of 10, 75, 150, and 250 ms and a
no-mask control. Correct response detection (i.e., hit rate) was the
dependent variable. They found that normal controls performed significantly
better, for each target duration, than the patients across all SOAs. Miller et al.
interpret these findings as showing a susceptibility of early visual information
processing to masking effects in relatively asymptomatic schizophrenia
patients.
Green and Walker (1984) studied backward masking deficits to
determine if this deficit was more pronounced in schizophrenia patients with
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24
positive or negative symptoms. Critical ISI was the dependent variable,
defined as the ISI where the patient was performing at 71% accuracy.
Results showed a significantly longer critical ISI for patients with negative
symptoms than for patients with positive symptoms.
Rund (1993) examined differences in normal controls and
schizophrenia patients, both chronic and nonchronic. There was a no-mask
control condition and two SOA conditions, one 16. 5 ms and the other 33. 0
ms. For masking conditions at both SOAs, normal controls performed
significantly better in terms of number of correct responses than the
schizophrenia patients. However, when the patients were split into the
chronic and nonchronic classification, nonchronic schizophrenia patients
performed just as well as the normal controls at both masking SOAs whereas
the chronic patients were significantly poorer in performance to both the
normal controls and the nonchronic patients. This suggests that backward
masking deficits in schizophrenia are susceptible to the time course of the
disease. However, Rund pointed out that it is difficult to interpret these
findings due to the chronic patients’ poorer performance on the no mask
condition (a condition which can be presumed to measure the ability to
apprehend and report on briefly presented stimuli). The effects of chronicity,
aging, or deteriorating cognitive status as the disease progresses could
possibly affect overall performance and thus be reflected in poorer
performance on backward masking trials as well as in the no mask condition.
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25
Unfortunately, Rund did not adjust for the poorer performance by the patients
in the no-mask condition in his analysis, further making interpretation of the
findings difficult.
Green et al. (1994a, Experiment 2) went another step further in
studying backward masking in schizophrenia patients to determine whether
their deficits were in integration or interruption mechanisms. Chronic
schizophrenia patients and normal controls were used. As mentioned above,
integration backward masking is best elicited by a high energy mask and
interruption backward masking is best elicited by a low energy mask and a
mask that does not spatially overlap the target.
A low energy mask was used and the mask was surrounded, but did
not overlap, the target. ISIs of 10, 20, 30, 40, 50, and 60 ms were utilized.
Results showed that the patients’ performance was poorer compared to the
controls across all ISIs. Furthermore, controls showed the expected U-
shaped curve that is expected of masking by interruption: this shows that
interruption, but not integration, was achieved by this manipulation.
Schizophrenia patients, however, only showed the initial descending portion
of the U-curve but not the ascending, recovery portion of the curve. These
results show a clear deficit in schizophrenia patients in recovery from
masking by interruption. Green et al. (1994a) described the results as
showing disruption was due to attention being diverted from the target to the
mask, possibly due to an abnormal transient/sustained interaction.
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26
Green et al. (1994b) further tried to separate the relative contribution
of transient and sustained mechanisms to backward masking. In order to
accomplish this, they attempted to use targets that limited the use of
sustained visual channels. Schizophrenia patients and normal controls were
studied under two conditions. The first condition used a target that was
blurred (which eliminated high spatial frequency information), a low energy
mask (i.e., one quarter the energy of the target), and ISIs of 5, 10, 20, 40, 70,
and 100 ms. In the second condition, the target was clear but subjects were
instructed to just name the location of the target (out of four possible
locations) but did not need to identify it; the energy of the mask and the
target were the same. ISIs for this condition were 5,10, 15, 20, 30, and 50
ms. The results for the “blurred” condition showed that schizophrenia patients
performed significantly worse (as defined by hit rate) than normal controls
across all ISIs except 10 and 40 ms. The results of the “location” condition
showed that the patients differed significantly across all ISIs compared to the
controls. Green et al. interpret these results as showing abnormalities in the
transient channels of schizophrenia patients; if they had abnormalities in the
sustained channel, there would have been no deficits found using targets
isolated to the transient channel. Furthermore, they hypothesize that these
backward masking deficits are probably due to sensorimotor gating deficits
as the subjects failed to attenuate the disruptive effects of the mask.
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27
Chapter 6
Converging Evidence Across Paradigms
Given the evidence that these paradigms seem to be tapping into a
sensorimotor gating process, it is surprising that there is only one experiment
that has attempted to measure backward masking and PPI in the same
recording session.
Perry and Braff (1994) examined information processing deficits in
schizophrenia using PPI and a backward masking paradigm and attempted
to relate these processing deficits to thought disorder. Fifty-two
schizophrenia patients underwent prepulse inhibition testing using acoustic
and tactile prepulses as well as undergoing testing on backward masking.
Subjects received various diagnostic interviews, one being the Rorschach-
derived Ego Impairment Index, which contains a human experience scale
and good and poor response subscales. It is thought that the Ego Impairment
Index taps into psychotic processing (Perry & Viglione, 1991). Results
showed significant negative correlations of the poor responses on the human
experience variable of the Ego Impairment Index with both backward
masking (at a 240 ms ISI) and with auditory and tactile PPI (at an SOA of
120 ms). Although correlations between backward masking and PPI were not
examined, the results provide support that both reflect information processing
deficits and specifically are related to thought disorder impairments.
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28
Although there has been only this one study assessing backward
masking and PPI in the same subjects, there are many other studies that
have assessed both paradigms in separate, but similar, populations.
Subjects exhibiting schizotypal traits have been shown to manifest deficient
PPI (e.g., Cadenhead, Geyer, & Braff, 1993) or deficient attentional
modulation of PPI (e.g., Schell et al., 1995) as well backward masking
deficits (e.g., Saccuzzo & Schubert, 1981). Similarly, adolescents with ADHD
show deficient PPI (e.g., Castellanos, Fine, Kaysen, Marsh, Rapoport, &
Hallett, 1996) as well as backward masking deficits (e.g., Rund, Oie, &
Sundet, 1996). These similarities are not limited to clinical populations, as
elderly subjects show deficits in PPI (e.g., McDowd, Filion, Harris, & Braff,
1993) as well as backward masking deficits (e.g., Walsh, Williams, &
Hertzog, 1979).
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29
Chapter 7
Summary and General Hypotheses
The purpose of this investigation was to gain a better understanding of
how prepulse inhibition is affected by stimulus parameters and directed
attention. Furthermore, the investigation was conducted to show that
prepulse inhibition and backward masking are related to sensorimotor gating,
and indeed reflect a single sensorimotor gating mechanism. The evidence
shown above led to the formulation of two separate experiments to test my
hypotheses.
In the first experiment, the effects of attention and modality on
prepulse inhibition and skin conductance orienting was tested using discrete
and continuous prepulses. This first experiment also attempted to show that
the same type of stimuli used in visual backward masking studies (i.e., very
brief visual stimuli) are similar to discrete visual prepulses, and in fact may be
acting in a similar manner. It was hypothesized that discrete prepulses, both
auditory and visual, would produce greater PPI than continuous prepulses, in
keeping with the theory that transient changes in the environment initiate
PPI. It was further hypothesized that attention would affect both discrete and
continuous prepulses, with greater PPI produced by attended prepulses
versus ignored prepulses. Lastly, it was hypothesized that a dissociation
between the orienting response, as indexed by skin conductance responding,
and the transient detecting response, as indexed by PPI, would be seen,
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30
providing support for Graham’s (1992) hypothesis regarding mechanisms
that respond differentially to discrete and continuous stimuli. This experiment
was conducted to gain a better understanding of how PPI is affected by
stimulus parameters, as the choice of parameters can influence the findings
in studying schizophrenia deficits in PPI (Braff et al., 2001). Most importantly,
it was conducted to show the similarity between a target in a backward
masking study and a visual prepulse and how that stimulus can result in PPI.
The second experiment was conducted to show that recovery from
backward masking effects is related to sensorimotor gating, as assessed by
PPI. It was hypothesized that the escape from backward masking effects is
due in part to a sensorimotor gating mechanism gating out the interruptive
effects of the mask. A significant negative relationship between PPI and
backward masking scores would show that recovery from backward masking
might indeed be due to sensorimotor gating. The same stimulus that served
as the target in the backward masking task served as the visual prepulse. In
addition, auditory PPI was elicited within an intermixed series of trials and
related to backward masking to show that the relationship between PPI and
recovery from backward masking was not modality-specific. Last, I was able
to assess the relationship between visual and auditory PPI. If there is a
single sensorimotor gating mechanism that is elicited solely by any transient
change in the environment, regardless if that change is auditory or visual,
visual and auditory PPI should be correlated.
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It was my hope to further our knowledge of sensorimotor gating, as
assessed using PPI, and our understanding of how that mechanism is
deficient in schizophrenia. I further hoped to show a clear relationship
between two paired stimulus paradigms, PPI and backward masking, in
which schizophrenia patients also are deficient. These findings attempted to
clarify the contribution of sensorimotor gating to the orderly processing of
information.
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32
Chapter 8
Experiment 1
Two aspects of the above mentioned issues were addressed in the
present study. First, although it has been shown that PPI is mediated by
transient onsets as opposed to sustained portions of stimuli, the only studies
that have directly compared continuous and discrete prepulses have used
auditory prepulses. The present experiment tested whether PPI is also
sensitive to transient versus sustained visual stimuli. It was hypothesized that
PPI for transient auditory and visual stimuli would be greater than for
sustained auditory and visual stimuli. If transient prepulses, whether auditory
or visual, both show greater PPI than sustained prepulses, this would provide
evidence that a single, non-modality-specific sensorimotor gating system,
initiated by the TDR, is responsible for prepulse inhibition. Moreover, since
both auditory and visual PPI are measured for each subject, I assessed
whether the two measures are correlated. It was expected that the two
measures of PPI would be positively correlated, reflecting a single
sensorimotor gating mechanism that is initiated by transient changes in
auditory or visual stimulation.
The second purpose of this experiment was to determine in greater
detail how attention can enhance PPI by examining the effects of attention on
discrete and continuous prepulses. That is, does attention act solely to
enhance the amount of inhibition initiated by the transient portion of a
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33
prepulse or does it also act to attenuate the amount of facilitation initiated by
the sustained portion of a prepulse? If attention acts to increase inhibition via
the transient system, attentional modulation of startle should be seen equally
with both discrete and continuous prepulses. If attention also acts to increase
inhibition by attenuating the facilitation initiated by the sustained portion of a
prepulse, then attentional modulation should be greater with continuous
prepulses than discrete prepulses. In light of the evidence reviewed earlier, I
hypothesized that attentional modulation of PPI would be observable with
both discrete and continuous prepulses. I also examined whether prepulse
inhibition acts to decrease the sensory representation of a startle stimulus, as
opposed to acting only to decrease the motor response to the startle, by
examining a system that does not have a strong motor component, namely
skin conductance. If skin conductance responses to the startle stimulus
behave in a similar manner to startle motor responses (i.e., if skin
conductance responding to a startle burst is inhibited by a prepulse), it would
strongly suggest that the sensory information of a startle noise burst is being
inhibited.
Methods
Subjects
Data on a total of 75 subjects were collected. There were 38 subjects
in the continuous group and 37 subjects in the discrete group. There were 59
females and 16 males. After exclusions (see Results) there were 4 males
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34
and 27 females in the continuous group and 9 males and 22 females in the
discrete group.
Design
This study used a2X2X2X2 mixed design. The first variable was
modality, with prepulses in either the visual or auditory modality, which was
varied within subject. The second variable was attention, with prepulses
either attended or ignored, which was varied within subject. The third variable
was lead interval, with lead intervals of 120 ms and 150 ms, varied within
subject. The last variable was prepulse duration, which was a between
subject factor, with groups receiving only continuous prepulses or only
discrete prepulses.
Procedure
Upon arrival to the laboratory, subjects were asked to read and sign a
consent form and given some brief instructions. After reading these items,
subjects were instructed to wash their hands with soap and water. Next,
electrodes were attached for recording of skin conductance and EMG
responses.
The first phase of the experiment began with the subject instructed to
match the intensity of two colored lights, specifically, to adjust the intensity of
a green light, using a rheostat, to match the intensity of a blue light. Both
lights were presented simultaneously for five seconds during which time the
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35
subject could adjust the intensity as needed. If the subject needed more time,
another five seconds were given
The next phase of the experiment had the subjects match the intensity
of the auditory stimuli to the intensity of the visual stimuli. For all subjects, the
blue light was always paired with the high pitch tone while the green light was
always paired with the low pitch tone. Subjects were instructed that they
would hear one tone and see one light simultaneously and were to report
whether that tone should be louder in order to be as loud as the light was
bright, should be softer, or if there should be no change. Any changes to tone
intensity were made in one decibel increments. Subjects could repeat this
task as many times as needed until they were comfortable with their choice.
After choosing their desired tone intensity, the light and tone were presented
again to ensure that subjects were comfortable with the choice they made.
After the first light and tone were matched, the same procedure was
repeated for the other light and tone. The matching of stimulus intensity was
performed to produce equally subjectively intense visual and auditory stimuli,
as it has been shown that stimulus intensity affects PPI (Blumenthal, 1996)
After the auditory and visual matching task, subjects were again
presented with the stimuli they were going to encounter. First, subjects were
instructed about and presented examples of the light and tone that were to
be attended during the task. Next, subjects were presented with the other
tone and light that they could simply ignore during the task.
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36
Finally, subjects were then given instructions about the task they were
to perform. Subjects were instructed that they would either see or hear their
attended stimuli followed by a target tone. They were told that for most trials,
the delay until the target tone was a normal length of 4 s. However, on a
subset of the trials this delay would be a longer-than-usual 7 s. Subjects
were instructed to count how many times they encountered a longer than
usual delay of 7 s. Subjects were given two examples of this task, both of
which contained one visual and one auditory to-be-attended stimulus, where
one trial had the normal delay-to-target of 4 s while the other trial had the
longer-than-usual delay of 7 s. After these two examples, subjects were
asked which trial had the longer-than-usual delay. If they responded correctly
they were given the choice to repeat the example once or to move on. If they
responded incorrectly, they were told which trial had the longer-than-usual
delay and were presented with one more example. Next, four trials consisting
of the presentation of all experimental stimuli were given to the subject. This
allowed the subjects to practice their task while encountering both their
attended and ignored stimuli and the startle burst.
To remind subjects of which stimuli they were to attend throughout the
session, a card with the names of the attended stimuli (e.g., “green, low”)
was placed on top of the light presentation box. In order to ensure that
subjects remained motivated to perform the task, they were offered a bonus
of $5 for perfect performance. However, subjects were told that for each
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37
number they were off from the correct number of longer-than-usual delays to
the target tone, $1 would be subtracted from their bonus.
For the experimental session, 36 trials were presented. These trials
were presented in a mixed, pseudo-random order. Each auditory and visual
prepulse was presented 9 times, resulting in 9 green prepulses, 9 blue
prepulses, 9 low pitch prepulses and 9 high pitch prepulses. Six out of the
nine prepulses of each type were probed with three startle probes each at
the 120 ms and 150 ms lead intervals. The other three trials were not probed
and were “clear” trials in order to record SCRs to uncontaminated prepulses.
Therefore, a total of 24 probed prepulse trials resulted. For each prepulse
type at each lead interval for both the attended and ignored stimuli, an
average of the responses to the three prepulses was calculated. There were
also a total of 18 intertrial interval (ITI) startle-alone white noise probes which
served as subjects’ baseline startle responding. Responses to these 18 ITI
probes were averaged to create a startle-alone baseline for each subject.
The experimental testing session lasted approximately 20 minutes.
Halfway through the experiment subjects were informed how much time they
had left in order to keep the subjects alert and focused on their task. At the
end of the experiment subjects were asked how many longer-than-usual
delays to their attended stimuli they encountered and were paid their
appropriate bonus. Out of the 36 prepulse stimuli, 12 (attended and
unattended) had a longer than usual delay of 7 s. Therefore, the correct
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38
number of longer-than-usual delays until the target for attended stimuli was
six. Out of these six, three occurred for both the auditory and visual stimuli.
Subjects were then given an opportunity to ask any questions they might
have and were then dismissed.
Experimental Stimuli
Prepulses were 50 ms in duration for the discrete group and 500 ms in
duration for the continuous group. The startle stimulus consisted of a 50 ms,
near instantaneous rise/fall time, 109 dB(A) white noise burst. The low pitch
and high pitch tones were 500 Hz and 1500 Hz, respectively, with a 10 ms
rise/fall time. These tones varied in decibel level, predetermined by the
subject. The target tone was a 2000 Hz, 10 ms rise/fall time, 80 dB(A), tone
that lasted for 1 s. Auditory stimuli were presented through binaural
Telephonies TDH-50P headphones. Decibel levels were recorded with a
Realistic sound level meter using a Quest Electronics earphone coupler.
Visual stimuli were presented from a custom made light box, situated
approximately 45” in front of the subject at eye level. The green light
appeared on the left and the blue light appeared on the right. The different
color lights were made by placing blue and green lighting gels behind an
opaque white gel and backlighting these gels. Each light field was
rectangular with a height of 8.5” and a width of 6.5”. The blue light was
backlit using a 75 W light bulb and remained at constant brightness. The
green light was backlit using a 100 W light bulb connected to a subject-
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39
controlled rheostat which controlled brightness. Brightness levels were
recorded using an Extech light meter set to ft/cd2 held approximately 0.5
inches from the front of the light box.
Recording and Scoring of Dependent Variables
Eye blinks were scored as EMG activity of the orbicularis oculi muscle
of the left eye. One small (4 mm) silver-silver chloride (Ag-AgCI) electrode
was placed on the left eyelid directly below the pupil while a second 4 mm
electrode was placed approximately 1 cm lateral to the first. The impedance
between the two electrodes was measured and was deemed acceptable
below 10 kOhms. A large (8 mm) Ag-AgCI electrode was placed behind the
left ear to serve as a ground. Skin conductance data were collected using
standard procedures (Dawson, Schell, & Filion, 2000) using two 8 mm Ag-
AgCI electrodes, filled with isotonic conductive paste, placed on the volar
surface of the distal phalanges of first and second finger of the left hand. A
constant 0.5 V DC was applied across the electrodes.
Stimulus presentation and data acquisition were controlled through
Contact Precision Instruments equipment and a computer running Psylab 7
software. Both the raw EMG (filtered at 10 Hz high pass and 500 Hz low
pass) and skin conductance signals were collected at a rate of 1000 Hz. The
data were stored and exported for analysis in microvolt values for EMG and
microSiemens (pS) for skin conductance. For analysis, the EMG signal was
software integrated using a 20 ms time constant. Startle response onset was
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40
set to be detected within a window of 20-120 ms while peak activity was set
within a window of 20-200 ms. Skin conductance data were first compressed
into a rate of 20 Hz and scored as a response beginning between 1-4
seconds after stimulus onset and having an amplitude of at least 0.05 pS.
Skin conductance responses were then square root transformed for data
analysis.
Prepulse inhibition was calculated as a percent change score:
[(prepulsed startle - startle alone) / (startle alone) * 100]. Percent change
units are preferred over difference scores (prepulsed startle - startle alone)
because difference scores in absolute pV units are correlated with baseline
startle blink amplitude whereas percent change scores are not, removing any
dependence on baseline startle amplitude (Jennings et al., 1996).
Results
Exclusions and Outlier Analysis
Before any statistical analysis, subjects scoring less than an average
of 1.0 pV to startle-alone trials (based on the average of all 18 ITI startle-
alone trials) were considered non-responders and were excluded from further
analysis. Furthermore, subjects scoring more than 5 errors on the task were
excluded due to an inability to perform the task adequately. A total of 13
subjects (7 from the continuous group and 6 from the discrete group) met
one of these criteria. This left a total of 31 subjects in both the continuous
prepulse group and the discrete prepulse group.
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41
After these exclusions, outlier analyses were conducted on the EMG
percent change values to the prepulse + startle trials and on startle amplitude
during startle alone trials. An outlier was considered to be any score greater
than three standard deviations above the mean and greater than two
standard deviation above the next highest score. Based on these rules nine
trials (0.6% of total trials) were considered to be outliers. These outliers were
replaced with the average of the other two similar trials.
Psychophysical Matching of Auditory and Visual Stimuli
As stated in the methods, subjects were instructed to match the
perceived intensity of the green light to that of the blue light, then match the
perceived intensity of the blue light with the high pitch tone and the green
light with the low pitch tone. The mean values in decibels for the tones and in
ft/cd2 for the lights are presented in Table 1. Although it was expected that
the values of the physical intensities of the two lights and two tones would
differ due to differential sensitivity of the ear and eye to frequency of sound
and light, it was important that the groups did not differ in the values. T-tests
were performed to compare each tone and each light across group and
showed that, in fact, the groups did not significantly differ from each other.
Group Comparisons for Startle-alone ITI and Task Errors
Startle-alone ITI values and task errors were compared between
groups to ensure that both groups performed equally well on the task and did
not differ in their baseline startle responding. The continuous group had an
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42
Table 1: Physical intensities for auditory prepulses (in dB(A)) and visual
prepulses (in ft./cd2 ).
Group Mean (S. D.) Total Mean (S. D.)
Low tone
Continuous 68.84 (3.78) dB(A)
68.89 (3.59)
dB(A) Discrete 68.94 (3.44) dB(A)
High tone
Continuous 70.61 (3.55) dB(A)
70.98 (3.39)
dB(A) Discrete 71.35 (3.24) dB(A)
Blue light
Continuous 57.89 (5.63) ft./cd2
58.79 (4.22)
ft./cd2 Discrete 59.69(1.69) ft./cd2
Green light
Continuous 53.58 (15.13) ft./cd2
53.36 (16.95)
ft./cd2 Discrete 53.15 (18.85) ft./cd2
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43
average ITI value of 11.06 |u iV whereas the discrete group had a value of
12.63 p,V; this difference was not significant (t(60) = 0.56, £ = 0.58). The
continuous group made an average of 1.74 errors on the task whereas the
discrete group made an average error of 2.32 and this difference was not
significant (t(60) = 1.61, £ = 0.11).
Startle Modification Analyses
Percent change startle modification scores were analyzed with a 2
(modality) X 2 (attention) X 2 (lead interval) X 2 (group) mixed ANOVA.
Results showed main effects of modality (F (1, 60) = 70.47, £ < 0.001), lead
interval (F (1, 60) = 15.43, £ < 0.001), and group (F (1, 60) = 12.53, £ =
0.001). Interactions between modality and attention (F (1, 60) = 6.40, £ <
0.02) and modality and lead interval (F (1, 60) = 7.51, £ < 0.01) were also
seen. There were no other main effects or group interactions.
Results from the ANOVA show a clear advantage for discrete
prepulses to produce greater PPI (average = -41.31%) than continuous
prepulses (-18.82%). It was also seen that auditory prepulses produced
significantly greater PPI (-50.00%) than visual prepulses (-10.13%).
Furthermore, the 150 ms lead interval produced significantly greater PPI
(-36.32%) than the 120 ms lead interval (-23.81%) (see Figure 3).
The significant modality x lead interval interaction is due to the visual
prepulse at a 150 ms lead interval producing greater PPI than at a 120 ms
lead interval, whereas the lead intervals did not produce a differential effect
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20
10
0
_ -10
0)
¥ -20
6 -30
no
K "40
^ -50
-60
-70
-80
Auditory-120 Auditory-150 Visual-120 Visual-150
Figure 3. Mean percent change PPI scores, averaged over to-be-attended and to-be-ignored prepulses,
as a function of group, modality, and lead interval. ‘ Significant difference between discrete and
continuous auditory prepulses at 120 and 150 ms lead intervals. “ Significant amount of prepulse inhibition
for the discrete visual prepulse at a 150 ms lead interval. Bars represent the standard error of the mean.
■ Continuous
■ Discrete
45
for the auditory prepulses. T-tests showed that visual prepulses at 120 ms
lead intervals (-0.14%) differed significantly from visual prepulses at 150 ms
lead intervals (-20.13%), t(61) = 4.41, £ < 0.001. The combined effects of
group, modality, and lead interval, seen in Figure 3, were such that although
both continuous and discrete auditory prepulses produced significant PPI at
both lead intervals, only discrete, but not continuous, visual prepulses
produced significant PPI, and only at the 150 ms lead interval. The mean for
the 150 ms lead interval discrete visual prepulse was -28.99%, and was
significantly different from zero, t(30) = 4.35, £ < 0.001.
The source of the significant modality x attention interaction can be
seen in Figure 4. For the auditory prepulse (averaging over prepulse
durations), PPI to the to-be-attended prepulse was significantly greater than
to the to-be-ignored prepulse (t (61) = 2.08, £ < 0.05, one-tailed), whereas for
the visual prepulse, PPI was non-significantly greater to the to-be-ignored
prepulse than to the to-be-attended prepulse.
Further Analysis by Group
Contrary to our initial hypothesis, attention only increased PPI when
the prepulse was continuous and in the auditory modality. Because we
hypothesized a priori that attentional modulation would be significant in both
the continuous and discrete groups, we examined attentional modification
separately for each group. Paired one-tailed t-tests were performed for each
attended versus ignored tone and light for both the continuous and discrete
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lission o f th e copyright owner. Further reproduction prohibited without permission.
10
0
-10
•a -2 0
O)
-30
O
£ ~4 0
£ -50
-60
-70
-80
Figure 4. Mean percent change PPI scores, averaged over lead interval, as a function of group,
modality, and attention. ‘ Significantly greater PPI to the to-be-attended prepulse compared to
the to-be-ignored prepulse for the continuous auditory prepulses. Bars represent the standard
error of the mean.
Attend
Continuous- Continuous-
Auditory Visual
Discrete-
Auditory
Discrete-
Visual
- P -
C D
47
groups. As can be seen in Figure 4, examining PPI to auditory prepulses
(averaged over lead intervals), only the continuous group showed greater
PPI to the attended prepulse (-40.70%) than to the ignored prepulse
(-28.24%), t(30) = 1.82, g < 0.04, one tailed.
Correlation between Auditory and Visual PPI
In order to assess the relationship between auditory and visual PPI,
PPI values produced by the auditory and by the visual prepulses at a lead
interval of 150 ms in the discrete group only were correlated (only the
discrete group and only this lead interval were analyzed because it was only
in this particular combination that any significant amount of PPI to a visual
prepulse was seen). As expected, there was a significant positive correlation
between auditory and visual PPI, Pearson’s r (31) = 0.52, g < 0.01.
SCR Analyses to Prepulse Alone Trials
SCRs to the prepulse alone trials were subjected to a 2 (modality) X 2
(attention) X 2 (group) mixed ANOVA. Results of this ANOVA showed a main
effect of attention, F (1, 60) = 6.40, g < 0.02, an interaction between attention
and modality, F (1, 60) = 8.10, g < 0.01, and a marginal main effect of
modality, F (1, 60) = 3.85, g < 0.06. There was no group effect or any other
interaction.
To-be-attended prepulses generated SCRs of 0.33 pS versus 0.28 pS
for to-be-ignored prepulses. For the marginal modality main effect, visual
prepulses generated SCRs of 0.32 pS versus 0.28 pS for auditory prepulses.
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48
Means for the attention x modality interaction can be seen graphically
represented in Figure 5. It can be clearly seen that this interaction is due to a
significant effect of attention on the SCR to the visual prepulse but not the
auditory prepulse.
Prepulse + Startle SCR Results
Percent change scores for the SCR elicited by prepulse + startle
combinations were generated in a similar manner to the blink analyses; that
is, the difference in the amount of skin conductance responding to the paired
prepulse + startle compared to the startle alone was computed as a ratio of
the startle alone response. SCRs were scored as responses occurring 1-4 s
after the onset of the startle stimulus in the prepulse + startle pair. These
results were analyzed with a 2 (modality) X 2 (attention) X 2 (lead interval) X
2 (group) mixed ANOVA. It is important to note that the SCRs represent the
amount of responding elicited by the prepulse combined with the amount of
responding elicited by the startle pulse (SCRs to the startle burst averaged
0.41 |iS) and that SCRs to the prepulse plus startle stimulus were greater
than to the startle stimulus alone (percent change scores were generally
positive).
Main effects of modality (F (1, 60) = 4.17, £ < 0.05), attention (F (1,
60) = 5.44, £ < 0.03) and lead interval (F (1, 60) = 6.96, £ < 0.02) were found.
There was no main group effect nor any significant interactions. It was found
that visual prepulses resulted in significantly larger SCR percent change
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0.5
0.4
^ 0.3
0.2
0.1
0
Attend
Ignore
*
Continuous Continuous Discrete Discrete
Auditory Visual Auditory Visual
Figure 5. Mean skin conductance responding to prepulses presented alone as a function
of group and attention.* Denotes a significant difference between to-be-attended and
to-be-ignored prepulses. Bars represent the standard error of the mean.
C O
50
scores (+37.78%) compared to auditory prepulses (+20.44%). Attended
prepulses resulted in significantly larger scores (+36.04%) compared to
ignored prepulses (+19.19%). These results simply replicate the findings with
SCRs to prepulses alone. More interestingly, startle pulses preceded by a
120 ms lead interval prepulse produced significantly larger scores (+40.07%)
than a 150 ms lead interval prepulse (+15.16%).
Discussion
First, this study replicates and extends the finding that a discrete
prepulse produces greater prepulse inhibition than a continuous prepulse
(Graham & Murray, 1977; Blumenthal & Levey, 1989; Wynn et al., 2000).
This is most clearly seen when using an auditory prepulse. However, the
transient advantage was also seen in the visual modality in that the discrete
visual prepulse at a 150 ms lead interval produced significant PPI while the
continuous prepulse did not.
It was also shown for the first time that visual PPI and auditory PPI
were significantly positively correlated (at least under conditions when
subjects exhibited significant amounts of PPI to the visual prepulse, as in the
discrete group at the 150 ms lead interval). This might be due only to the
commonality of the motor neural circuits involved in inhibiting startle,
independent of input from transient detection systems in the different
modalities. However, this result may also indicate that the sensitivity of the
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51
auditory and visual transient detecting circuits are correlated, reflecting a
general sensitivity to this type of environmental stimuli.
The finding that discrete prepulses elicit greater PPI than continuous
prepulses supports the proposal by Graham (1992) that the transient
detection response is responsible for initiating prepulse inhibition. Moreover,
a clear dissociation can be seen between the orienting response and the
transient detecting response. That is, auditory prepulses produced the
greatest amount of PPI and attentional modulation, whereas visual prepulses
produced the greatest amount of skin conductance responding but the least
amount of PPI.
It was also found that attention acted to enhance PPI only when the
prepulse was continuous and in the auditory modality. There were no
attentional effects found for the visual prepulses, most likely due to the fact
that these prepulses simply were not very effective at generating PPI. For the
auditory prepulses, the results are consistent with the hypothesis that
attention acts largely to enhance PPI by decreasing the facilitatory effects of
the continuous portion of the prepulse, not by increasing the inhibitory effects
of the discrete portion of the prepulse. If attention directly affected transient
inhibition, attentional modulation would have been equal for discrete and
continuous prepulses. These findings are consistent with Graham’s (1975)
findings that the sustained portion of a prepulse can facilitate sensory
processing and input. By directing attention towards the prepulse, attention is
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52
now mainly focused on processing of the prepulse and this reduces sensory
input from following stimuli in order to allow processing the prepulse without
interruption. Although it was seen in this experiment that only the continuous
prepulses showed attention modulation, it is doubtful that attention acts only
on the continuous portion in all situations. As seen in DelPezzo and Hoffman
(1980), Hackley and Graham (1987), and Hazlett et al. (2001), greater PPI
was seen during discrete prepulses that were attended than during discrete
prepulses that were ignored.
It is important to note that the discrete auditory prepulses in the
present study produced large amounts of inhibition (averaging over lead
interval and attended and ignored prepulses, -66%). It is possible that
modulation of PPI failed to occur with these prepulses because subjects
were reaching a ceiling where they could not produce any more PPI. Another
possibility why discrete auditory prepulses failed to show attentional
modulation of startle may have been that the discrete auditory prepulses
used in this study were less discriminable in the short amount of time
presented, and subjects took longer to make the distinction between
attended versus ignored prepulses. In the previous studies reviewed earlier
that did show attentional modulation of startle using discrete prepulses,
recognition during or immediately after prepulse presentation would have
been possible in a very short amount of time (e.g., spatial location or
numerical stimuli).
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53
This study also seems to indicate that visual prepulses, in general, are
not as inherently effective at producing PPI as auditory prepulses, even when
those visual prepulses were matched in psychophysical intensity to auditory
prepulses. This may be due in part to visual prepulses requiring longer lead
intervals than were used in this study to produce maximum PPI. For
example, Graham (1980) reviewed two studies where auditory and visual
prepulses were matched psychophysically to be equally intense, and visual
inspection of the figure (Figure 35.2) clearly shows that visual prepulses,
regardless whether the prepulse is discrete or continuous, are not as
effective at producing PPI as auditory prepulses. It can also be seen that
visual PPI became more pronounced at the later lead interval (240 ms)
whereas auditory PPI began to attenuate by the later lead interval. It may
have been the case that subjects were exhibiting a floor effect with visual
prepulses; without any significant amounts of visual PPI, attention would not
have been able to modulate what wasn’t there. These results are similar to
those reported by Bohmelt, Schell, and Dawson (1999) whose results also
showed visual prepulses to be less effective in a within-subjects design
similar to that employed in this study.
Another possible reason that visual prepulses were not effective in
producing PPI is that this study may not have utilized as arousing or difficult
a task as would be required for visual prepulses to produce strong PPI. As
seen in this study, discrete visual prepulses probed at a lead interval of 150
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54
ms, averaged over to-be-attended and to-be-ignored prepulses, produced
only -29% PPI. The visual stimuli used during the CPT in the Hazlett et al.
(2001) study would have been much more engaging and the task more
difficult as compared to the visual stimuli and task in this study. A CPT study
requires one to be constantly vigilant for target stimuli that not only appear
fairly rapidly in a stream of non-target stimuli, but are also only present for
just 50 ms. Upon examining Figure 2 in the Hazlett et al. study, PPI produced
by target and non-target stimuli averaged approximately -45%, a value much
larger than that seen with the discrete visual stimuli in this experiment.
Therefore, possible reasons that this study tended to show very poor PPI
using visual prepulses include non-optimal lead intervals or lower than
optimal arousal and/or lower attentional demands during the task.
The results presented here, and shown in the previous studies
described earlier, are consistent with the view that PPI is a two-stage
process. The first stage in the generation of PPI is automatically triggered by
a transient change in a stimulus, producing the initial amounts of prepulse
inhibition seen at around 60 ms. At this early stage, PPI cannot be modified
by attentional processes (see Dawson et al., 1997, for a review; however,
see Hackley and Graham, 1987). The next stage is a “modification” stage. In
this stage, several variables can modulate the initial amounts of inhibition
produced by the transient change. Two clear examples, as seen in this study,
are the facilitatory effects on blink of the sustained portion of a prepulse (i.e.,
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55
the continuous group showed less PPI than the discrete group) and the
further inhibitory effects of attention directed towards the prepulse. The
discrete advantage must occur because the continuous portion of the
prepulse has facilitatory rather than inhibitory effects.
The final interesting finding of this study lies in the examination of how
lead interval affects skin conductance responding to a prepulse + startle
burst combination. The response to the prepulse plus startle combination
was clearly dominated by the startle burst, since overall response to the
prepulse alone (averaged over modality, duration, and attended versus
ignored prepulses) was 0.30 pS while responses to the startle burst alone
averaged 0.41 pS, and responses to the prepulse plus startle combined
averaged 0.48 pS. A 150 ms lead interval produced smaller SCRs to the
combination compared to a 120 ms lead interval, paralleling the finding for
PPI with startle eyeblinks. Because skin conductance does not have a strong
motor component, the results imply that sensorimotor gating is at least partly
gating out of the sensory input of the startle pulse and not just the motor
response to that startle burst; and, since this gating was greater on average
in this study at 150 ms than at 120 ms, the SCR generated by the prepulse +
startle combination was smaller at 150 ms. These latter results are consistent
with the findings of Filion and Ciranni (1994) and Norris and Blumenthal
(1996) who found that greater startle inhibition results in more accurate
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56
processing of the sensory information of the prepulse, suggesting that the
actual sensory information of the startle pulse is being gated out.
This experiment successfully extended the finding of an auditory
transient advantage in prepulse inhibition to the visual modality and again
replicated the finding that transients, whether auditory or visual, are
responsible for initiating startle inhibition. Furthermore, our understanding of
the effects of attention on startle inhibition has been made more clear by the
finding that it may act at least in part by reducing the facilitation that would
otherwise be produced by the continuous portion of a prepulse.
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57
Chapter 9
Experiment 2
Given the finding that a discrete visual prepulse also exhibits a
transient advantage, that is a discrete visual prepulse produces greater PPI
than a continuous visual prepulse, it seems plausible that a similar type of
stimulus in another paradigm, backward masking, might elicit a sensorimotor
gating response.
Due to the very similar nature of the time course of the development of
PPI and the recovery from backward masking effects, it is possible, as Green
et al. (1994b) suggest, that recovery from backward masking is due to
sensorimotor gating. This experiment attempted to determine whether
recovery from backward masking effects is due to a sensorimotor gating
mechanism, as assessed by PPI, by measuring and manipulating both
backward masking and PPI within an intermixed series of trials. It was
hypothesized that recovery from backward masking and PPI reflect a
common sensorimotor gating mechanism. That is, the escape from the
effects of the mask on the target is due to a sensorimotor gating mechanism
becoming fully active.
It is predicted that at around 30 ms, backward masking effects as well
as PPI will begin to manifest. At around 30-60 ms, backward masking effects
will be maximal and PPI will continue to develop. At around 120-150 ms,
backward masking effects will no longer be present but PPI will be maximal.
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58
It is hypothesized that at this point the gating mechanism initiated by the
target (which is serving as a visual prepulse) will have developed fully
enough to gate out the interrupting effects of the mask and the interfering
effects of the startle pulse. If recovery from backward masking is related to
sensorimotor gating, a negative relationship should be seen between the
number of correct hits at each SOA (with recovery from masking resulting in
a greater number of correct hits) and the amount of PPI seen at each SOA. I
hypothesized that across subjects, development of PPI would be correlated
with recovery from backward masking, supporting the hypothesis that there is
a common sensorimotor gating mechanism which both phenomena index in
different ways. In addition, I hypothesized that auditory and visual PPI are
positively correlated, providing further support for a common sensorimotor
gating mechanism.
Methods
Subjects
Data on a total of 52 subjects were collected. There were 37 females
and 15 males. After exclusions (see Results) there were a total of 36
subjects, 11 males and 25 females.
Design
This study used a 2 X 5 within-subject repeated measures design to
assess auditory and visual PPI. The first independent variable was modality,
with prepulses in either the visual or auditory modality. The second
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59
independent variable was lead interval, with lead intervals of 30, 45, 60, 120
and 150 ms.
Backward masking was assessed using a one-way repeated
measures design, with SOA as the independent variable. Six SOAs were
used: 0 ms (no mask), 30, 45, 60, 120 and 150 ms.
Procedure
Upon arrival to the laboratory, subjects were asked to read and sign a
consent form and some brief instructions. After reading these items,
electrodes were attached for recording of electromyographic (EMG)
responses.
Subjects were instructed that they would see one of 15 letters (A, E, F,
H, I, K, L, M, N, T, V, W, X, Y, and Z) presented very briefly on the wall in
front of them. These letters served as targets in the backward masking task
and as visual prepulses to assess PPI. They were told that they were to try to
name the letter that they were shown as quickly as possible. They were also
warned that on some occasions, another slide with a random pattern of lines
would be shown shortly after the slide with the letter and that this slide could
be ignored. If they did not clearly see the letter or were unsure of the letter,
they were to choose one of the fifteen letters which were typed on a list and
posted in front of the subject.
Subjects were also told that they would hear a low-intensity, brief
static-like noise on certain trials (this served as the auditory prepulse). They
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60
were instructed to say “heard” whenever they heard this noise. Furthermore,
subjects were instructed that they would hear a much louder static-like noise
(serving as the startle stimulus) and that this noise could simply be ignored.
To familiarize the subjects with the low-intensity and high-intensity noises,
two examples of each were presented. Subjects were asked if they could
discern the difference between the two; if not, they were presented with one
more of each noise.
In order to ensure that subjects remained motivated to perform the
task, they were offered a bonus of $5. However, subjects were told that for
each incorrect letter identified, $0.10 would be subtracted from their bonus.
For the experimental session, 100 trials were presented: 10 visual
target alone trials (Figure 6, top), 50 masking trials (Figure 6, middle), 20
visual PPI trials (Figure 6, bottom), and 20 auditory PPI trials. These trials
were presented in a mixed, pseudo-random order. Backward masking trials
were presented 10 times at each SOA. Auditory and visual prepulses were
presented 4 times at each SOA. None of the visual PPI trials co-occurred
with a mask.
There were also a total of 20 intertrial interval (ITI) startle-alone white
noise probes which served to indicate the subjects’ baseline startle
responding. Responses to these 20 ITI probes were averaged to create a
startle-alone baseline for each subject. A total of 60 startle probes were
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61
A
SOA— ►
Target Alone
A
SOA— ►
Target + Mask
A
Startle Burst
SOA— ►
Visual Prepulse + Startle Burst
Figure 6. Examples of visual stimuli used in Experiment 2.
Top: target alone; Middle: target + prepulse;
Bottom: target + startle probe.
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62
presented: 20 ITI startle-alone probes and 4 startle probes at each of the 5
SOAs following the auditory and visual prepulses.
The entire experimental testing session lasted approximately 25
minutes. Halfway through the experiment subjects were informed how much
time they had left in order to keep the subjects alert and focused on their
task. At the end of the experiment subjects were told how many correct
letters they identified and were paid their bonus. Subjects were then given an
opportunity to ask any questions they might have and were then dismissed.
Experimental Stimuli
Auditory prepulses and visual prepulses (letters presented alone and
with the mask) were 15 ms in duration. Pilot data showed that 15 ms was
ample time to correctly detect the target presented alone. Auditory prepulses
were 15 ms 75 dB(A) white noise bursts with near instantaneous rise/fall
times. The startle stimulus consisted of a 50 ms, near instantaneous rise/fall
time, 109 dB(A) white noise burst. Auditory stimuli were presented through
binaural Telephonies TDH-50P headphones. Decibel levels were recorded
with a Realistic sound level meter using a Quest Electronics earphone
coupler.
Visual stimuli were presented from two Kodak Carousel 4600 slide
projectors, stacked on top of each other using a Pacific Innovations slide
projector stacking system, housed in a sound proof chamber situated behind
the subject. Visual stimuli were presented on the wall in front of the subject.
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63
The distance from the wall to the subject was approximately 55”. The
brightness of the visual stimuli, as measured by placing an Extech light meter
directly on the wall area receiving the stimuli, was 23.3 ft./cd2.
Recording and Scoring of Dependent Variables
Eye blinks were scored as EMG activity of the orbicularis oculi muscle
of the left eye. One small (4 mm) silver-silver chloride (Ag-AgCI) electrode
was placed on the left eyelid directly below the pupil while a second 4 mm
electrode was placed approximately 1 cm lateral to the first. The impedance
between the two electrodes was measured and was deemed acceptable
below 10 kOhms. Stimulus presentation and data acquisition were controlled
through Contact Precision Instruments equipment and a computer running
Psylab 7 software. The raw EMG signal (filtered at 10 Hz high pass and 500
Hz low pass) was collected at a rate of 1000 Hz. The data were stored and
exported for analysis in real microvolt values. For analysis, the EMG signal
was software integrated using a 20 ms time constant. Startle response onset
was set to be detected within a window of 20-120 ms while peak activity was
set within a window of 20-200 ms.
Prepulse inhibition was calculated as a percent change score:
[(prepulsed startle - startle alone) / (startle alone) * 100]. Percent change
units are preferred over difference scores (prepulsed startle - startle alone)
because difference scores in absolute pV units are correlated with baseline
startle blink amplitude whereas percent change scores are not, removing any
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64
dependence on baseline startle amplitude (Jennings et al., 1996). Effects of
backward masking were assessed by determining the amount of correct hits
at each SOA (ranging from 0-10 for each target-to-mask SOA and target-
alone).
Results
Exclusions and Outlier Analysis
Before any analysis, any subject scoring less than an average of 2.0
pV for his/her eyeblink response to startle-alone trials (based on the average
of all 20 ITI startle-alone trials) was considered a non-responder and was
excluded from analysis. Furthermore, any subject scoring more than 2 errors
on the letter-naming task when the letter was presented alone was excluded
from analysis due to an inability to perform the task adequately. A total of 16
subjects met one of these criteria (11 with an average ITI less than 2.0 pV, 2
with more than 2 misses to the target presented alone, and 3 with a
combination of both). This left a total of 36 subjects (11 males, 25 females).
After exclusions, the average ITI startle amplitude was 12.34 pV (s.d. = 12.15
p V ).
After these exclusions, outlier analyses were conducted on the EMG
values to the prepulse + startle trials and to startle alone trials. An outlier was
considered to be any score greater than three standard deviations above the
mean and greater than two standard deviation above the next highest score.
Based on these rules, there were no outliers among the data of the prepulse
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65
+ startle trials or the startle alone trials. Next, percent change scores were
screened for outliers. Based on the above rules no percent change scores
were considered outliers. For all analyses Greenhouse-Geisser (s)
corrections were used for repeated-measures analyses of variances with
more than one degree of freedom. We report uncorrected degrees of
freedom, corrected £ values, and s values. Rom’s procedure (Rom, 1990)
was used for all groups of post hoc t tests to control for experiment-wise
Type I Error.
Backward Masking Analysis
A one-way repeated measures ANOVA was used to assess the
effects of backward masking. Accuracy of identification of the target
presented alone (SOA-O ms: mean 9.69, s.d. 0.62; a perfect score was 10)
as well as for the other five targets followed by the mask (SOAs 30, 45, 60,
120, and 150 ms) were used as the repeated measure. Means and standard
errors are reported in Table 2 and shown in Figure 7 (note that in Fig. 7, the
scores have been converted to % correct to bring them in scale with the PPI
scores for visual comparison only).
A main effect of SOA was shown, F (5, 175) = 225.754, £ < 0.001, s =
0.575. The number of correct hits was minimal when the target was followed
by a mask at an SOA of 30 ms. From 30 to 150 ms, the number of correct
hits for the target increased, with asymptote being reached at 120 ms.
Compared to the target presented alone, post hoc analysis of SOAs of 30,
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66
Table 2: Means (standard errors) for the three dependent variables at each
SOA
30 ms 45 ms 60 ms 120 ms 150 ms
Auditory PPI
(% Change)
-37.42
(4.69)
-66.41
(3.34)
-82.39
(2.72)
-80.22
(3.08)
-82.30
(3.16)
Visual PPI
(% Change)
72.44
(12.77)
36.12
(6.70)
-3.07
(5.92)
-32.27
(6.98)
-53.28
(5.18)
Backward
Masking
(# Correct
Hits)
1.94
(0.28)
5.08
(0.35)
7.25
(0.23)
9.58
(0.12)
9.64
(0.11)
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100
80
Auditory PPI
Visual PPI
Backward Masking
60
c n
I
40
o
( 1 )
20
O
O
C L
0 L
< D
O)
c
(0
x:
O
-20
-40
-60
-80
-100
30 60 45 120 150
SOA
Figure 7. Visual and auditory PPI and percent correct hits in the backward masking task
as a function of SOA.
CD
68
45, and 60 ms revealed significantly fewer correct hits (all t’s (35) > 10.454, £
< 0.001).
Auditory PPI Analysis
Percent change startle modification scores for auditory prepulses were
analyzed with a one-way repeated measures ANOVA. The five SOAs (30,
45, 60, 120, and 150 ms) were used as the repeated measure. Results
showed a main effect of SOA, F (4, 140) = 41.168, £ < 0.001, s = 0.661.
Means and standard deviations are listed in Table 2 and shown in Figure 7.
As can be seen in Figure 7, PPI increased as the SOA increased from 30 to
60 ms and leveled off from 60 to 150 ms. Post hoc analyses revealed that all
auditory PPI scores were significantly different from zero (all t’s (35) < -7.983,
£ < 0.001).
Visual PPI Analysis
Percent change startle modification scores for visual prepulses were
analyzed with a one-way repeated measures ANOVA. The five SOAs (30,
45, 60, 120, and 150 ms) were used as the repeated measure. Results
showed a main effect of SOA, F (4, 140) = 51.765, £ < 0.001, e = 0.439.
Means and standard deviations are listed in Table 2 and shown in Figure 7.
As can be seen in Figure 7, percent change PPI scores steadily changed
from facilitation to increasing inhibition from an SOA of 30 ms to an SOA of
150 ms. However, the post hoc analysis of visual PPI scores revealed that
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69
only SOAs of 120 and 150 ms produced significant amounts of PPI (t’s (35) <
-4.626, £ < 0.001).
Assessing the Relationship Between Backward Masking and Prepulse
Inhibition
Using a Hierarchical Linear Model
In order to assess whether prepulse inhibition is a predictor in
recovery from backward masking, hierarchical linear modeling (HLM) was
used due to the multilevel nature of the data collected. HLM models are very
similar to multiple regression equations; however, the assumptions are more
flexible and the model is more statistically sophisticated (Bryk &
Raudenbush, 1992). The data collected are applicable to an HLM analysis as
the repeated measure (SOA) can be considered the level-1 model (where
there are five responses per individual corresponding to the dependent
variables measured at each SOA) and individuals with different PPI values at
each SOA can be considered the level-2 models (36 individuals). As noted
by Goldstein, Healy, and Rasbash (1994), repeated measures data are
suitable for a two-level random coefficients model.
A random coefficient regression model was used to test the
hypothesis that PPI is a predictor in recovery from backward masking.
Separate equations were used to assess this relationship for auditory and
visual prepulse inhibition. As the relationship between recovery from
backward masking and PPI was the only hypothesis to be tested, a
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70
significant slope (or y coefficient) had to be shown to be able to reject the null
hypothesis. This y coefficient is the pooled level-1 slope between recovery
and PPI and the significance test tests whether or not there is a significant
relationship between recovery and PPI.
Furthermore, the HLM procedure can provide an estimate of the level-
1 variance in recovery from backward masking accounted for by PPI. This is
done by taking a ratio of the within group variance (ct2 w s; where group is the
individual) in recovery from backward masking to the level-1 residual
variance (a2 w s). This is shown mathematically below:
R2 for Level-1 model = ( g 2ws - cy2res) / cr2 w s
The first stage of the HLM analysis partitions the variance in backward
masking into between and within individual components. As there are no
predictors at this stage, backward masking for each subject at each SOA is
predicted to be the overall mean. The analysis provides an estimate of the
within subject variance, referred to here as o2 w s, which is then used in the
equation noted above.
The second stage of the HLM model now takes the predictors into the
equation and provides a direct test of the hypothesis that PPI is related to
recovery from backward masking effects. This stage of the analysis provides
the y estimate, which is simply the mean of the regression slopes across the
individuals. The HLM analysis provides a t-test which indicates whether this
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71
slope is significantly different from zero. This test determined whether or not,
on average, there was a significant relationship between recovery from
backward masking and PPI. This phase of the analysis also estimates the
level-1 residual variance. As the random regression model includes level-1
predictors, a2 will now equal the level-1 residual variance, referred to here as
C T 2re s - Comparing this level-1 residual variance to the variance in the first
stage of the analysis will provide an estimate of the level-1 variance in
recovery from backward masking accounted for by PPI, R2 (refer to the
equation above) (Hoffman, 1997).
Each of the steps involved will be shown along with the equations
associated with each of the steps. The results will be shown in relation to the
equations. This will only be done for the first analysis (i.e., visual PPI and
backward masking). A table (Table 3) of the entire results of the HLM
analysis is included after this page so the reader can refer to a brief summary
of the results. Subsequent analyses will only show the results.
HLM Analysis of Visual PPI and Recovery from Backward Masking
First Stage
The equations that make up this analysis are:
Level 1: masking# = B o y + a #
Level 2: Boy= yoo + Uoy
Where yoo is equal to the grand mean number of correct hits, r# is the within
individual variance in masking (a2 w s ), and Uoy is the between individual
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Table 3: Summary of the results of the HLM analysis for Experiment 2.
Visual PPI
T o o Y io Y 20
a2 R2
First
Stage
(no
predictors)
6.70 10.502
Second
Stage
PPI 6.739 -0.034** 6.094 41.97%
PPI +
SOA
2.684 -0.007* 0.050** 3.130 70.20%
Auditory PPI
Y o o Y io Y 20
a2 R2
First
Stage
(no
predictors)
6.70 10.502
Second
Stage
PPI 1.250 -0.076** 6.631 36.86%
PPI +
SOA
0.321 -0.036** 0.047** 2.377 77.40%
* denotes p < 0.05, ** denotes p < 0.001
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variance in masking. The analysis showed a yoo equal to 6.70 and a2 w s was
equal to 10.50236.
Second Stage: Visual PPI as Predictor
This model tests the hypothesis that recovery from backward masking is
related to prepulse inhibition (the amount of inhibition exhibited at each SOA
is a mean, collapsed over the four PPI trials for each SOA). A significantly
different from zero y, or the mean of the slopes across individuals, will allow
for the null hypothesis to be rejected. The equations that make up this
analysis are:
Level 1: masking# = B0 y + B-iy(PPI) + a #
Level 2 : Boy= yoo + U o y
Biy = yio + U-iy
yoo is the mean of the intercepts across individuals, the mean of the
slopes for PPI is equal to yio (i.e., PPI is related to recovery from backward
masking), /# • is the residual variance (a2 re s ), U 0 y is equal to the variance in the
intercepts for each individual, and Uiy is the variance in the slopes for each
individual. Uncentered (raw) values were used for the predictors (centering
becomes useful when there is no meaningful zero for the predictor values;
however, we know what zero is in PPI and SOA, therefore the values can be
used in the analysis as they are).
Results showed a yo o equal to 6.739. The mean of the slopes relating
PPI to recovery from backward masking was significantly negative, t (35) =
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-9.860, g < 0.001. The percentage of the variance in backward masking
accounted for by visual PPI was 41.97%.
Second Stage: Visual PPI and SOA as Predictor
This model tests the hypothesis that recovery from backward masking is
related to prepulse inhibition taking into account the effect of SOA.
Significantly different from zero y’s will allow for the null hypothesis to be
rejected. However, if y for visual PPI becomes nonsignificant, the relationship
between recovery from backward masking and visual PPI is due solely to
SOA. The equations that make up this analysis are:
Level 1: masking,; = Boy + B 1 y (PPI) + B 2 y(S O A ) + nj
Level 2: Boy = yo o + Uoy
Biy = yio + U-iy
B 2 y = J20 + U 2 y
yoo is the mean of the intercepts across individuals, the mean of the
slopes for PPI is equal to y1 0 (i.e., PPI is related to recovery from backward
masking), the mean of the slopes for SOA is equal to y2o , H j is the residual
variance, and U o y is the variance in the intercepts whereas the other U
parameters are the variances in the slopes for each individual. Uncentered
(raw) values were used for the predictors.
Results showed a yo o equal to 2.684. The mean of the slopes for PPI
was significantly negative, t (35) = -2.564, £ < 0.02, indicating a significant
negative relationship between recovery from backward masking and visual
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75
PPI. The mean of the slopes for SOA (y2 0 ) was significantly positive, t (35) =
11.625, £ < 0.001, indicating a significant positive relationship between
recovery from backward masking and SOA. The percentage of the variance
in backward masking accounted for by PPI and SOA was 70.2%. Most
importantly, a significant relationship remained between visual PPI and
recovery from backward masking effects even when the contribution of SOA
to recovery is separately extracted. That is, PPI predicts recovery from
backward masking independently of their mutual relationship to SOA.
HLM Analysis of Auditory PPI and Recovery from Backward Masking
The first stage of the analysis is exactly the same as for the visual PPI
analysis. Therefore, a2 w s is still equal to 10.50236.
Second Stage: Auditory PPI as Predictor
Results showed a yo o equal to 1.250. The mean (of the slopes for PPI (y1 0 )
was significantly negative, t (35) = -8.352, £ < 0.001, indicating a significant
negative relationship between recovery from backward masking and visual
PPI. The percentage of the variance in backward masking accounted for by
auditory PPI was 36.86%.
Second Stage: Auditory PPI and SOA as Predictors
Results showed a yoo equal to 0.321. The mean of the slopes for PPI (yio)
was significantly negative, t (35) = -6.098, £ < 0.001, indicating a significant
negative relationship between recovery from backward masking and visual
PPI. The mean of the slopes for SOA (y2o) was equal significantly positive, t
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(35) = 12.992, 2 < 0.001, indicating a significant positive relationship between
recovery from backward masking and SOA. The percentage of the variance
in backward masking accounted for by PPI and SOA was 77.4%. As with the
visual PPI results, a significant relationship remained between auditory PPI
and recovery from backward masking even when the contribution of SOA to
recovery is separately extracted.
Evidence that Greater PPI Results in Greater Hits
If sensorimotor gating is partially responsible for escape from
backward masking, then it would follow that those who show greater PPI at
an earlier SOA than at a later SOA should show greater correct hits at an
earlier stage. To assess this possibility, auditory prepulse inhibition scores
were used to split subjects into groups based on those exhibiting more
inhibition at 60 ms than at 150 ms. Auditory PPI was chosen as nearly all
subjects exhibited some amount of PPI at a 60 ms SOA. Those who
exhibited greater inhibition at 60 ms should show a higher correct detection
rate at earlier SOAs than those showing greater inhibition at 150 ms.
There were 19 subjects who exhibited greater PPI at 60 ms than at
150 ms (“early inhibitors”), and 17 subjects who exhibited the reverse pattern
(“late inhibitors”). The early inhibitors exhibited significantly greater PPI at 60
ms (-88.91%) than at 150 ms (-73.69%), t (18) = -3.946, e < 0.01;
conversely, late inhibitors exhibited significantly greater PPI at 150 ms
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77
(-91.94%) than at 60 ms (-75.09%), t (16) = 3.958, £ < 0.01. Early inhibitors
tended to show greater correct hit rates (2.37) than late inhibitors (1.47) at an
SOA of 30 ms, t (34) = 1.665, £ < 0.06, one-tailed.
Correlations Between Auditory PPI and Visual PPI
To determine whether visual PPI and auditory PPI were related, the
average amount of PPI produced by visual prepulses across all SOAs was
correlated with the average amount of PPI produced by auditory prepulses
across all SOAs. A significant positive relationship was seen, Pearson’s r =
0.378, £ < 0.05, n = 36, reflecting that as the amount of visual PPI increased
the amount of auditory PPI also increased.
Performing an HLM analysis on the relationship between auditory and
visual PPI yielded similar results. The one-way ANOVA using auditory PPI as
the outcome resulted in o2 = 686.144. Using visual PPI as a predictor, a
significant y was found, y = 0.210 (0.036). Using the equations above to
calculate R2, visual PPI accounted for 26.21% of the variance in auditory
PPI.
Discussion
The results of this study provide evidence that PPI, using both
auditory and visual prepulses, is related to recovery from backward masking
effects. These results lend strong support to the hypothesis that escape from
backward masking is indeed related, if not due, to sensorimotor gating.
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78
The analyses showed a clear and strong relationship between
recovery from backward masking and prepulse inhibition. Using PPI as the
sole predictor in the HLM analyses showed a significant relationship in that
recovery from backward masking (i.e., greater hit rate) was related to greater
inhibition. This was true for both auditory and visual PPI. In the next step of
the HLM analyses, it was determined whether this relationship was due
purely to the effects of the timing between the first stimulus and the second
stimulus, i.e. the SOA. If the relationship was due purely to a mutual
dependence of PPI and recovery from backward masking on SOA, adding
SOA as a predictor in the equation would account for all of the shared
variance and PPI would no longer show a separate significant relationship to
recovery from backward masking. In essence, was SOA entirely explaining
the relationship between PPI and recovery from backward masking?
However, as the results have shown, even accounting for SOA, a significant
relationship remained between both auditory and visual PPI and recovery
from backward masking. Higher levels of PPI (a more negative number) were
associated with greater recovery from backward masking.
The theory that these results suggest a single sensorimotor gating
mechanism is supported by the design of the study, the cross-modal
relationship seen between recovery from backward masking and auditory
PPI, and the significant positive correlation between auditory and visual PPI.
By using the same visual stimulus that served as the target as well as the
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79
visual prepulse, it is clear that whatever form of processing occurs to this
stimulus has to be occurring when both of the second stimuli, either the mask
or the startle pulse, are presented.
The theory is also supported by the fact that the relationship between
recovery from backward masking and PPI is seen using both visual and
auditory prepulses. If there were a separate gating mechanism that affected
visual stimuli and auditory stimuli, there would be no relationship between
auditory PPI and recovery from visual backward masking. However, as the
results have shown, this is not the case. The theory of a single sensorimotor
gating mechanism is made all the more strong by the positive correlation
between auditory and visual PPI. Though auditory and visual PPI have been
studied extensively, it is believed that this is only the Second study to show a
direct relationship between the two in the same subjects and in the same
testing session (the first being Experiment 1 of Wynn, Dawson, and Schell, in
preparation).
The results of this study are consistent with and help to explain the
findings that schizophrenia patients are deficient in both PPI and backward
masking. It has been shown that schizophrenia patients show deficient PPI
as compared to normal controls, reflecting poor or defective sensorimotor
gating (Braff et al., 1978; 1992). Furthermore, schizophrenia patients show
greater susceptibility to backward masking effects (Saccuzzo et al., 1974;
Green et al., 1994a, b). In reviewing the backward masking and PPI literature
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80
in schizophrenia, all evidence strongly suggests that schizophrenia patients’
sensorimotor gating mechanism is somehow impaired. Moreover, as
schizophrenia patients exhibit deficits in both backward masking and PPI,
this would point to a common sensorimotor gating mechanism reflected in
the two phenomena, one that is dysfunctional in schizophrenia patients.
In conclusion, the results are promising in showing that backward
masking and prepulse inhibition are indeed related and may reflect a
common sensorimotor gating mechanism. These results have several
implications for normal functioning and for those who are afflicted with
information processing deficits, and more specifically for schizophrenia
patients. First, if backward masking and PPI both reflect sensorimotor gating,
there are implications for conducting sensorimotor gating experiments of a
practical nature. It is much easier and far less costly to conduct a backward
masking experiment using only behavioral measures than to conduct a PPI
study, which involves very specific and expensive physiological monitoring
equipment. Thus, being able to evaluate and determine the effectiveness of
neuroleptic medications in improving gating for schizophrenia patients would
be made much easier. If a drug were in fact effective at alleviating
sensorimotor gating deficits, we would expect to see improvements in both
backward masking scores and PPI. These are intriguing results with
implications not only for normal functioning but for populations with
information processing deficits as well.
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81
Chapter 10
General Discussion
Summary of Experimental Findings
Experiment 1 was conducted to gain a better understanding of the
nature of prepulse inhibition and how stimulus parameters can affect PPI. It
was also conducted to gain a better understanding of how attention
modulates PPI.
First, it was shown that attention modulates PPI in part by decreasing
the facilitatory effects of the sustained portion of a continuous prepulse.
Though it was initially hypothesized that attention also modulates PPI by
increasing the inhibitory effects of the transient portion of a prepulse, this
effect was not supported by the results. However, this hypothesis cannot be
entirely discounted as it appears possible that the subjects were exhibiting
ceiling effects in PPI with regards to the auditory prepulse and floor effects
with regards to the visual prepulse. These limiting effects may have reduced
the opportunity for the top-down modulation of PPI by attentional set.
As reviewed earlier, separate studies have shown that schizophrenia
patients exhibit deficient automatic sensorimotor gating (Braff et al., 1992)
while other studies contend that schizophrenia patients exhibit normal
automatic sensorimotor gating but that their attentional modulation of PPI is
deficient, reflecting controlled attentional deficits (Dawson et al., 1993). Braff
and his colleagues utilize a passive paradigm while Dawson and his
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82
colleagues use an active attention paradigm, as described earlier, and this
different use of attention may be contributing to the conclusions drawn by
both labs. It was important to know in an active attention task which
mechanism attention was affecting: the inhibitory mechanism initiated by the
discrete prepulse or the facilitatory mechanism initiated by the continuous
prepulse.
Experiment 1 also demonstrated that auditory PPI and visual PPI are
significantly positively correlated, indicating a common sensorimotor gating
mechanism that is initiated by a transient change in the environment,
regardless of stimulus modality.
The skin conductance results from Experiment 1 revealed two
findings: (1) there is a dissociation between the transient detecting response
(as evidenced by PPI) and the orienting response (as evidenced by skin
conductance orienting), in keeping with Graham’s (1992) hypothesis that
transient detection and orienting reflect two separate mechanisms and (2)
skin conductance responding showed evidence that the sensory information
of the startle burst was being gated out. This finding is important as it
confirms the theory that the prepulse prevents successive stimuli from
entering the processing stage and thus interrupting processing of the
prepulse.
The final interesting finding was that a transient advantage for PPI
was extended to the visual modality in addition to the already known
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83
advantage in the auditory modality. The results reported in Experiment 1
showed that discrete visual prepulses produced significant amounts of PPI
whereas continuous visual prepulses did not, showing that it is the transient
nature of the visual prepulse that is responsible for PPI.
This finding was important in terms of implications for Experiment 2. A
discrete visual prepulse is very similar in nature to a target used in a visual
backward masking paradigm. Given that a discrete visual prepulse initiates
prepulse inhibition, a measure of sensorimotor gating, perhaps a visual target
initiates a gating response that is responsible for escaping from the effects of
visual backward masking.
The second experiment reported above tested this hypothesis and did
show that prepulse inhibition, reflecting sensorimotor gating, and recovery
from backward masking effects are related. By using the same visual
stimulus as the prepulse and the target of backward masking, this
relationship was made all the more significant. Processing of the same visual
stimulus presented in a prepulse-startle pair and in a target-mask pair
evidently initiated the same gating mechanism to produce both prepulse
inhibition and recovery from backward masking.
That escape from backward masking was also related to auditory PPI
implies that a general gating mechanism that is non-modality-specific is
initiated by any transient change in the environment and acts to gate out
subsequent competing or interrupting stimuli. The notion of a single
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84
sensorimotor gating mechanism is also substantiated by the finding of a
significant positive correlation between auditory and visual PPI, also seen in
Experiment 1.
Implications for the Study of Prepulse Inhibition
The first experiment was a replication and extension of Wynn et al.
(2000). That study attempted to clarify the role of discrete prepulses and
prepulse characteristics on prepulse inhibition. It was also meant to address
the different findings of PPI deficits in schizophrenia patients seen in two
different labs. In the lab of David Braff and colleagues, discrete white noise
prepulses, generally presented over a very noticeable background noise
(e.g., 70 dB) and using short-lead intervals, are used. The findings of Braff
and colleagues have consistently shown that schizophrenia patients exhibit
significantly less PPI than normal controls, reflecting an automatic
sensorimotor gating deficit.
Alternatively, in the lab of Michael Dawson and colleagues, continuous
tone prepulses, generally presented with no noticeable background noise
and with lead intervals of 60 and 120 ms, are used. However, Dawson and
colleagues employ an attentional manipulation task as described earlier. The
findings of Dawson and colleagues have consistently shown that while
schizophrenia patients exhibit significant PPI and PPI during nonattended
prepulses that does not differ from that of normal controls, they fail to exhibit
attentional modulation of PPI. These findings suggest that schizophrenia
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85
patients are normal in their automatic sensorimotor gating but have
deficiencies in their controlled attentional modulation of PPI.
Experiment 1 attempted to determine how attention is able to increase
PPI to an attended prepulse. It was found that attention only produced
modulation when the prepulse was continuous, indicating that attention acts
in part to decrease the facilitatory effect of the continuous portion of the
prepulse. In terms of the findings of Dawson and colleagues, this would
indicate that patients are unable to devote additional resources to attenuate
the facilitatory effect of the sustained portion of a prepulse. However, it does
not address why Dawson’s lab finds normal amounts of PPI in schizophrenia
patients in response to nonattended stimuli while Braff s lab does not.
Moreover, other investigators have found attentional modulation of PPI using
discrete stimuli in tasks with high attentional demands (e.g., Hazlett et al.,
2001).
Two of the most obvious differences between the two labs is the use
of background noise in Braff’s lab and the use of an attentional task in
Dawson’s lab. First, the use of background noise may very well be impacting
the results of the Braff group. Most of the prepulses used in their studies are
only a few decibels above background noise. As Blumenthal (1996) showed,
PPI is related to prepulse intensity: the greater the intensity of the prepulse,
the greater the amount of PPI. However, the use of background noise does
not seem to make the prepulses undetectable by the schizophrenia patients.
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86
Braff et al. (1992) have shown that the patients have the same latency
facilitation to a startle following a prepulse as normal controls, indicating that
the patients heard the prepulse just as well as the normal controls. It may
simply be the case that the slight change in the background noise (i.e., the
prepulse) is not sufficient enough for schizophrenia patients’ TDR to initiate
sensorimotor gating.
The Dawson group’s use of an attentional task presents its own
problems with interpreting the results. The to-be-attended and to-be-ignored
prepulses are intermixed within session and are not compared by block. This
makes it difficult to say that the to-be-ignored prepulse is being automatically
processed as there must be some level of attentional processing to
determine if it is indeed the stimulus to be ignored. It may be that the
attention task is more arousing than a passive task and this may be why
schizophrenia patients are showing normal levels of PPI to the nonattended
prepulses. In fact, there is evidence that arousal, or activation, can lead to
enhanced PPI. Acocella and Blumenthal (1990) showed lower startle
probability during a block requiring directed attention to a prepulse compared
to a trial block that did not require any attention. Similarly, McDowd et al.
(1993) found greater PPI when subjects were instructed to push a button
compared to when they were not pushing a button. The results could be due
to either the attentional requirements of the tasks or to the arousal
differences. As Kahneman (1973) noted, activation during a task is indicative
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87
of the use of processing resources for that task and that increasing activation
can increase processing resources. Perhaps because of the activating task
used in the Dawson group’s studies, patients are able to devote enough
processing resources to initially inhibit the startle burst but are deficient in
extra processing resources to additionally offset the facilitatory effects of the
sustained portion of the prepulse.
The studies reported herein seem to indicate that activation can affect
PPI. There is some circumstantial evidence that PPI was not exhibited to the
visual prepulses in general in Experiment 1 of the studies reported here due
to lower attentional demands. If activation does increase PPI, simply
comparing lights did not seem to activate the subjects as much as comparing
two very brief tones, as there was little to no PPI exhibited in response to
visual prepulses in Experiment 1. The theory that activation might increase
PPI is even more striking in light of the fact that subjects exhibited significant
visual PPI at the 120 and 150 ms lead intervals in the backward masking
task, which was more difficult and engaging than the task in Experiment 1.
Implications for Sensorimotor Gating
The experiments reported herein attempted to address the notion of a
single sensorimotor gating mechanism by 1) showing a correlation between
auditory and visual PPI and 2) showing that one paradigm that is well-known
to index sensorimotor gating, PPI, is related to another paradigm where it
has not been shown to index sensorimotor gating. Moreover, both
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88
Experiment 1 and 2 showed significant positive correlations between auditory
and visual PPI. This result alone is interesting as it provides evidence for the
first time that sensorimotor gating is a single mechanism that can be initiated
by any transient change in the environment.
The results of Experiment 2 successfully addressed the hypothesis
that sensorimotor gating is related to backward masking and showed that
backward masking and PPI, generated by both visual and auditory
prepulses, are related. Experiment 2 showed that there may indeed be a
single mechanism that functions to gate out any competing stimulus, whether
that be a startling noise (in PPI) or an interruptive mask (in backward
masking).
Though this significant relationship was seen to the two different
paradigms, it remains puzzling why PPI and backward masking were not
correlated at each individual SOA. It may have been the case that a simple
correlation at each SOA could not detect the relationship between PPI and
recovery from backward masking. The hierarchical linear regression should
be a statistically stronger technique to detect the relationship since there are
multiple data pairs for each subject.
The results of Experiment 2 would suggest a single sensorimotor
gating mechanism, which, when studied in PPI and backward masking
paradigms in schizophrenia patients, is dysfunctional. By showing the
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89
relationship between the two, it is more clear that backward masking most
likely indexes sensorimotor gating, as hypothesized by Green et al. (1994b).
The notion of a single sensorimotor gating mechanism is strengthened
by the fact that both PPI and backward masking seem to be linked to the
same neurotransmitters. As reviewed by Braff et al. (1991), dopamine
hyperactivity is related to both PPI and backward masking deficits and
antipsychotics reverse those effects.
Although the observed relationship between backward masking and
PPI was suggestive of a single sensorimotor gating mechanism, this
hypothesis has not always been supported. Data using other purported
measures of sensorimotor gating, most notably the P50 suppression
response, are ambiguous. As seen by Light and Braff (2001) and
Schwarzkopf, Lamberti, and Smith (1993), P50 suppression was
uncorrelated to PPI. However, others (e.g., Oranje, van Berckel, Kemner,
van Ree, Kahn, & Verbaten, 1999) have shown a correlation between PPI
and P50 suppression, suggesting a similar sensorimotor gating mechanism.
Moreover, the relationship of schizophrenic symptoms to deficits are
different for backward masking and PPI. On the one hand, Green and Walker
(1986) found that negative symptoms were related to backward masking
deficits while Dawson, Schell, Wynn, & Nuechterlein (1997) and Perry and
Braff (1994) found that positive symptoms were related to PPI deficits. If
there is a single sensorimotor gating deficit, it is strange that the deficits seen
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90
in the two paradigms are related to different symptomatology. This is
obviously an area of research that needs to be further explored.
Given these results and the hypothesis that PPI and backward
masking share a similar gating mechanism, it would be of great import to
determine where the breakdown in gating in schizophrenia appears and how
that breakdown manifests itself. It seems plausible that there is a malfunction
in the transient system in schizophrenia that results in a breakdown in
sensorimotor gating processes. There are two possible reasons for this
deficit: 1) a simple defect in the TDR failing to initiate a sensorimotor gating
response or 2) a “hyper-transient” advantage. First, schizophrenia patients
might have underactive transient detectors which are not efficient at initiating
the sensorimotor gating mechanism via the TDR. Therefore, while patients
are obviously normal at detecting stimuli, they are abnormal in being able to
gate out competing stimuli.
The second possibility why patients show deficient PPI may be what I
refer to as a hyper-transient advantage. Recall that the transient system
detects all changes in the environment. Given this, it is plausible to think that
patients might be have a normal TDR that initiates gating but a second,
competing transient is overpowering the negative feedback loop which
Graham (1975) believes is part of the sensorimotor gating mechanism. This
detection of all stimuli is reflected in the visual and auditory flooding
schizophrenia patients report. In other words, the negative feedback loop
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91
may be in effect but may be attenuated by the second transient being
detected, due to a hyper-transient advantage. Whatever effects the gating
mechanism has on the second stimulus, the second stimulus can have such
a large transient advantage that it overwhelms the gating mechanism.
Schwartz, Mallott, and Winstead (1988) suggest that a GABA antagonist
would make all transient and sustained cells behave in a more sustained
manner. Perhaps using a GABA antagonist would provide a way to block the
effects of this proposed transient hyper-advantage.
There remains much more work to gain a firmer understanding of
sensorimotor gating, as it is assessed by prepulse inhibition and backward
masking. With regards to PPI, it is still my belief that attention acts to
increase the amount of inhibition initiated by the transient onset of a
prepulse. It appeared that subjects were exhibiting floor effects, in regards to
the discrete auditory stimuli, and ceiling effects, in regards to the continuous
and possibly discrete visual stimuli, and thus were unable to demonstrate
attentional modulation of startle. If somewhat less effective auditory stimuli
and somewhat more effective visual stimuli were used, attention effects
might have been seen to discrete prepulses. If attention were seen to affect
PPI to discrete prepulses then it would be clear that attention acts to increase
PPI by increasing the initial amounts of inhibition produced by the transient
portion of a prepulse.
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92
Alternatively, if the continuous portion of a prepulse facilitates startle
due to the sustained prepulse activating the subject to sensory input, would
repeated presentations of a continuous prepulse actually increase PPI? As
Wynn et al. (2000) demonstrated, the continuous portion of a prepulse
initiates an orienting response; after repeated presentations, the orienting
response should habituate. As the continuous portion of a prepulse
habituates, it should no longer carry significant information and the subject
would no longer need to be as receptive to sensory input. This would
possibly eliminate the facilitation of startle due to the sustained portion of the
prepulse and result in greater PPI.
The effect of arousal, or activation, on PPI is clearly another area to
be researched. Previous studies have implied that activation can increase
PPI, and the results with visual PPI seen in the two experiments reported
herein would also imply that activation can increase PPI. It was seen in
Experiment 1 that visual PPI was non-existent or very small. In Experiment 2,
where the task was much more demanding, visual PPI at the 120 and 150
ms lead intervals was significant and quite robust. It appears that the
stronger task demand in Experiment 2 activated the subjects more than in
Experiment 1, thus resulting in greater visual PPI. It would be interesting to
understand why and how arousal affects PPI.
Another area in PPI to follow up is in how a prepulse can affect skin
conductance responding to a startle burst. The skin conductance results of
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93
Experiment 1 reported herein imply that the sensory information of a startle
burst is being inhibited. As this was the first time skin conductance was used
to examine sensorimotor gating, it is obvious that replication needs to be
accomplished. Furthermore, it may be possible to use other physiological
measures, such as heart rate, to assess if the sensory information of the
startle burst is being inhibited.
The backward masking results of this paper also pose their own
questions to be resolved and findings to be replicated. The first question to
be resolved is why wasn’t PPI and backward masking correlated at each
individual SOA? Another follow-up study would be to determine what
happens to PPI when a visual prepulse is masked. Would PPI be decreased
if a visual prepulse is masked or would it be unaffected?
Another issue that should be investigated is whether or not there is an
active sensorimotor gating mechanism in forward masking. If the first
stimulus of a pair initiates a gating response, then a mask that is presented
prior to a target (i.e., forward masking) should disrupt accurate detection of
the target, via gating, at SOAs where gating is usually seen (e.g., 100 ms or
greater). However, as seen in Hellige, Walsh, Lawrence, and Prasse (1979),
Figure 3 of their experiment would suggest that the mask has no effect on
target detection at SOAs of -48 ms and lower (negative SOAs are used to
show that the mask precedes the target). That is, if the mask were initiating a
gating mechanism, under conditions of forward masking this gating
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94
mechanism has no effect on target detection at SOAs where strong gating
should be seen. It is odd that under conditions of backward masking as seen
in Experiment 2 of this dissertation there appears to be an effect of gating in
correctly identifying a target but under the conditions of forward masking as
seen in Hellige et al. it would appear that gating has no effect on correctly
identifying a target. This difference could possibly be due to different
mechanisms underlying backward and forward masking (e.g., integration vs.
interruption). It would be beneficial to conduct a follow-up study and
determine if gating is in effect under forward masking conditions.
Translating the backward masking and PPI study to schizophrenia
patients would also be an area that would be of great interest. As it was
hypothesized that backward masking and PPI both reflect a single
sensorimotor gating mechanism, schizophrenia patients should show deficits
in both. Furthermore, measuring both PPI and backward masking within the
same recording session in schizophrenia patients would also be a stronger
way to detect important deficits in sensorimotor gating. However, if, as is
proposed in this paper, there is a deficit in a single sensorimotor gating
mechanism in schizophrenia patients, would they exhibit no relationship
between PPI and backward masking, or would they exhibit the relationship
but at a longer SOA? That is, are patients so dysfunctional in PPI and
backward masking their scores would either be very restricted or vary too
much to detect a relationship between the two paradigms? Alternatively, it
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95
may simply require more time for schizophrenia patients to exhibit normal
amounts of PPI and show recovery from backward masking effects and by
measuring at later SOAs (at least greater than 150 ms) a significant
relationship between the two paradigms would then possibly be seen.
Obviously, future research remains to gain a better understanding of the
gating deficits in schizophrenia and other populations.
The present experiments attempted, and mostly succeeded, at gaining
a better understanding of how PPI is affected by stimulus parameters and
attention, and showing that backward masking might be reflecting a
sensorimotor gating mechanism due to its relationship to PPI. By gaining a
better understanding of the properties of PPI and its relationship to other
paradigms, it is hoped to come closer to gaining a clearer understanding of
the nature of the cognitive and sensorimotor gating deficits associated with
normal functioning and the cognitive dysfunctions exhibited by schizophrenia
patients.
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96
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Appendix A: Experiment 1 Informed Consent Form
University of Southern California
Department o f Psychology
INFORMED CONSENT FOR NON-MEDICAL RESEARCH
- —
CONSENT TO PARTICIPATE IN RESEARCH
Tone and Light Judgments
You are asked to participate in a research study conducted by Jonathan K. Wynn,
MA, from the department of psychology at the University of Southern California.
This research is part of a dissertation conducted by Jonathan K. Wynn. You were
selected as a possible participant in this study because voluntarily signed up to
participate in this study. A total of 40 subjects will be selected from undergraduate
psychology classes to participate. Your participation is voluntary.
□ PURPOSE OF THE STUDY
This study will attempt to gain a better understanding of how the brain is able to
process stimuli that are received in rapid succession
□ PROCEDURES
If you volunteer to participate in this study, we would ask you to do the following
things:
1) You will read and sign this informed consent form.
2) You will be asked to wash your hands with soap and water.
3) After a brief introduction, small sensors will be attached just below the eyelid
of your left eye, on the index and middle fingers of your left hand, and just
behind your left ear. These measure your physiological responses to the
stimuli that you will see and hear. These sensors simply rest on the surface of
your skin and do not harm you or affect you in any way.
4) You will be assigned a specific task during this experiment. You will hear
one of two different tones or see one of two different lights at any given time.
Approximately 4 or 7 seconds after these stimuli come on, you will hear
another tone (the target tone). This target tone is usually presented 4 seconds
after the first stimulus; however, sometimes this target tone will be presented
7 seconds after the first stimulus. Your task is to count how many longer-
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105
than-usual target tone delays there are. However, you are only to perform this
counting task after you hear one specific tone and one specific light (the
experimenter will tell you to which tone and light you are to attend). In order
to increase your accuracy, you will be given a bonus. You will be given an
initial $5.00 bonus, but for each incorrect guess on your counting task your
bonus will decrease by $1.00. You will be given examples of each tone and
light and examples of your counting task. You will also occasionally hear
some loud, static-like noises. However, you will simply ignore these noises.
5) After this part of the experiment is completed the sensors will be removed
and you are free to ask any questions or leave.
The total length of this study is approximately 1 hour (but not more than 1 hour).
You will only be asked to participate once in this study. The location of this study is
in SGM 906.
□ POTENTIAL RISKS AND DISCOMFORTS
There are no risks associated with measuring your physiological responses with the
sensors. Although the static-like noise is loud, there is no risk to the health of your
ears.
□ POTENTIAL BENEFITS TO SUBJECTS AND/OR TO SOCIETY
Although you will not benefit directly from this research, the knowledge gained from
this study may potentially increase our understanding of how the brain processes
information and to potentially gain a better understanding of how this processing is
abnormal in certain people with psychological disorders.
□ PAYMENT FOR PARTICIPATION
As stated above, you will receive a bonus of $5.00, minus $1.00 for each incorrect
response you give on your tone counting task. If you decide to withdraw from the
experiment you will receive the $5.00 bonus minus $1.00 for however many
mistakes you made up until that point. You will be paid immediately after
completion of the study and will be asked to sign a receipt to acknowledge payment
received.
□ CONFIDENTIALITY
Any information that is obtained in connection with this study and that can be
identified with you will remain confidential and will be disclosed only with your
permission or as required by law. You will only be identified by a unique subject
number and your subject pool PIN assigned to you. When the results of the research
are published or discussed in conferences, no information will be included that
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
106
would reveal your identity. Although we monitor you with a video camera from an
adjacent room, we DO NOT record any portion of the session. This monitoring is
only to view your movements which may interfere with the physiological recordings.
□ PARTICIPATION AND WITHDRAWAL
You can choose whether to be in this study or not. If you volunteer to be in this
study, you may withdraw at any time without consequences of any kind. You may
also refuse to answer any questions you don’t want to answer and still remain in the
study. The investigator may withdraw you from this research if circumstances arise
which warrant doing so. Such an instance may be equipment malfunction or
uncorrected sight- or hearing-impairment.
□ IDENTIFICATION OF INVESTIGATORS
If you have any questions or concerns about the research, please feel free to contact
Michael E. Dawson, Ph.D. or Jonathan Wynn, M.A. at (213) 740-2297, Dept, of
Psychology, SGM 501, Los Angeles, CA, 90089-1061.
□ RIGHTS OF RESEARCH SUBJECTS
You may withdraw your consent at any time and discontinue participation without
penalty. You are not waiving any legal claims, rights or remedies because of your
participation in this research study. If you have questions regarding your rights as a
research subject, contact the University Park IRB, Office of the Vice Provost for
Research, Bovard Administration Building, Room 300, Los Angeles, CA 90089-
4019, (213) 740-6709 or upirb@usc.edu.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
107
SIGNATURE OF RESEARCH SUBJECT, PARENT OR LEGAL
REPRESENTATIVE.
I understand the procedures described above. My questions have been answered to
my satisfaction, and I agree to participate in this study. I have been given a copy of
this form.
Name of Subject
Name of Parent or Legal Representative (if applicable)
Signature of Subject, Parent or Legal Representative Date
Sl(;NATl R I-: OI: l \ \ l-SMCATOK
I have explained the research to the subject or his/her legal representative, and
answered all of his/her questions. I believe that he/she understands the information
described in this document and freely consents to participate.
Name of Investigator
Signature of Investigator Date (must be the same as subject’s)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
108
Appendix B: Experiment 1 Instructions
Light Intensity Matching
First, you are going to perform a task where you will match the
brightness of one light to the brightness of another light. These two lights will
be shown from a box in front of you. Both lights will be on at the same time,
with one light on the left side and another light on the right side. With the dial
provided, adjust the light on the left so it matches the brightness of the light
on the right.
Sound Intensity Matching
Now you will perform a task where you will match the loudness of a
sound to the brightness of a light. You will be hearing two different sounds
and seeing two different lights. Your task is to match the loundess of one
sound to the brightness of one light and to match the other sound to the other
light. You will first see one light and I will play the sound you need to match
to it. You will tell me if that sound should be louder than the light is bright or
softer than the light is bright, or if there should be no change. You will then
perform this same task for the other light and the other sound. You can
repeat this process until you are comfortable with the choices you have
made.
Judgment Task Demonstration
Now you are going to hear [attended tone-pitch] and see [attended
light]. After you hear and see [attended tone-pitch and attended light] you
will hear the target tone. [Play demo] Do you need to hear those again?
Now you are going to hear and see the other stimuli; however, these
stimuli you can simply ignore throughout the experiment. [Play the ignored
stimuli]
Now we are going to give you two examples of the task you are to
perform. You will hear [attended tone-pitch] or see [attended light] followed
by the target tone. Most trials will have a usual delay until the target of 4
seconds. However, on a few trials the delay to the target tone will be a
longer than usual 7 seconds. Tell me which example had the longer-than-
usual delay to the target. [Play the demo]. Do you want those examples
again?
Now you are going to get 4 example trials of the experiment. Perform
the task while you receive these practice trials. Remember, your attended
stimuli are [attended stimuli], [play the demo] How many longer-than-usual
delays did you count?
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
109
Experiment
We are ready to begin the experiment. Remember that your attended
stimuli are [attended stimuli]; refer to the card in front of you if you forget.
Keep a running count of how many delay-to-targets you encounterd after you
attended stimuli and we will ask you for this at the end of the experiment. In
order to help you with the task we will offer you a bonus of $5. We will ask
you at the end of the experiment how many longer-than-usual delays to the
target following your attended stimuli there were. One dollar will be
subtracted from your bonus for how ever many answers you are off from the
correct amount of respones. If you have any questions ask them before the
experiment begins as it cannot be paused once begun. Here we go.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
110
Appendix C: Experiment 1 Data
Subnum Low High Blue Green AbsErr Group Attend
1 73 68 47.6 55.1 4 1 1
2 75 76 36.3 19.5 3 1 2
3 69 66 46 28 2 1 1
4 65 68 47.8 72.2 2 1 2
5 75 70 61.1 64.9 18 1 1
6 73 70 59 49.2 3 1 2
7 60 65 61.7 58.4 1 1 1
8 68 68 60 59.4 3 1 2
9 68 72 59.4 51.9 1 1 1
10 65 65 59.9 49.6 3 1 2
11 71 67 59.7 62.1 0 1 1
12 65 79 60 40.3 2 1 2
13 74 70 64.4 66.7 8 1 1
14 65 74 57.8 81.1 1 1 2
15 74 73 59.9 59.7 4 1 1
16 68 69 58.4 62.6 0 1 2
17 68 71 58.4 68 1 1 1
18 68 65 58.4 47.3 4 2 2
19 68 75 56.4 81.8 2 2 1
20 68 73 55.2 53 1 2 2
21 69 71 57.7 62.4 2 2 1
22 69 71 59.7 69.2 0 2 2
23 71 73 59.7 76 2 2 1
24 69 73 58.4 44.3 1 2 2
25 73 71 58.4 36.5 3 2 1
26 69 73 58.7 42.2 2 2 2
27 61 65 60.8 42.5 1 2 1
28 64 66 60.1 37.1 6 2 2
29
L 6 6
66 57.8 32.8 3 2 1
30 64 70 59.7 43.6 1 2 2
31 74 68 58.3 73.5 1 2 1
32 66 72 59.4 44.6 2 2 2
33 66 70 58.8 37.1 1 2 1
34 64 68 59.3 41.3 1 1 2
35 66 76 60.7 58.1 3 1 1
36 68 70 60.1 62.5 1 1 2
37 72 70 61 49.5 0 1 1
38 70 78 60.1 38.8 1 1 2
39 66 68 60.6 46.6 5 1 1
40 72 74 60.8 88.2 2 1 2
41 76 72 61.1 90.3 1 1 1
42 72 74 60.9 39.6 2 1 2
43 64 71 60.5 48.3 2 1 1
44 67 69 61 49.5 8 1 2
45 67 71 61.2 84.3 20 1 1
46 69 69 61.4 43.5 3 1 2
47 71 69 61 58.8 1 1 1
48 69 73 61.1 53.8 0 1 2
49 74 75 61.6 61.6 0 2 1
50 74 77 60.1 41.7 3 2 2
51 68 71 61.4 61.4 0 2 1
52 68 72 61.4 44.4 8 2 2
53 75 71 63.3 52.3 4 2 1
54 68 69 62.1 49.7 4 2 2
55 66 69 61.9 17.5 3 2 1
56 66 72 60.7 22.8 4 2 2
57 64 69 60.5 23.8 4 2 1
58 68 66 60.5 30.8 2 2 2
59 66 70 60.5 62.7 7 2 1
60 73 78 60.6 83.3 3 2 2
61 66 72 59.7 46.9 2 2 1
62 70 68 60 40.3 3 2 2
63 70 72 60.4 79.7 1 2 1
64 73 67 59.4 56.1 0 1 1
65 70 70 60.6 62.5 5 2 2
66 67 71 59.7 72.3 2 2 1
67 75 67 62.3 52.5 6 1 2
68 70 68 58.1 40.9 8 2 2
69 66 72 58.8 28.8 1 1 1
70 68 72 61.1 74.6 4 2 1
71 68 70 58.4 62.9 0 1 2
72 68 77 59.8 80.5 1 2 2
73 73 73 59 47.9 0 1 1
74 75 75 59 34.4 5 2 1
75 69 71 60.4 70.9 3 1 2
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
111
Subnum ATTA120 ATTA150 ATTV120 ATTV150 IGNA120 IGNA150 IGNV120 IGNV150 ITI
1 17.49 -69.19 -12.01 -37.34 -59.53 11.49 -77.81 -34.20 1.28
2 14.08 -7.78 2.30 -53.07 30.32 -31.48 58.34 -53.55 4.86
3 -0.45 -64.46 -75.88 -67.21 -71.61 -74.08 -1.24 -8.39 29.95
4 -66.84 25.75 7.82 91.08 -18.46 -100.00 9.05 -28.78 1.36
5 8.31 -18.95 -100.00 -79.80 -84.44 -53.00 -72.09 -94.18 5.10
6 -92.39 -80.01 7.02 -44.98 -76.80 -95.06 40.42 -27.45 12.23
7 -21.45 -100.00 7.35 -38.97 -64.95 -72.41 -84.89 -8.86 1.66
8 -59.89 -69.29 12.43 2.21 -57.15 -81.67 70.28 24.62 11.84
9 -27.83 -3.63 77.11 9.30 -10.86 -12.21 2.56 8.19 16.80
10 -81.55 -72.96 44.69 -91.28 -27.92 4.09 17.00 -58.70 7.64
11 -23.05 -36.93 18.00 -35.57 -15.48 -38.86 32.66 -28.78 13.26
12 -100.00 -44.39 -100.00 -27.09 14.32 101.44 76.42 -100.00 1.08
13 -43.84 -100.00 -46.37 57.64 -71.14 -79.45 -6.49 -6.19 6.86
14 -64.61 -65.52 -8.46 -20.45 -39.70 -53.38 -40.53 -36.35 20.10
15 7.12 -100.00 82.99 -22.72 22.46 -54.28 32.30 -9.21 3.08
16 -27.94 -23.47 59.48 104.45 103.38 -46.15 113.33 -24.10 5.29
17 -18.22 -40.76 -12.74 -51.04 -18.75 -40.09 19.12 4.10 32.70
18 -81.58 -84.66 58.76 -25.73 -49.45 -88.96 31.92 -14.73 9.39
19 -98.33 -95.21 -19.58 -59.32 -81.33 -94.04 -14.93 -32.58 4.59
20 -96.77 -87.32 -84.72 -65.74 -96.62 -62.54 -76.23 -87.29 11.15
21 -64.50 -86.10 -54.62 -16.02 -67.68 -79.75 15.74 -15.24 4.72
22 -21.12 -10.20 -0.57 10.29 -19.22 -65.90 -4.27 -10.34 16.85
23 -9.33 -47.63 -16.53 -75.14 -53.76 -61.21 48.20 -28.16 6.80
24 -90.81 -77.83 -17.65 -12.37 -84.37 -83.74 16.77 -54.28 3.16
25 -67.53 -78.64 18.02 -23.45 -40.49 -55.16 23.55 15.91 6.63
26 -31.95 -59.29 19.76 8.04 -65.71 -71.99 31.52 -23.11 7.43
27 -73.07 -93.51 -35.89 -63.74 -100.00 -96.08 -59.20 13.71 8.68
28 -53.59 -59.98 106.47 -21.58 -17.30 -78.16 -39.26 -51.48 5.37
29 -63.69 -15.52 5.47 28.95 -25.51 -5.26 -12.73 -9.70 22.44
30 -92.60 -57.85 -10.38 -69.04 -73.62 -81.87 -37.27 -76.68 18.37
31 -79.62 -77.41 153.85 82.41 -100.00 41.16 -3.76 57.12 1.36
32 -33.33 43.39 -16.93 -48.68 -4.23 56.61 105.29 92.59 0.63
33 -28.20 -37.00 -24.61 -21.04 -21.72 -50.99 13.54 17.64 39.68
34 -56.95 -33.95 -21.01 -25.13 -19.64 -48.03 -23.11 12.30 15.53
35 -56.20 7.58 2.89 151.99 -85.92 -42.00 55.48 57.04 2.77
36 -87.38 -25.82 -33.95 9.97 -40.52 -64.90 -20.46 -38.27 1.93
37 21.24 -42.21 182.75 141.36 25.58 -56.82 77.53 79.04 3.54
38 -91.01 -66.07 -28.67 -33.24 -73.58 -63.25 -18.31 -75.85 22.92
39 -75.82 -72.24 -27.59 -72.59 -82.19 -84.27 -24.58 -37.54 2.88
40 443.05 -44.37 54.97 264.24 -100.00 -100.00 -48.34 690.73 0.25
41 -23.67 -63.79 113.12 63.07 -85.95 45.81 -15.92 72.02 4.77
42 -45.79 -81.68 -16.52 -26.05 -47.50 -58.30 -5.57 9.77 4.48
43 -94.18 -98.38 -64.74 -92.58 -40.92 -93.33 -91.43 -77.42 13.34
44 -30.99 -63.67 85.47 52.27 8.47 -42.29 86.69 109.46 1.92
45 -100.00 -37.17 -100.00 -65.84 -97.04 -97.19 -33.83 -44.47 6.99
46 -37.88 -48.85 27.61 30.93 1.05 -17.28 -2.03 -43.03 6.93
47 -17.35 -53.49 -15.86 -1.55 -49.93 11.90 21.84 -65.69 9.83
48 34.96 -54.87 13.90 1.43 75.79 56.02 36.25 1.43 0.78
49 -71.32 -62.31 54.78 48.81 -70.64 -52.81 41.49 -45.42 4.92
50 -70.72 -44.09 -12.43 -11.99 -80.15 -90.65 3.06 -36.80 8.41
51 -67.12 -81.62 -29.14 -34.24 -77.46 -21.85 -28.60 -25.90 18.92
52 -57.97 -51.71 -15.55 -19.08 -72.33 -85.25 -58.63 -20.64 2.55
53 -83.51 -67.95 59.75 22.95 -75.85 -60.11 43.56 82.73 17.42
54 -39.23 -57.38 -100.00 4.11 2.66 -100.00 -100.00 -100.00 1.61
55 -52.79 -94.90 -37.41 -47.10 -94.65 -26.63 -11.59 -50.86 3.99
56 -87.29 -79.17 25.17 -60.98 -85.75 -100.00 -18.89 -65.36 1.52
57 -71.71 -59.74 23.08 15.14 -52.98 -62.09 -22.24 11.46 23.12
58 38.59 20.00 -21.41 25.07 -100.00 -21.41 78.31 -100.00 0.39
59 -64.12 -38.21 92.91 -63.01 77.85 -48.17 101.11 12.51 1.51
60 -100.00 -42.65 22.65 -90.44 -79.56 -91.24 -34.16 -100.00 1.26
61 -78.67 -92.70 -53.80 -61.40 -89.54 -61.90 -22.96 -79.01 8.77
62 -78.88 -54.06 -29.89 -49.59 -46.94 -61.64 -37.82 -89.91 48.45
63 -61.51 -83.82 -53.47 -50.25 -78.82 -93.22 -18.41 -47.20 7.66
64 -9.70 -23.02 11.97 13.62 -23.32 -22.99 6.75 -2.14 34.90
65 -55.53 -48.26 35.17 -18.80 -47.97 -60.50 11.48 -27.26 9.26
66 -87.63 -82.98 -19.41 -48.49 -74.59 -78.33 -27.90 -16.60 30.67
67 -34.87 -82.23 3.09 3.07 -38.94 -55.43 -18.41 •4.45 19.55
68 -87.21 -29.84 -50.09 -37.46 -1.11 -49.27 -15.97 -6.77 6.12
69 -15.90 -61.10 -54.87 -7.72 8.78 -33.91 -2.81 -26.14 15.05
70 -63.46 -75.58 -8.58 -77.31 -60.98 -57.84 -4.56 -60.74 14.10
71 -25.45 8.28 15.97 -2.28 159.31 51.02 -38.78 -51.98 1.39
72 -39.93 -28.41 3.18 -18.04 -14.58 -26.80 12.28 -8.55 28.45
73 21.96 2.59 -13.08 35.45 -52.80 14.06 -73.01 -6.92 4.77
74 -92.60 -100.00 39.60 -45.53 -66.69 -84.49 9.81 -64.92 1.89
75 8.83 12.02 9.82 -5.54 15.35 -22.78 -5.63 -19.63 34.59
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
112
Subnum ATAAMP12 ATAAMP15 IGAAMP12 IGAAMP15 ATVAMP12 ATVAMP15 IGVAMP12 IGVAMP15
1 0.59 0.36 0.38 0.74 0.65 0.58 0.61 0.43
2 0.39 0.39 0.42 0.37 0.37 0.32 0.49 0.33
3 0.49 0.64 0.61 0.64 0.47 0.59 0.40 0.50
4 0.08 0.06 0.00 0.00 0.13 0.21 0.18 0.08
5 0.52 0.35 0.10 0.20 0.40 0.25 0.10 0.10
6 1.13 0.70 1.15 0.78 0.86 0.71 0.97 0.72
7 0.75 0.77 0.77 0.65 0.70 0.26 0.47 0.10
8 0.22 0.35 0.47 0.35 0.42 0.52 0.65 0.51
9 0.81 0.97 0.96 0.72 0.98 1.01 0.98 0.86
10 0.13 0.28 0.37 0.31 0.28 0.32 0.45 0.32
11 0.31 0.00 0.24 0.18 0.54 0.20 0.48 0.15
12 0.00 0.00 0.00 0.00 0.00 0.06 0.00 0.00
13 0.00 0.15 0.14 0.00 0.00 0.00 0.08 0.00
14 0.59 0.45 0.67 0.29 0.62 0.32 0.42 0.35
15 0.44 0.00 0.37 0.40 0.12 0.40 0.33 0.00
16 0.25 0.60 0.74 0.26 0.24 0.10 0.44 0.14
17 0.50 0.43 0.41 0.58 0.63 0.69 0.55 0.42
18 0.93 0.95 1.01 0.22 1.14 0.68 1.00 0.93
19 0.24 0.00 0.00 0.21 0.26 0.16 0.44 0.27
20 0.44 0.47 1.20 1.15 1.55 1.54 1.05 0.87
21 0.55 0.00 0.00 0.08 0.00 0.00 0.29 0.10
22 0.56 0.59 0.85 0.39 0.77 0.57 0.84 0.97
23 0.13 0.44 0.08 0.22 0.65 0.44 0.22 0.34
24 0.27 0.31 0.35 0.19 0.61 0.32 0.58 0.29
25 0.49 0.27 0.40 0.26 0.41 0.08 0.35 0.00
26 0.30 0.39 0.43 0.24 0.54 0.22 0.20 0.10
27 0.96 0.53 0.16 0.65 0.90 0.50 1.56 0.83
28 0.30 0.14 0.79 0.17 1.03 0.42 0.08 0.38
29 0.23 0.50 0.27 0.00 0.59 0.37 0.53 0.22
30 0.84 0.78 0.79 0.72 0.90 0.85 1.13 0.90
31 0.24 0.19 0.40 0.23 0.53 0.60 0.36 0.47
32 0.00 0.00 0.13 0.00 0.00 0.00 0.00 0.00
33 0.31 0.08 0.00 0.00 0.36 0.17 0.36 0.08
34 0.10 0.16j 0.19 0.28 0.15 0.28 0.28 0.34
35 0.65 0.00 0.00 0.00 0.00 0.98 0.00 0.00
36 0.22 0.40 0.33 0.10 0.33 0.27 0.24 0.10
37 1.04 0.65 0.78 0.97 1.09 1.01 0.72 0.89
38 0.66 0.58 0.76 0.52 0.74 0.65 0.55 0.49
39 0.69 0.40 0.16 0.94 0.76 0.82 0.67 0.55
40 0.14 0.55 0.13 0.00 0.17 0.00 0.10 0.08
41 0.31 0.00 0.00 0.00 0.15 0.16 0.21 0.15
42 0.10 0.39 0.28 0.47 0.14 0.06 0.25 0.55
43 0.39 0.54 0.82 0.49 0.50 0.51 0.68 0.59
44 0.83 0.98 1.06 0.52 0.86 0.74 0.54 0.29
45 0.44 0.00 0.00 0.06 0.36 0.00 0.34 0.48
46 0.97 1.42 1.25 0.20 1.40 0.93 0.97 1.14
47 0.68 0.58 0.79 0.55 0.76 0.72 0.44 0.79
48 1.26 0.97 1.15 1.28 1.03 1.26 1.01 1.33
49 0.79 1.16 0.92 0.88 1.03 1.00 1.08 0.87
50 0.48 0.69 0.28 0.62 0.67 0.71 0.61 0.54
51 0.64 0.50 0.56 0.77 1.04 0.74 0.93 0.75
52 0.22 0.19 0.31 0.26 0.59 0.34 0.42 0.16
53 0.60 0.38 0.67 0.30 1.00 0.62 0.82 0.70
54 0.13 0.59 0.56 0.10 0.43 0.22 0.38 0.00
55 0.45 0.06 0.13 0.32 0.08 0.22 0.00 0.22
56 0.24 0.30 0.17 0.24 0.41 0.28 0.27 0.18
57 0.97 0.49 0.46 0.59 0.94 0.93 0.44 0.57
58 0.79 1.58 1.26 1.17 0.96 1.60 1.18 1.86
59 0.36 0.66 0.16 0.44 0.54 0.12 0.28 0.18
60 0.40 0.48 0.41 0.31 0.67 0.73 0.33 0.91
61 0.30 0.13 0.19 0.10 0.18 0.25 0.32 0.14
62 0.06 0.00 0.26 0.00 0.13 0.00 0.08 0.06
63 0.80 0.98 0.93 0.86 1.06 1.09 0.81 0.83
64 0.20 0.10 0.06 0.16 0.14 0.28 0.13 0.18
65 0.00 0.52 0.58 0.47 0.73 0.00 0.08 0.16
66 1.02 0.75 0.99 0.63 0.96 1.12 1.11 1.00
67 1.06 0.89 1.22 1.00 0.95 0.81 0.60 0.51
68 0.50 0.52 0.48 0.40 0.24 0.41 0.34 0.73
69 0.35 0.00 0.78 0.46 0.26 0.28 0.26 0.00
70 0.85 1.04 0.58 0.61 1.23 1.03 1.16 0.54
71 0.94 0.67 0.75 0.47 0.34 0.73 0.81 0.42
72 0.88 0.64 0.37 0.22 0.47 0.28 0.37 0.00
73 0.00 0.21 0.08 0.12 0.15 0.34 0.08 0.00
74 1.05 0.77 0.99 0.53 0.57 0.90 0.88 0.68
75 0.10 0.23 0.37 0.32 0.19 0.06 0.24 0.31
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Subnum ATAUDAMP IGAUDAMP ATVISAMP IGVISAMP WNAMP
1 0.00 0.24 0.49 0.32 0.39
2 0.12 0.49 0.12 0.37 0.16
3 0.44 0.47 0.27 0.12 0.40
4 0.00 0.06 0.12 0.00 0.07
5 0.21 0.41 0.39 0.00 0.41
6 0.51 1.01 0.78 0.66 0.78
7 0.00 0.27 0.15 0.24 0.51
8 0.14 0.00 0.12 0.00 0.35
9 0.71 0.44 0.86 0.43 0.62
10 0.14 0.06 0.37 0.38 0.29
11 0.18 0.10 0.37 0.00 0.22
12 0.00 0.00 0.00 0.00 0.06
13 0.39 0.00 0.00 0.00 0.30
14 0.59 0.14 0.30 0.22 0.26
15 0.34 0.00 0.24 0.00 0.45
16 0.00 0.36 0.29 0.13 0.28
17 0.50 0.37 0.40 0.29 0.38
18 0.55 0.63 0.44 0.50 0.85
19 0.22 0.00 0.24 0.00 0.18
20 0.50 0.44 0.61 0.50 0.94
21 0.00 0.00 0.00 0.00 0.03
22 0.37 0.31 0.44 0.00 0.52
23 0.00 0.00 0.47 0.15 0.21
24 0.15 0.43 0.18 0.14 0.17
25 0.00 0.08 0.00 0.00 0.39
26 0.20 0.30 0.29 0.22 0.22
27 0.45 0.54 0.52 0.65 0.91
28 0.00 0.19 0.00 0.40 0.37
29 0.27 0.35 0.57 0.40 0.26
30 0.62 0.48 0.59 0.79 0.77
31 0.32 0.43 0.40 0.25 0.35
32 0.00 0.00 0.00 0.00 0.06
33 0.12 0.00 0.36 0.00 0.20
34 0.23 0.19 0.06 0.12 0.20
35 0.00 0.36 0.24 0.00 0.12
36 0.00 0.44 0.08 0.28 0.22
37 0.72 0.64 0.53 0.39 0.74
38 0.65 0.50 0.66 0.45 0.53
39 0.33 0.45 0.48 0.22 0.52
40 0.10 0.06 0.10 0.00 0.15
41 0.22 0.00 0.20 0.20 0.27
42 0.14 0.21 0.10 0.10 0.22
43 0.18 0.37 0.40 0.08 0.46
44 0.00 0.08 0.14 0.35 0.64
45 0.00 0.00 0.15 0.00 0.35
46 0.30 0.37 0.67 0.67 0.74
47 0.20 0.49 0.14 0.24 0.62
48 1.06 0.69 1.06 1.03 1.05
49 0.35 0.00 0.31 0.10 0.69
50 0.36 0.51 0.52 0.48 0.30
51 0,45 0.38 0.83 0.40 0.79
52 0.22 0.38 0.08 0.12 0.16
53 0.26 0.17 0.22 0.29 0.50
54 0.46 0.34 0.18 0.42 0.28
55 0.00 0.00 0.32 0.15 0.28
56 0.00 0.25 0.29 0.17 0.17
57 0.92 0.17 0.66 0.87 0.77
58 0.84 0.48 0.97 0.69 1.08
59 0.00 0.00 0.00 0.00 0.20
60 0.69 0.15 0.29 0.00 0.47
61 0.00 0.18 0.22 0.06 0.16
62 0.00 0.00 0.00 0.00 0.12
63 0.94 0.79 0.84 0.65 0.85
64 0.00 0.00 0.00 0.13 0.03
65 0.00 0.00 0.31 0.00 0.25
66 0.67 0.89 1.14 0.49 1.03
67 0.84 0.43 0.61 0.50 0.69
68 0.24 0.20 0.17 0.18 0.60
69 0.00 0.33 0.45 0.68 0.28
70 0.74 0.72 0.85 0.37 0.75
71 0.28 0.21 0.69 0.57 0.34
72 0.00 0.00 0.00 0.30 0.69
73 0.22 0.00 0.55 0.32 0.22
74 0.66 0.47 0.99 0.90 0.68
75 0.16 0.00 0.00 0.00 0.28
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114
Appendix D: Experiment 2 Informed Consent Form
University of Southern California
Department o f Psychology
INFORMED CONSENT FOR NON-MEDICAL RESEARCH
CONSENT TO PARTICIPATE IN RESEARCH
Did you see that?
You are asked to participate in a research study conducted by Jonathan Wynn, M.A.,
and Michael Dawson, Ph. D., from the Department of Psychology at the University
of Southern California. This research is part of a dissertation conducted by Jonathan
Wynn. You were selected as a possible participant in this study because of your
willingness to participate and because English was your native (first language
learned) language. A total of 50 subjects will be selected from undergraduate
psychology classes to participate. Your participation is voluntary.
□ PURPOSE OF THE STUDY
This study aims to determine how the brain processes information received in rapid
succession and how that information processing can be disrupted.
□ PROCEDURES
If you volunteer to participate in this study, we would ask you to do the following
things:
1) Read and sign this consent form.
2) Have electronic sensors attached underneath your left eyelid and behind your
left ear to measure your physiological responses to stimuli.
3) View a series of slides which contain a letter and attempt to name the letter
just presented.
4) You will hear random loud bursts of static noise to which you do not need to
respond.
This experiment will last approximately 30 minutes and you need participate only
once. This experiment is conducted in SGM 907.
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115
□ POTENTIAL RISKS AND DISCOMFORTS
There are no risks associated with measuring your physiological responses with the
sensors. Although the static-like noise is loud, there is no risk to the health of your
ears.
□ POTENTIAL BENEFITS TO SUBJECTS AND/OR TO SOCIETY
Although you will not benefit directly from this research, this line of research allows
us to further our understanding of how the brain processes information and how that
processing is affected by certain psychological disorders.
□ PAYMENT FOR PARTICIPATION
You will receive $0.10 for each correctly identified slide for a maximum award of
$7.20 if all slides are identified correctly. You will be paid immediately at the end of
the experiment. You will also receive one hour of extra credit to the psychology
course of your choice which allows extra credit for participation in studies.
□ CONFIDENTIALITY
Any information that is obtained in connection with this study and that can be
identified with you will remain confidential and will be disclosed only with your
permission or as required by law. Although we monitor your movements with a
video camera, we do not record the testing session. When the results of the research
are published or discussed in conferences, no information will be included that
would reveal your identity.
□ PARTICIPATION AND WITHDRAWAL
You can choose whether to be in this study or not. If you volunteer to be in this
study, you may withdraw at any time without consequences of any kind. You may
also refuse to answer any questions you don’t want to answer and still remain in the
study. The investigator may withdraw you from this research if circumstances arise
which warrant doing so. Such an instance may be equipment malfunction or
uncorrected sight- or hearing-impairment. If you or the experimenter does not wish
to continue the experiment you will receive your full one hour of extra credit but will
only receive the $0.10 bonus per slide for however many slides you saw and
identified correctly.
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116
□ IDENTIFICATION OF INVESTIGATORS
If you have any questions or concerns about the research, please feel free to contact
Michael E. Dawson, Ph.D. or Jonathan Wynn, M.A. at (213) 740-2297, Dept, of
Psychology, SGM 501, Los Angeles, CA, 90089-1061.
□ RIGHTS OF RESEARCH SUBJECTS
You may withdraw your consent at any time and discontinue participation without
penalty. You are not waiving any legal claims, rights or remedies because of your
participation in this research study. If you have questions regarding your rights as a
research subject, contact the University Park IRB, Office of the Vice Provost for
Research, Bovard Administration Building, Room 300, Los Angeles, CA 90089-
4019, (213) 740-6709 or upirb@usc.edu.
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117
SIGNATURE OF RESEARCH SUBJECT, PARENT OR LEGAL
REPRESENTATIVE.
I understand the procedures described above. My questions have been answered to
my satisfaction, and I agree to participate in this study. I have been given a copy of
this form.
Name of Subject
Name of Parent or Legal Representative (if applicable)
Signature of Subject, Parent or Legal Representative Date
SIGNATURE OF INVESTIGATOR
I have explained the research to the subject or his/her legal representative, and
answered all of his/her questions. I believe that he/she understands the information
described in this document and freely consents to participate.
Name of Investigator
Signature of Investigator Date (must be the same as subject’s)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
118
Appendix E: Experiment 2 Instructions
Your task is to name the letter presented on the slides shown in front
of you. Keep your eyes focused on the spot on the wall; this is where the
slides will be shown. Do your best to name as quickly as you can the letter
seen on the first slide. Most of the slides with letters will be followed by
another slide of a random pattern of lines. If you could not tell what letter it
was, take your best guess from the 15 letters listed in front of you. Most of
the time slides will be presented but there will be times when no slides are
presented. In this case, you will hear a brief noise. I will play you this noise
now. Whenever you hear this noise I want you to say “heard”; there will be no
letter presented with this noise. You will also hear loud bursts of noise; you
can simply ignore this sound. In order to help you with your task, we will
present you with a $5.00 bonus. However, for each incorrect or missed letter
you say we will subtract 10 cents from your bonus. If you have any questions
please ask them now as the experiment can’t be paused once begun. The
experiment lasts approximately 25 minutes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix F: Experiment 2 Data
119
Subnum SOAO SOA30 SOA45 SOA60 SOA120 SOA150 ITI
23 10 3 6 8 9 9 0.18
24 10 0 1 8 10 10 9.17
25 10 1 3 8 10 10 9.16
26 8 0 2 5 8 9 0.59
27 9 4 8 8 9 10 1.39
28 8 6 4 7 10 10 2.86
29 10 1 1 4 10 10 5.60
30 10 1 6 8 10 9 1.56
31 10 1 4 6 9 10 4.48
32 10 2 4 5 10 10 2.29
33 9 1 8 8 9 10 4.01
34 10 1 5 7 10 10 15.54
35 6 2 2 2 8 6 0.51
36 10 1 6 8 10 10 2.32
37 9 2 4 5 9 10 0.83
38 10 2 5 9 8 10 39.38
39 9 0 5 6 10 10 11.34
40 10 1 4 7 7 9 3.40
41 9 0 4 7 10 9 20.03
42 9 1 6 9 8 10 2.04
43 10 1 8 9 9 10 3.57
45 10 1 5 8 10 10 53.84
46 9 1 6 8 10 10 1.01
47 9 2 1 4 10 10 6.57
48 10 6 6 8 10 10 10.16
49 10 8 7 9 9 10 1.10
50 10 3 2 5 9 9 2.34
51 10 4 7 8 10 8 3.87
52 10 2 6 8 10 10 0.69
53 10 1 5 9 10 10 22.24
54 10 2 4 6 10 10 5.14
55 7 5 6 8 9 10 24.72
56 10 6 9 7 10 8 3.96
57 10 2 8 9 10 10 3.12
58 9 4 8 9 10 10 1.05
59 10 1 7 7 10 9 16.96
60 7 3 1 6 10 9 0.43
61 10 2 6 8 10 8 4.60
62 9 1 1 8 10 10 1.72
63 10 2 3 7 10 10 28.16
64 7 1 4 9 10 9 1.44
65 8 3 6 6 9 9 37.58
66 6 0 1 7 7 8 3.28
67 10 0 1 4 10 7 0.26
68 10 1 5 6 10 10 11.70
69 10 3 9 9 9 10 15.39
70 10 1 3 7 10 10 27.47
71 10 3 4 8 10 9 2.96
72 10 5 7 9 10 9 8.46
73 10 2 6 7 10 10 10.20
74 8 0 6 7 9 10 20.05
75 10 1 6 8 9 10 14.24
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
120
Subnum PVIS30 PVIS45 PVIS60 PVIS120 PVIS150 PAUD30 PAUD45 PAUD60 PAUD120 PAUD150
23 50.80 360.40 -100.00 721.26 -100.00 -100.00 -100.00 6.73 -100.00 106.94
24 93.71 135.39 121.71 50.41 -46.04 -16.35 -50.06 -37.58 -96.06 -95.10
25 219.99 48.96 -12.62 -53.82 -79.69 -52.98 -75.59 -93.26 -96.65 -97.46
26 229.92 364.66 28.78 -100.00 -61.96 152.81 -100.00 167.80 63.10 123.91
27 131.64 108.72 -43.85 63.31 -52.82 -13.08 155.43 -28.69 -87.39 -77.97
28 7.51 20.26 3.12 -25.25 -61.72 -65.86 -90.21 -89.49 -47.89 -85.08
29 -1.10 33.11 1.62 -73.30 -67.78 -53.06 -73.93 -96.02 -91.00 -94.70
30 149.97 1.01 112.26 97.05 133.44 96.35 149.34 10.93 -42.71 -62.33
31 190.02 29.03 4.33 -82.98 -82.16 -55.14 -81.26 -86.40 -99.04 -100.00
32 34.02 -57.88 -42.46 -52.38 -74.10 -81.02 -67.00 -100.00 -83.04 -87.93
33 158.35 87.27 -32.43 -67.67 -79.38 1.52 -49.87 -88.71 -78.71 -80.46
34 3.48 -3.89 -50.12 -39.87 -65.30 -63.67 -100.00 -98.45 -94.32 -77.85
35 1016.17 236.53 -25.78 -28.14 -100.00 9.74 11.50 -0.53 491.90 446.96
36 91.67 82.28 24.20 -37.56 -79.34 -62.74 -92.74 -91.54 -96.42 -94.91
37 16.26 -100.00 187.09 -100.00 63.35 -86.29 -8.53 -82.65 -84.51 -100.00
38 12.31 8.00 -0.73 2.38 -20.89 -29.15 -68.25 -84.31 -92.39 -85.77
39 31.80 67.21 12.84 30.48 2.59 20.17 -27.43 -53.41 -57.90 -85.72
40 341.17 128.01 15.55 14.46 -44.26 -35.40 -47.98 -87.07 -57.73 -81.34
41 73.08 70.60 41.99 -54.69 3.88 -68.05 -65.92 -76.94 -61.11 -69.30
42 62.73 17.10 -19.64 -61.67 -71.71 -67.32 -95.88 -97.14 -95.05 -91.51
43 32.74 30.05 -55.35 -58.74 -67.70 -47.87 -87.54 -70.54 -75.37 -82.64
45 10.59 2.50 -7.43 -16.67 -5.48 -20.24 -78.95 -100.00 -98.67 -76.36
46 87.80 -27.07 -73.76 -20.97 -22.57 21.36 -92.66 -75.77 -85.20 -64.31
47 37.88 -52.25 -55.47 -66.90 -68.03 -32.12 -79.11 -83.63 -100.00 -94.85
48 225.49 61.63 39.92 -16.71 -81.58 -32.33 -71.74 -93.59 -98.21 -59.03
49 137.78 -74.76 114.35 -36.54 -100.00 -11.79 26.73 -100.00 -77.37 -51.33
50 185.18 67.90 -58.63 -71.92 -84.85 -81.84 -73.44 -77.85 -74.12 -11.15
51 30.76 66.78 34.89 63.19 -29.40 13.70 -64.58 -89.23 -81.10 -77.23
52 -90.27 -66.13 20.61 -91.40 -100.00 -100.00 -88.71 -100.00 -100.00 -79.89
53 16.13 40.96 -6.99 17.36 -40.30 -35.00 -67.43 -71.38 -76.70 -91.61
54 98.15 52.35 -21.46 -62.07 -94.23 -45.44 -66.88 -81.34 -84.55 -91.79
55 41.83 13.29 -37.39 -29.02 -84.06 -65.82 -85.42 -100.00 -98.15 -93.41
56 31.06 7.26 -37.25 -85.83 -91.39 -72.64 -50.33 -100.00 -100.00 -100.00
57 22.21 33.94 56.87 -43.97 -66.07 -0.87 -83.90 -93.50 -72.32 -95.97
58 552.27 65.72 41.16 -91.61 -85.62 -43.24 -62.54 -82.23 -87.29 -100.00
59 62.15 56.81 -11.23 -51.01 -64.97 -19.99 -89.29 -93.04 -69.89 -93.11
60 733.78 265.15 597.93 285.18 93.00 334.86 120.49 6.37 -100.00 -100.00
61 86.51 32.15 -1.20 10.23 -13.49 -26.14 -34.93 -56.60 -79.28 -85.96
62 235.63 5.14 -40.00 -69.43 -100.00 13.54 -55.42 -86.24 -100.00 -100.00
63 27.45 0.11 -3.53 7.79 -19.61 -45.51 -66.85 -93.15 -91.81 -88.51
64 141.36 -67.44 -62.37 90.04 56.35 -44.57 70.45 -75.29 84.48 -40.33
65 6.53 -0.53 -19.03 -58.20 -9.84 -19.95 -41.67 -73.19 -73.19 -59.28
66 229.34 114.37 -17.68 -34.16 -72.84 -74.98 -70.63 -86.12 -69.56 -55.37
67 19.65 534.24 -100.00 -44.55 -55.25 -100.00 -100.00 11.87 -100.00 -56.23
68 114.51 30.07 5.61 -41.29 -40.39 -42.93 -57.10 -70.90 -36.10 -33.40
69 54.93 46.99 1.06 4.71 -64.69 -29.53 -17.83 -31.80 -58.65 -87.79
70 46.53 28.14 9.45 -21.21 -65.95 -41.40 -71.29 -84.25 -100.00 -100.00
71 51.54 -1.28 -9.57 -77.86 -91.55 -13.03 -44.73 -90.28 -100.00 -100.00
72 48.97 44.19 -3.40 45.55 -19.59 -78.38 -93.26 -85.29 -80.36 -82.81
73 32.67 61.19 12.46 -17.48 13.04 28.41 -46.44 -73.74 -73.27 -80.23
74 17.09 6.95 -50.21 -86.00 -55.31 -52.44 -45.66 -92.07 -87.18 -91.04
75 49.91 19.14 2.46 -83.21 -90.73 -22.41 -71.76 -80.28 -29.67 -53.04
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

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Asset Metadata

Creator Wynn, Jonathan Kajzovar (author)

Core Title Prepulse inhibition: Stimulus parameters and its relationship to visual backward masking

Degree Doctor of Philosophy

Degree Program Psychology

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,psychology, experimental

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-547174

Unique identifier UC11339229

Identifier 3094387.pdf (filename),usctheses-c16-547174 (legacy record id)

Legacy Identifier 3094387.pdf

Dmrecord 547174

Document Type Dissertation

Rights Wynn, Jonathan Kajzovar

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA