University of Southern California Dissertations and Theses

Monocular Acquisition And Interocular Transfer Of Two Types Of Discriminations In Normal And Corpus Callosally-Sectioned Guinea Pigs

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Monocular Acquisition And Interocular Transfer Of Two Types Of Discriminations In Normal And Corpus Callosally-Sectioned Guinea Pigs

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Content LEVINSON, Daniel Mayer, 1941-
MONOCULAR ACQUISITION AND INTEROCULAR TRANSFER
OF TWO TYPES OF DISCRIMINATIONS IN NORMAL AND
CORPUS-CALLOSALLY-SECTIONED GUINEA PIGS.
University of Southern California, Ph.D., 1970
Psychology, experimental
j
University Microfilms, A X ER O X Com pany, Ann Arbor, Michigan '
t
THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED
MONOCULAR ACQUISITION AND INTEROCULAR TRANSFER
OF TWO TYPES OF DISCRIMINATIONS IN NORMAL
AND CORPUS -CALLOSALLY-SECTIONED
GUINEA PIGS
by
Daniel Mayer Levinson
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)
June 1970
UNIVERSITY O F SOUTHERN CALIFORNIA
TH E GRADUATE SC H O O L
UNIVERSITY PARK
LO S A N G ELES, CA LIFO R N IA 9 0 0 0 7
This dissertation, written by
Daniel Mayer Levinson
under the direction of A„i.S„ Dissertation Com-
mittee, and approved by all its members, has
been presented to and accepted by The G radu
ate School, in partial fulfillm ent of require
ments of the degree of
D O C T O R O F P H I L O S O P H Y
f 7 7 1 ‘
Dtan
D a t e J M . . . t f . A ± . 1 3 : 7 . 0 .
'aXf O
94
DISSERTATION COMMITTEE
!
DEDICATION
This dissertation is dedicated to the
memory of my beloved parents, Dr. and
Mrs. Sidney 0. Levinson.
ii
TABLE OF CONTENTS
Page
LIST OF TABLES.................................. Iv
LIST OF FIGURES............ v
ACKNOWLEDGMENTS................................ vi
ABSTRACT vili
Chapter
I. INTRODUCTION ........................... 1
II. METHOD ............................... 18
III. RESULTS .......... 34
IV. DISCUSSION . . ....................... . 56
V. SUMMARY ............................. 82
REFERENCES 87
APPENDICES 93
APPENDIX A .............. 94
APPENDIX B ............................ 97
ill
LIST OF TABLES
Table
1. Outline of Experimental Design .............
2. Trials to Avoidance Criterion on Pretraining
3. Trials to Criterion for Original Acquisition
Homolateral Reacqulsition* and Interocular
Transfer ..............................
4 . Factorial Analysis of Variance: Summary
Table ..............................
Page
1
I
iv
LIST OP FIGURES
Figure Page
1. Top view of apparatus; entryway cue doors . .
2. Side view of apparatus ......................
3. Histographic representation of pretraining
data .....................................
4. Mean number of trials to criterion on pre-
training avoidance, first eye;acquisition,
and second eye acquisition ...............
5. Photographs of brain sections .............
v
ACKNOWLEDGMENTS ]
I wish to thank my Chairman, Dr. Gary Galbraith, j
for his patience, encouragement and understanding during
the course of my doctoral studies. I am also grateful j
to the other members of my dissertation Committee,
|
Drs. Jacek Szafran and John E. Holmes, as well as the
other members of my guidance committee, Drs. T. J. Teyler
and Henry Slucki, for their suggestions and assistance
in preparing the dissertation. I also wish to thank
|
Dr. Ernest G. Greene for his enthusiasm and concern; this j
proved to be invaluable during the final stages of my ;
research. I am indebted to Dr. John S. Williston for
many reasons, not the least of which is his friendship.
For reasons too numerous to list here, I express |
my heartfelt gratitude and appreciation to Dr, Don R.
Justesen. of the Veterans Administration Hospital, Kansas j
City, Missouri, and to Dr. Charles L. Sheridan, University
of Missouri, Kansas City. Over the years, their constant
support, encouragement and advice have proved to be a
tremendous source of inspiration and was largely respon
sible for the completion of my dissertation research and
graduate studies. It is an honor and a privilege to call
them "friend" and now "colleague."
vi
This research was supported in part by 8200 Funds
from the Kansas City Veterans Administration Hospital
Neuropsychology Laboratories, and in part by funds from
Grant NB-07455-03 from the United States Public Health
Service to Charles L. Sheridan.
vii
! ABSTRACT
i
Monocular acquisition and Interocular transfer of
; an active avoidance task involving the discrimination of
; j
i (a) stripes differing in orientation (tilt), or (b) a
; triangle and a square (shape), were measured in guinea j
; pigs which had undergone either surgical sectioning of
: the corpus callosum or control surgery involving making i
of a bone flap and slitting of dura. A reliable deficit j
in learning an active avoidance task during pretraining
was observed in callosally-sectioned animals (_t(26) =
2.321, p <.03), possibly due to clngulate damage which j
invariably accompanied sectioning of the callosum. How
ever, commissurotomy did not impair acquisition of either
i
discrimination. It was found that guinea pigs were able j
to learn the tilt discrimination within approximately 150 j
i
trials, and the shape discrimination within about 350 j
1
: trials. These data are similar to published data obtained
, from other species.
Although sham-operated animals exhibited interocular
j decrements ranging from 1-3 percent, callosally-sectioned
j animals showed decrements ranging from 25-50 percent;
i
j these were reliably greater than those seen in the
1
controls (P(l,2 3) = 1 3.15, £^. 005). Yet the observed
decrements for callosally-sectioned animals are
viii
Intermediate to the values predicted by anatomical studies
on the crossed and uncrossed visual fiber systems in the
i
guinea pig. It was hypothesized that these intermediate j
i
values might be due to facilitation on second-eye acquisi-]
i
tion resulting from the carry-over of nonspecific learn
ing on the original eye. Sectioning of the corpus cal- ;
losum resulted in a larger reduction in transfer in the j
i
guinea pig than that which was previously reported for
the hooded or albino rat; this finding is interpreted
as indicating a greater dependence in this species on
i
intact callosal fibers.
A discussion of the behavioral data and neuro-
anatomical implications, as well as data from the liter
ature, provided answers to several important questions:
1. How does the guinea pig compare with the rat j
i
in terms of learning capabilities? The guinea pig ex- ;
j
hibited more difficulty in avoidance training than did I
I
the rat, but acquired discrimination tasks as rapidly
or more rapidly than hooded or albino rats. j
2. Does the guinea pig show good transfer of dif-
i I
flcult tasks as well as easy ones? The sham-operated
j
: guinea pigs showed near-perfect interocular transfer of
both the tilt (96 percent) and the shape (97 percent)
; discriminations.
; 3. How large is the interocular decrement in the
| guinea pig; indirectly, how much transfer is accounted for
by the corpus callosum? The callosally-sectioned guinea
j
pigs showed interocular decrements ranging from 25-50
percent. The corpus callosum may account for more specific;
interhemispheric transfer of visual cue information than
suggested by these data, since much transfer of general
("nonspecific") information may take place via intact
transverse fiber systems.
4. How does callosal functioning in guinea pigs
compare with callosal functioning in other species? The
guinea pig is more dependent on an intact corpus callosum
for interhemispheric communication than is either the
hooded or albino rat, perhaps even as dependent as is
the cat or monkey. Several studies were suggested which
could further clarify the degree and roles of callosal
functioning in the guinea pig.
5. Does callosum-sectioning facilitate acquisition |
in guinea pigs? Although the callosally-sectioned guinea
pigs required fewer trials to meet criterion on both
tasks, their performance did not differ reliably from j
; that of the sham-operated animals. |
6. Can commissurotomized guinea pigs learn to !
j discriminate shape? All of the callosally-sectioned
i
I guinea pigs which were assigned to the experimental
| treatment involving the discrimination of shape were
able to successfully master this task.
x
A brief consideration of the merits of the guinea
pig as a subject for neuropsychological research suggests
that this animal has several characteristics which uniquely
qualify it for research on interhemispheric communication
and related phenomena.
CHAPTER I
INTRODUCTION
The corpus callosum Is the largest and phylogenet-
ically most recent fiber bundle in the central nervous
system, containing in man over 300 million fibers. It
first appears in mammals, and as one ascends the mammalian
scale, its function seems to increase in importance.
There had been a long-standing speculation that the
corpus callosum arose phylogenetlcally from the anterior
commissure; however, Abbie (1939) showed that In early
mammals there is no anatomical relationship between the
two commissures to support this viewpoint. In general,
callosal fibers connect a given area of one hemisphere
with an homologous area in the other. Ontogenetically,
It develops from the telencephalic lamina termlnalis,
an embryonic bridge of fibers crossing from one hemis
phere to the other (Truex and Carpenter, 1964, p. 2 1).
All of the functions of the corpus callosum are not
known, but in recent times it has become clear that It
does more than merely hold the two hemispheres together,
as was once facetiously proposed.
' 2
i
I
i
; Spilt-Brain Research
Within the last decade* some interesting work has
been done with the so-called "split-brain'' preparation.
; In this research* the subject has undergone surgical
; !
sectioning of the corpus callosum* and sometimes the optic j
chiasm and related commissural structures. Sperry (1 9 6 4) j
; has discussed much of the work done with split-brain
animals, mainly cats and monkeys. Among the functions
of the corpus callosum he listed are correlation of images
in the left and right halves of the visual field* inte
gration of sensations from paired limbs (or learning that
requires motor coordination of the limbs), and unifica
tion of the cerebral processes of attention and awareness.
Sperry believes the corpus callosum has more general
influences: e.g.* It exerts* as do other large nerve-
flber tracts, a general tonic effect upon the brain cells
to which it feeds Impulses.
A split-brain animal is "essentially a divided or
ganism with two mental units* each with its own memories
and its own will--competing for control over the organ
ism," (Sperry, 1964, p. 5 2). Holmes* in a discussion of
i
!split-brain studies, stated that ". . . the corpus callosum
does Indeed transfer information from one hemisphere to
t
jthe other" (1966* p. 88). He also feels that the split-
i
[brain technique Is valuable in research on cortical
1
function.
3
Sperry and his associates reported findings on
several neurosurgical patients (Gazzaniga, Eogen, and
i
Sperry, 1965; Gazzanlga and Sperry, 1966, 1967; Gazzaniga, j
1967); these reports are summarized in Sperry, 1968. j
!
i
The patients all suffered from epilepsy and underwent |
neurosurgical sectioning of the corpus callosum and
1
related commissures in an attempt to alleviate convulsive i
i
seizures. Sperry reported (1968) that the most striking j
outcome of commissurotomy was the apparent lack of behav- j
ioral effects. However, using more sensitive and appro
priate tests, he was able to demonstrate symptoms which i
he referred to as "the syndrome of the neocortical com-
1
mlssures" (Sperry, 1968, p. 7 2 4). The various tests used
j
(e.g., stereognosis, crossed topognosls in the hands,
tests for laterality of function, ipsllateral motor j
control) indicate the "presence of two minds in one j
i
body. ..." (Sperry, 1968, p. 7 2 4). In many ways the j
1
two hemispheres seem to be independent of one another;
; there is a lack of cross integration. Hemispheric later-
iality of function was also studied in these patients, and
it was found that although the minor hemisphere is aphasic
land agraphic, it can mediate higher reasoning. This in-
'eludes abstract (symbolic) thought, some comprehension of
i
;writing and speech, perception of spatial relationships,
iand emotional responses.
In some earlier work on cats, Myers (1 9 5 5) studied
Interocular transfer (transfer of training as learned via
one eye and tested via the other). The studies tested
transfer with the brain intact, with sectioned optic
chiasm alone, with sectioned corpus callosum alone, or j
with both sectioned. When both were sectioned, no trans- j .
fer occurred, whereas with either sectioned alone, trans- ;
j
fer did occur. Myers also assessed effects of partial j
sectioning of the callosum. With anterior lesions the
animals transferred, while with posterior lesions they j
did not. |
The split-brain technique has enabled investigators !
surgically to isolate various structures in the nervous j
system without completely disrupting the organism.
Sperry, Stamm, and Miner (1956), using split-brain cats,
[
removed all cortical material unilaterally except the j
i
striate cortex— producing what they termed a "visual-
island" preparation. They found the animals did poorly
on visual discrimination tasks. They repeated the pro
cedure with the somasthetlc area. Animals with a somas-
thetic island discriminated better via tactile cues than
did the visual island preparation via visual cues. In
another study on the effects of lesions involving the
split-brain technique, Downer (1962) performed unilateral
temporal lobectomies in commissurotomized M. mulatta.
j With the eye to the intact side open, the animals behaved
! normally; with the eye to the damaged side open, they
exhibited the KlCfver-Bucy syndrome (Kluver and Bucy, 1939)-
Rutledge and Kennedy (i960), in an investigation of
1
:transcallosal volleying in cats, electrically stimulated
one hemisphere and recorded from the homologous portion
of the other hemisphere. With the corpus callosum intact, I
transcallosal volleying was observed; if the callosum
was transected, the volleying disappeared. After commis
surotomy, however, another type of response was observed— ;
it was delayed in comparison to the transcallosal volley
ing and was therefore termed an interhemispheric delayed
response (IDR). The IDR resembles a response through a
diffuse system, possibly through the brain stem reticular
formation.
!
Trevarthen (1962) tested for simultaneous learning |
of opposite habits in split-brain monkeys. The animals ;
i were trained to make tactile responses to stimuli viewed
through polarized filters. His experimental group did \
ilearn both habits simultaneously; however, his one control
ianimal showed no learning. When the split-brain monkeys
iwere later tested for retention of the discriminations,
!each of these mutually conflictive habits was found to
1 be retained via opposite eyes. With both eyes open, one
or the other habit always dominated.
The Mature of Interhemlspheric Transfer
It Is apparent that the bilateral symmetry of the
central nervous system does not dictate bilaterality of
I
i
function. The brain functions as a single and Integral j
unit in the intact organism; the two halves are in har- j
monious communication. Information to one hemisphere j
I
does transfer to the other; but how does this transfer
affect or influence the opposite hemisphere? Several '
interocular transfer studies (Levine, 1945 , with pigeons; j
Sheridan, 1965, with albino rats) indicated that the
learning of a discrimination task in one eye can facil
itate the learning of the reversal (conflicting) problem ;
in the opposite eye. Mello (1965)^ investigating stimulus |
generalization, trained pigeons to discriminate stripes l
tilted 45° left vs. 45° right. The problem, was learned j
monocularly. When the birds were tested for generallza- j
i
I
tion via input to the "untrained" eye, they responded :
j
more frequently to the incorrect stimulus. |
Levinson and Sheridan (1967b) hypothesized that if
Information transfers in a polarized fashion, it should
facilitate reversal learning via the second eye, and
second-eye reversal learning should in turn enhance re
tention of the original habit via the first eye. If,
however, an irrelevant habit is learned via the second
i eye, retention of the original habit, as measured via
j
1 7
i
j the first eye, should not be enhanced, but reduced. In
an experiment designed to test this hypothesis, albino
rats received first-eye training on either pattern (45°
: tilted stripes) or brightness discriminations, followed
: by second-eye training on either the reversal or the Irrel-j
| evant discrimination. The rats were then tested for reten-j
tion via the eye of original training. All groups showed
good retention, regardless of the Interposed second-eye
problem, and the reversal animals showed some facilita
tion. In addition, the brightness discrimination was
learned reliably faster following pattern training than
originally; the same was true for the conditions of pattern;
following brightness. Thus savings on reversal training
might not be due to polarizing of transferred information, :
but merely from transfer effects of prior general learning.
Much of the interaction between the cerebral hemis
pheres could well be of an Inhibitory nature. Jung (1962)
: states that without cortical inhibition ". . .we would j
j
all be epileptics. . without interhemispheric inhib
ition one could not develop skill and coordination of
! voluntary movements. He found that unilateral stimulation
! of the motor cortex fires a transcallosal volley which
| leads to Inhibition of single neurons in the homologous
j portion of the other hemisphere. Some recent research
Indicates that learning can be enhanced by freeing one
hemisphere (the one mediating the learning) from the in
fluences of its counterpart; this hemispheric liberation
has been accomplished by various methods. Creel and
]
Sheridan (1966) placed unilateral lesions in the striate j
cortex of albino rats and found that animals with input j
to a single intact hemisphere acquired a pattern discrim- I
1
ination more readily than did bilaterally intact controls, j
To investigate further the enhancement phenomenon, Boles i
and Sheridan in 1967 (reported in 1969), unilaterally i
ablated the posterior cortex in albino rats and observed j
i
that the animals were able to learn a monocularly mediated j
i
pattern discrimination faster than unoperated or uni- !
I
laterally enucleated controls (p. < .02). Subsequently,
in a similar study, Levinson, Hottman, and Sheridan (in
preparation) observed that albino rats with unilateral ;
striate lesions acquired a monocular pattern discrimin- j
I
at ion more rapidly than cortically intact controls,* how- j
: ever, there were no differences between sham-operated
animals and those with striate lesions learning a light-
dark task. These data are not in conflict, as light-dark
(luminous-flux) discriminations can be mediated sub-
cortically, and their acquisition should not necessarily
1
I be influenced by small unilateral lesions at the cortical
i level.
In a study of interocular transfer in commissur-
! otomized hooded rats, Levinson and Sheridan (1969)
observed a measurable degree of enhancement in acquisi-
' tion of both tilt and shape discriminations for the 1
i
callosum-sectioned animals when compared to their sham- j
operated counterparts. This enhancement was statistically j
1
of doubtful reliability, but nonetheless suggested a
diminution in interhemispheric inhibition due to section- I
ing of the corpus callosum.
Factors Influencing Interocular Transfer
Interocular transfer Is one good method of measur-
i
Ing the function of the corpus callosum; however, such i
transfer is affected by certain environmental aspects, j
as is usually the case when behavior Is used as an Indi- j
I
cant of neurophysiologlcal events. Whether Information j
I
; gets across the midline, and how much, depends on certain !
j
factors. Meikle and Sechzer (i960) found that the type j
i
; of discrimination used is one of these factors. Working
; with split-brain cats, they obtained interocular transfer
: for a brightness, but not for a pattern, discrimination.
These results could be due to luminous-flux cues, whose
1
j discrimination can be mediated subqortically. Ingle
; (1965) trained goldfish (a naturally acallosal animal)
| to discriminate redundant pattern and color cues; I.e.,
red-horizontal correct, green-vertical Incorrect. He
l
1
! . - ~
j 10
then removed the redundancy of cues. On the originally
: trained eye, Ss used pattern cues; on the other (untrained)
: eye they used color cues. These data indicate that even
in acallosal animals some Interocular transfer does occur;
in this case, of color. \
i
i
That the type of response can determine whether |
I
interocular transfer occurs was first identified in gold- j
fish. McCleary (i960) used two different types of con- j
ditioning procedure. In a classical conditioning paradigm,!
i
light was paired with shock and the response was a change |
in heart rate. Here, there was good transfer of training j
I
between two eyes. When the problem was changed to one of |
conditioned avoidance, however, there was no transfer. |
Pish monocularly trained to avoid shock showed no evidence !
|
of learning when tested with the other eye. Meikle, |
Sechzer, and Stellar (1962), using split-brain cats, j
also obtained transfer for autonomic (respiratory), but j
!
not skeletal (limb flexion) responses. I
j
Sechzer (1964) studied motivational effects on
transfer. Split-brain cats were trained to make a hor
izontal-vertical discrimination either to avoid shock
or obtain food. The animals learning the problem under
: shock motivation showed 80 percent interocular transfer,
I whereas animals rewarded by food showed less than 1 per-
| cent. Sechzer noted that pattern discriminations are
! likely mediated at the mid-brain level when the animal j
' is under aversive motivation— since pain fibers are
: bilaterally distributed to the superior colliculi,
transfer could occur subcortically.
Interocular transfer studies such as those already
cited indicate that there are species-dependent differences
in the function of the corpus callosum. Bianki (1959)*
working with rabbits and a flashing-light discrimination,
found that callosum-sectioned animals showed incomplete
transfer of the brightness task, while intact animals
transferred it completely. Sheridan (1965) measured
interocular transfer in intact pigmented (hooded), and
in intact or callosum-sectioned albino rats. His albino
controls showed 90 percent transfer of the light-dark
habit and 60 percent of the striped (the 45° tilt) habitj
the hooded animals, however, showed 100 percent transfer
of both discriminations. The callosum-sectioned albino
animals showed 40 percent transfer of light-dark, and 32
percent of pattern, habits. The albino-hooded differ-
i ences In transfer were unexpected, and Sheridan stated,
! "The structural basis for such a difference is far from
j
I clear, but it may be in variations In the visual input
I systems of the two strains. Perhaps the paucity of un-
! crossed fibers that characterized rodents In general is
|
!even further reduced In the albino" (1965* p.-2 9 4).
I This hypothesis was verified anatomically by Lund (1965)
in a histological study of the crossed and uncrossed
fiber systems of hooded and albino rats. His results
showed that in the hooded rat 95 percent of the fibers
cross, whereas in the albino more than 99 percent cross.
Sheridan and his colleagues (Sheridan arid Shrout, 1966;
1
Creel and Sheridan, 1966) found in behavioral studies !
|
that albino rats restricted to using uncrossed fibers
displayed only limited learning ability when compared ;
i
to hooded rats under similar conditions, or when compared I
j
to albino rats using crossed fibers. Blank! and Morozova I
(196*0 were unable to train callosum-sectioned albino
rats to distinguish geometric shapes; however, Levinson
and Sheridan (1969) found that callosum-sectioned hooded j
rats could not only discriminate shape, but also showed
I
90 percent interocular transfer of this as well as 84 j
percent of a pattern (tilt) habit. Sham-operated hooded
rats showed 100 percent transfer of both habits. j
The interocular decrement (percentage loss of trans- j
I
fer) following callosum-sectioning is fairly similar for j
,
hooded and albino rats, suggesting that the corpus cal-
i
:losum is responsible for 10-30 percent of the transfer
■ in these animals. The dissimilarities in original transfer
measures would appear due to the differences in the visual
input systems and not in callosal functioning as such.
13
Levinson and Sheridan (1967a) found that guinea
pigs learned difficult discriminations (e.g., upright vs.
inverted triangles) and that these habits reliably trans
fer interocularly. In a developmental study, Petre and j
1
|
Sheridan (1966) measured transfer of a pattern discrim- j
i
ination in albino and pigmented guinea pigs that ranged !
from 1 to 37 days of age at the onset of training. No |
j
relation of albinism to transfer was observed; however, I
1
the older animals showed reliably more transfer than did S
the piglets. This difference could be due to the develop- j
j
ment of either the interhemispheric communication system, j
i
or the ipsllateral visual fiber system, or both. I
Polyak (1957) stated that the guinea pig has a
sparser uncrossed fiber system than the rat (March! j
stain), whereas Hess (1958) holds that fully 25 percent
of the visual fibers of the guinea pig are uncrossed
(Toluidine blue and other stains). If Polyak is correct,
i
then the high level of transfer in guinea pigs could be
: due to advanced callosal development. If Hess is correct,
the high level transfer would be due to a superior ipsi-
lateral visual system. Polyak (1 957) also feels that the
guinea pig is lower phylogenetically than the rat and
j therefore might be processing more information sub-
: cortically.
j A recent study by Creel (1969) indirectly supports
: Polyak's views. Creel measured visual evoked responses
: (VERs) in various mammalian species ranging from rats
' through man. He assessed VERs as a function of crossed
and uncrossed visual fiber input. In albino and hooded
rats, the VERs reflected the differences in uncrossed
; fiber systems suggested by the earlier studies cited
above. In guinea pigs, there was no such difference as
a function of pigmentation; both albino and visually
pigmented strains showed negligible ipsilateral VERs in
contrast to strong contralateral VERs. Creel's data
suggest that the guinea pig has few functioning uncrossed
fibers, certainly less than the 25 percent of the total
optic fibers estimated by Hess (1958). Creel believes
that if Hess is correct, the Ipsilateral VER may be
processed subcortically (in' keeping with Polyak's low
estimation of the guinea pig's phylogenetic status) and
thus cortical VERs would not accurately reflect ipsi-
i lateral input. However, the meager cortically evoked
potentials obtained ipsilaterally in Creel's study
i suggest that Polyak's characterization of the guinea
| pig's uncrossed fiber system Is more accurate than Hess's.
In summary, then, many salient feat vires of research
j
| bearing on the function of the corpus callosum have been
1
j discussed: species; effects of callosum-sectioning on
15
interhemispheric communication; difficulty and modality
of discrimination; and form of motivation. It is to some
of these variables that this study is addressed; speci
fically, to the problem of measuring interocular transfer
in callosum-sectioned and sham-operated guinea pigs on
tilt and shape discriminations under conditions of shock
avoidance motivation.
There are several reasons for the choice of the
guinea pig as the subject of this research. First, the
function of the corpus callosum appears to be species-
dependent and to increase in importance as one ascends
the phylogenetic scale— perhaps paralleling the process
of encephalization. As a transitional species, seemingly
quite low on the phylogenetic scale, the guinea pig can
yield data that will bear on subsequent comparative
studies. Second, the guinea pig appears to have a highly
decussated visual input system and therefore, with exper
imental sectioning of the corpus callosum, should provide
an elegant split-brain preparation. With this makeup,
the guinea pig has similar characteristics to, but may
be a more viable subject than, the albino rat. Lockard
(1968) has objected to the ubiquitous use of the albino
rat in psychological research. He feels that this animal
has been inbred to the point of artificiality. The gener
ality of data obtained from it would therefore be limited,
and other species should be utilized, especially when
there are more suitable animals available for the problem
being studied. This study is designed to assess the
suitability of the guinea pig for research on Interhemls-
pherlc transfer. Third, the cortex of the guinea pig is
more highly developed than that of the rat, and therefore
one would expect that the pig might have better learning
abilities. Several studies (Petre and Sheridan, 1966;
Levinson and Sheridan, 1967a) confirm this possibility.
It is believed that the measurement of acquisition rates
of difficult discriminations will shed light on the ques
tion of relative ability to learn.
The rationale for using tilt and shape discrimin
ations is twofold: to be able to compare acquisition of
these habits by guinea pigs with existing rat data, and
to test further the generality of the research of Bianki
and Morozova (1964), which suggested that commissuro-
tomized organisms are incapable of learning spatial dis
criminations, Since most of the interocular transfer
work with rats and cats has utilized shock avoidance
motivation, the present research also utilizes It; this,
to make possible more exacting comparisons of inter
species data.
The findings of this Investigation should provide
insight Into the following problems:
17
1. How does the guinea pig compare with the rat in
terms of learning capabilities?
2. Does the guinea pig show good transfer of diffi
cult tasks as well as easy ones?
3. How large is the interocular decrement in the
guinea pig; indirectly, how much transfer is accounted
for by the corpus callosum?
4. How does callosal functioning in guinea pigs
compare with callosal functioning in other species?
5. Does callosum-sectioning facilitate acquisition
in guinea pigs?
6. How general are the findings of Bianki and
Morozova (1964); i.e., can commissurotomized guinea
pigs learn to discriminate shape?
f
CHAPTER II
METHOD
Subjects. The subjects were 32 male pigmented
guinea pigs (Cavia porcellus) of the American Brown
strain, approximately 6 months old at the onset of the
study. They were naive to both the apparatus and pro
cedure. The animals were obtained from Small Stock In
dustries, Pea Ridge, Arkansas, and Redwood Game Farms,
Salt Lake City, Utah, and were housed in the Vivaria on
the main campus of the University of Southern California.
Apparatus. The discrimination apparatus utilized
in the study is similar to that described by Thompson
and Bryant (1955). Constructed of 3/^" thick plywood,
it consists of a start box, runway, choice point with
two alternative entryways, and a goal box. The interior
is painted gray. The floor of the startbox, runway, and
alternative entryways is composed of aluminum grids spaced
3/4" apart; the grid floor is flush with the wooden floor
of the goal area and provides shock motivation. The dis
tance from the floor to the top of the apparatus is 12
inches. The start box area is 12" x 12"; the runway area
(including the entryway grids, which comprise the last 6"
18
of the runway) is 12" x 24"; the goal area is 12" x 12".
In all, there are four sets of independently electrifiable '
shock grids: one set for the startbox, one set for the
runway, and one set in front of each goalbox entryway
extending 6 inches into the runway. The last two sets
are separated by a plywood partition which serves as a
choice point. The cue doors, 4-|" square, are constructed
of 1/4" thick black Plexiglas which has been sanded to
remove its gloss. For the pattern task 1/2" white stripes
tilted 45° left or 45° right have been painted on two of
these black doors; for the shape task a white triangle
and a square of equal area (4 square in.) have also been
painted on two black doors. The apparatus and discrim-
inanda are illustrated schematically in Figures 1 and 2.
To facilitate cleaning of the grids, the apparatus Is
constructed In two parts. The upper portion consists
of the walls of the startbox, runway, and goal box, as
well as the goal box floor; it also includes the guillo
tine door (which separates the startbox and runway), the
choice-point partition, and the clear Plexiglas lids
covering the three areas. The lower portion (base)
contains the grids; the top portion fits securely into
the base and rests on the grids. Motivational shock
in the startbox, runway, and choice areas is approximately
250 milliwatts of constant current provided by a
20
Figure 1. Top view of apparatus; entryway cue doors.
START BOX
RUNWAY
GOAL BOX
GUILLOTINE DOOR
12 "
ELECTRIFIABLE
GRID SETS
CHOICE ENTRYWAYS
CHOICE ENTRY D I S C R I tt I N A N D A '
ro
H
20134700377513
Figure 2. Side view of apparatus
GUILLOTINE
V * T
T fci- ’V -' - - ^ ^ . '4
r o
u>
24
Lehigh-Valley Electronics sine-wave shocker, Model 1311j
powered by a Sola 24 volts dc power source (catalogue
no. 281024-1). Shock is delivered to the grids via
knife switches operated by the experimenter.
Reversible monocular occlusion was achieved via
black acrylic contact occluders similar to those of
Schuck and Copolla (1963) except that they were larger,
fitting all the way to the base of the eye. These are
similar to contact lenses, and were worn by the animals
during the training sessions. They are inserted into
the eye in the same manner as contact lenses; Barnes
and Hind wetting solution is normally used for this
procedure. These occluders have been used in several
studies (e.g., Sheridan, 1965; Levinson and Sheridan,
1967a) without any known experimental confounding or
deleterious effects to the animals. Muntz and Sutherland
(1964) described a tendency of a tendency of monocularly
occluded rats to approach the side of the exposed eye
and hypothesized that this might account for the inter
ocular transfer decrement in normal albino rats; however,
they used bandage-type blinders. Sheridan and Shrout
(1965), using contact occluders, found no such tendency,
although they did obtain the interocular decrement for
albino rats on both brightness and pattern discrimin
ations.
25
Procedure. The study was run in two replications
of the paradigm illustrated in Table 1. This paradigm
calls for a sample of 32 animals, allowing for an n of 8
in each of the four main groups comprised from the com
bination of the first two factors (surgical condition X
type of discrimination problem). The other two factors
(correct cue door, first eye occlusion) were counter
balanced as they might be a source of extrinsic vari
ability. Preoperatively, subjects were randomly assigned
to each of the 16 subgroups resulting from the combination
of the four factors. Due to mortality, however, for the
four main conditions in Table 1 the resultant ns were
7, 7, 6, and 7, respectively. The procedure entailed
several phases:
Habituation. After their arrival in the laboratory,
the guinea pigs were given 7 days to adjust (habituate)
to their new environment.
Surgery. The callosum-sectioning technique used
is similar to that described by Sheridan (1965). Each
animal was anesthetized with Chloral Hydrate (50 mg/100
g body weight) and placed in a stereotaxic instrument.
A bone flap extending from 10 mm posterior to the bregmal
suture to a point 5 mm anterior to bregma, and 1-2 mm
bilateral to the midsagittal suture, was excised from
TABLE 1
Outline of Experimental Design
Surgical
Condition
Type of
Discrimination
Correct
Cue Door
Laterality of
1st Eye
Occlusion
Corpus Callosum Section (Exp) Sham Operation (Control)
Tilt (n = 7) Shape (n = 7) Tilt (n = 6) Shape (n = 7)
III
\W A □ ///
\W
< i
□
' ~ 1
R L R L R L R L R L R L R L H L
27
the skullj the dura was slit 1 mm on either side of the j
superior sagittal sinus, and then both the dura and sinus
i
were reflected aside by a needle held in the stereotaxic
instrument. The needle was interhemispherically lowered
to a depth of 5 nim below the cortical surface and passed
from behind the corpus callosum to the anterior limit of
the skull opening. The coordinates for the guinea pig
corpus callosum were determined by means of several pilot
operations and reference to Tindal's atlas of the guinea
pig forebrain (Tindall, 1965). The sham operation was
identical to the commissurotomy except that the needle
was not lowered into the brain and the callosum was not j
sectioned. Wound clips (Autoclips) were used to close
the incisions. Postoperatively the animals were given
antibiotics, both topically (Neopolycin) and systemlcally
(Longicil). Since it was impossible to perform more than
a few operations per day, seven days were allotted for
the surgical phase of the study.
Only one animal succumbed to the surgery (i.e., \
i
expired on the operating table), and it was immediately j
[
replaced. Although the guinea pig has a relatively thick j
dura, there are adhesions of the superior sagittal sinus
to the cranium at bregma. Due to these adhesions, the
i
sinus was either cut or nicked in several animals and
rather profuse bleeding resulted. However, with the use
of Xylocaine and gauze sponges, this bleeding was stopped
28
fairly quickly in all cases with no observable ill effects
to the animals.
Recovery. The post-operative recovery period began
on the day following completion of surgeries and lasted
for 14 days. During this period a general antibiotic
(Terramycin) was added to the drinking water.
Due to several outbreaks of Salmonella in the guinea
pig colony, data were obtained on 27 animals only. Of the
first and second groups of 16 Ss, 14 and 13 respectively
completed all phases of the study. Although quite a few
animals were lost to this illness, efforts in combatting
it did not prove entirely futile, as several infected
animals did respond to treatment and eventually recover.
Subsequently, they were able to participate in the study.
Inspection of their data revealed that earlier illness
did not measurably affect pretraining, acquisition, or
transfer measures. The treatment of the Illness involved,
in addition to the aforementioned antibiotics, giving the
guinea pigs fresh lettuce daily and dusting Suloptic
sulfa powder into infected eyes. Once a cure was effected
it was complete; no outbreaks of Salmonella occurred after
the 11th day of post-operative recovery in either the
first or second group. Mortality was randomly distributed
over the four treatment conditions; as a result, the ns In
29
each of the treatment groups remained fairly even (7, 7j
6, and 7). Recovery was relatively rapid (as compared
to the rat); the wound clips used to close the incisions
were removed two weeks post-operatively, and by 3-4 weeks
after surgery the fur over the original wound site had
regrown and the animals appeared normal.
Discrimination training. The procedure used in
this research is similar to that described elsewhere
(Sheridan, 1965; Levinson and Sheridan, 1967b). Generally
it consists of several steps.
1. Pretraining avoidance. Unoccluded Ss were
first pretrained to avoid shock by knocking over gray
doors which blocked the goal-box entryways. On Day 1,
each animal was allowed a 1/2-hour habituation period in
the discrimination apparatus. Prom Day 2 on, the animals
received actual pretraining trials. S_ was placed in the
startbox, and the guillotine door was raised. If £ 3 did
not leave the start area within 10 sec., that area was
electrified until £ 3 moved into the runway and then the
guillotine door was lowered behind him. Twenty seconds
were allowed for avoidance of runway and entryway shock.
S i then had to move to one or the other entryway and into
the goal box. Three consecutive avoidances from startbox
to goal box (with no doors in place) constituted the first
phase of pretraining; during the second and third phases
was not required to avoid shock in the startbox. The
second phase entailed knocking down gray doors which were
placed 1/2" behind the goal box entryways (the doors are
actually leaned up against the wing nuts which are later
used to lock them— this is termed the "half-door” phase).
Three consecutive avoidances in the runway and entryways
with half-doors in place must be made to complete this
phase. The final phase of pretraining called for five
consecutive runway and entryway avoidances with the gray
doors in their normal position, fully blocking the goal
area from the animal's view (full doors). During pre
training both doors were unlocked and neither set of
entryway grids was electrified. S was given 25 trials
per day until the criterion of 5 consecutive avoidances
with full doors in place had been reached. The day after
all Ss had reached criterion, they were again run to the
criterion of 5 full-door avoidances.
2. Acquisition mediated via the original eye ("first
eye” learning).Discrimination learning commenced on the
day following completion of pretraining. The discrimin
ative doors were in their normal (fully upright) position;
the incorrect door was locked and the set of grids in
front of it electrified. Any contact with these grids
;was scored as an error; detection of such contact was
'31 1
facilitated by neon lamps which were connected to both
sets of entryway grids. When the current to the grids
was oiij the lamps glowed brightly, whereas contact with
the grids caused the lamps to dim appreciably. The left-
right position of the correct door was varied randomly
across trials (Fellows, 1967); this random left-right
series comprised the data sheets for the study (see
Appendix A). The monocularly occluded animals were
normally run 25 trials per day until a criterion of 18
correct responses in 20 consecutive trials was met. Six
hundred trials were used as the cutoff point for first-
eye acquisition.
3. Homolateral reacquisition. Having met criterion,
S underwent removal and replacement of the occluder in
the same eye (homolaterally) and was overtrained until he
met the same criterion a second time (reacquisition).
This procedure was incorporated for two reasons. First,
removal and replacement of the occluder in the homolateral
eye serves as a control measure for the upcoming occluder
shift to the contralateral eye; i.e., the degree of dis
ruption of performance on acquisition via the second eye
which might result from the contralateral occluder shift
can be estimated by observing the disruption of first-
eye performance (if any) following the homolateral
occluder shift. Second, the number of trials required
32
to meet criterion a second time yields an asymptotic j
1
measure of learning which indicates the degree to which
acquisition of the discrimination has stabilized. By !
comparing second-eye acquisition with homolateral reac-
quisition, one can estimate the degree of disruption of
second-eye learning which might be due to instability of
orlginal-eye acquisition.
4. Acquisition mediated via the opposite eye
("second-eye1 1 learning). The occluder was then shifted
to the opposite eye (contralaterally) and the animal was
again trained to criterion (18/20 correct). Savings on
second-eye acquisition reflected the degree of interocular|
transfer.
Histology. At the conclusion of the study, all
callosum-sectioned Ss and two sham-operates were perfused ;
with saline and then 10 percent formalin, and the brains
were examined for completeness of lesion and incidental
damage. It was expected that callosum sections would be I
accompanied by some cingulate damage; however, it was also!
S
expected that the sham-operates would probably exhibit
slight cingulate damage due to cortical exposure resulting j
from cutting of the dura (as reported by Sheridan, 1965,
and Levinson and Sheridan, 1969). Photographs were taken
of representative brain sections of callosum-sectioned
and sham-operated animals.
33
As mentioned earlier, the research was carried out
In two stages, as dally available time prohibits running
more than 20 animals 25 trials/day in a Thompson-Bryant
apparatus. The animals were run in squads of three or
four, and each squad contained commissurotomized and
sham-operated Ss. This procedure allowed for counter
balancing of such variables as time of day, running order
of Ss in the experiment, and experimenter fatigue.
CHAPTER III
RESULTS
Fretralnlng avoidance. Pretraining data were ob
tained on 28 animals and are presented in Table 2 and
histographically in Figure 3. The mean number of trials
to a criterion of five full-door avoidances for callosally-
sectioned Ss was 87.00, while for the sham-operated Ss it
was 48.79. A t test revealed that these means differed
reliably /1[(26) = 2.321, p <.037. As reflected in
Figure 3, the majority of the sham-operated Ss had reached ,
criterion within the first 50 trials, while the majority
of the callosally-sectioned Ss required more than 50
trials to reach criterion.
Discrimination training. One of the sham-operated
animals developed an eye infection after 125 discrimin
ation trials and therefore had to be dropped from the
study. Thus, acquisition and transfer measures were
obtained on 27 animals, each of them meeting criterion
within 600 trials. The data for these animals on the
above measures, as well as the homolateral control
measure, are presented in Table 3.
34
35
Table 2
Trials to Avoidance Criterion on Pretraining
Callosum-sectioned S s Sham-operated Ss
S # trials S # trials
1 46 2
89
3 135
4 50
5 39
6
37
7
72 8 52
9 57
10 40
13 105
12 44
15 47
14
45
17 53
16
37
19
94 18 40
21 198 20 40
23
60 22 84
25
44 24
17
29 47
26
47
31
221 30
71
X=87.00 X=48.79
Figure 3. Histographlc representation of frequency
of Ss reaching pretraining avoidance criterion within
50 trials, between 50 and 100 trials, and over 100 trials.
Callosally-sectioned Ss are represented by the dark bars,
shara-operated Ss by the light bars.
TRIALS TO PRETRAINING AVOIDANCE CRITERION
NUMBER OP SUBJECTS MEETING CRITERION
38
Table 3
Trials to Criterion for Original Acquisition,
Homolateral Reacquisition, and
Interocular Transfer
Discrimination Surgical
treatment
S Eye 1 Homolateral Eye 2 # dec.
control
Callosal
sectioning 1 70 0
29
41.4
3
127 0 48
37.7
9 145
0 48 32.6
21 217
2 62
28.5
23
148 0
71 47.9
29
20 0 10 50.0
31 107
0 34
31.7
"Tilt" X = 119.14 0.28 43.14 38.54
Sham 4
55
0 0 0.0
sectioning 10 130 0 0 0.0
12 88 0
3
3.4
22
95
0
5
5.2
24
313
2 12 3.8
32
248 0
35
14.1
X=
154.83 0.33
9.14 4.14
Callosal
sectioning
5
220 2 61
27.7
7
318 0 126 39.6
13
144 0
17
11.8
15 379
6 90 23.7
17
256 0
63
24.6
19 257
2 108 42.0
25 175
0 56 32.0
"Shape" x= 249.86 1.43 74.43 28.77
Sham 6 301 2
9 2.9
sectioning 8 150 2
13
8 .6
14
321
0 0 0.0
16 485 0 0 0.0
18 74 0 6 8 .1
20 400 10 4 1 .0
26 426
3 1 0.2
X= 308.14 4.74
4.71 2.97
39
Although the mean rate of flrst-eye acquisition
was superior for callosally-sectioned Ss on both discrim
inations, the difference in rates was not statistically
reliable. Performance on the stability measure (homo
lateral control) was similar for operates and controls.
An analysis of variance for a three-factor experiment
with repeated measures on one factor (Winer, 1962, p. 337)
was used to analyze the full set of data; the first and
second factors were surgical condition and type of dis
crimination, and the third (repeated measures) factor was
training via the first, and then the second, eye.
As mentioned earlier, the ns of the four main treat
ment conditions were 7j 7j 6, and 7. Since this statis
tical procedure cannot be used with unequal sample sizes,
the ns had to be equalized. There were two methods of
accomplishing this: (1) randomly eliminate one S from
each of the three groups containing 7 Ss, or (2) add a
phantom mean (a score equal to the mean of the n defici
ent condition). It was felt that the loss of information
from discarding data on 3 Ss would be too great to
warrant the first procedure; thus, the latter method
was chosen and the degrees of freedom adjusted accord
ingly. A summary table for the analysis of variance
is presented In Table 4. The following sources of
variance were found to be reliable at or below the .01
Table 4
Factorial Analysis of Variance:
Summary Table
Source df MS
f
£
Between Ss
Surgical Condition (A) 1 82.56
--
Discrimination (B) 1
84553.13 9.70 .005
AxB 1 151.16
—
Error (between) 23*
8712.29
Within Ss
First then second _c
eye training (C) 1 429450.00 165.10 10 5
AxC 1 34204.58
13.15 .005
BxC 1
57857.15‘
22.24 .001
AxBxC 1 2972.56 1.14
Error (within) 23* 2601.10
* adjusted df
41
level: type of discrimination, first then second eye
training (repeated factor), the interaction of first
and second eye training with the surgical condition,
and the interaction of first and second eye training
with the type of discrimination task. A graphic illus
tration of the main effects and interactions is shown
in Figure 4.
Several 1 ; tests using pooled scores revealed that
callosum-sectioned Ss did not differ reliably from sham-
operated Ss on first-eye acquisition (all ts C 1.00; all
g . s . ^.35)* however, on second-eye training, the callosum-
sectioned Ss required significantly more trials to meet
criterion / t ( 2 5 ) = 5.499* P ^.00l7- For type of dis
crimination, the converse held: on first-eye training
the shape discrimination proved to be significantly more
difficult than the tilt /t(25) = 3.61, p { . 0 0 2 /, whereas
on second-eye training the type of discrimination did not
reliably affect mean trials to criterion, even though
the shape task was somewhat more difficult than the tilt.
Callosum-sectioned Ss showed similar losses in transfer
on both tasks, while sham-operated Ss showed a high
degree of transfer of both tasks. These t tests were
performed in order to illustrate further the reliable
effects found in the analysis of variance.
Figure 4. Mean number of trials to criterion on
pretraining avoidance, first-eye acquisition, and second-
eye acquisition. CC = callosally-sectioned animals;
Sh = sham-operated animals; T = tilt discrimination;
S = shape discrimination.
TRIALS TO CRITERION
^3
0 ~ m = cc - T
# 4
= cc - S
o-o
= Sh - T
4-4
= Sh - S
300 .
100 ^
PRETR.
AVOID
EYE 1
ACQUIS.
EYE 2
ACQUIS,
The Interocular decrement for each £ 3 was computed
by dividing the number of trials to criterion on the
second eye by the number of trials to criterion on the
first eye; this decrement is usually expressed as a
percentage and reflects extent of loss in transfer from
one hemisphere to the other. It can also be used as a
measure in savings on second-eye learning; i.e., 100
percent minus the percent decrement is equal to the
perc ent saving s.
The interocular decrements for each S are also
presented in Table 3. Analysis of these data indicated
that the mean decrement for callosum-sectioned Ss was
|
much larger than that for sham-operated Ss, regardless
of the task /t(25) = 9.715, £ <. 0027.
To test for occluder leakage, upon completion of
second-eye training two Ss (one bn tilt, the other on
shape) were given 10 free trials with both eyes occluded. :
The doors were unlocked and no shock was delivered to the
entryway grids. With binocular occlusion, the animals'
|
performance fell to chance levels.
Histology. The brains of all callosum-sectioned
and two sham-operated animals were removed, and sections
i
were taken at approximately levels A 15.0, A 11.0-12.0,
and A 7*5-9.0 (Tindal, 1965). These coordinates correspond
fairly well to the anterior (genu), central (body), and
; " I
posterior (splenium) portions of the corpus callosum.
Photographs were taken of these sections for each
animal and are presented in Figure 5. All callosum-
sectionings were complete at the posterior and central
levels except in the case of S 15; at the anterior level
there were two animals with incomplete lesions: 23
and S 25. Nevertheless, the performance of these animals
did not differ greatly from the mean performance of their
respective groups, either on acquisition or transfer
measures.
Callosum-sectioning was inevitably accompanied by
unilateral damage to the cingulate cortex, and in several
cases damage was bilateral and fairly massive (e.g.,
S 3 at levels 15.0, 7.5; S 31 at level 15.0). Sham
surgery produced incidental cortical damage; close
inspection of the brains of S 4 and S 8 revealed slight
unilateral damage which appears to have resulted from
cutting of the dura. A slight bilateral edema of the
cortex near the midline also appeared in these brains,
as well as in the brains of the callosum-sectioned
animals, and was probably due to cortical exposure con
comitant with surgical removal of the bone flap. It was
also noted during histology that at the site of excision
of the bone, the dura and periosteum had grown together
Figure 5. Photographs of brain sections taken at
three anterior (A) levels from all callosall-sectioned
Ss and two sham-operated Ss. L = left; R = right;
CC = corpus callosum-section; SHAM = sham-operation.
The "metric1 ' rule shown In the photographs is scaled
in centimeter units.
FIGURE 5
47
S 1: R CC S .3: R CC
A 15.0
A 11.0
A 7.5
v I
kI
48
FIGURE
S 4: R SHAM
(continued)
S 5: L CC
49
FIGURE 5 (continued)
S 7s L CC
S 8: L SHAM
A 15.0
A 11.0
A 7.5
METRIC 1
50
FIGURE 5 (continued)
S 9: R CC
S 13: L CC
A 15.0
A 11.0
A 7.5
51
FIGURE 5 (continued)
52
FIGURE
S 19: R CC
(continued)
S 21:
L CC
53
FIGURE 5 (continued)
S 23: L CC S 25: R CC
A 15.0
A 11.0
A 7.5
• v ? ^
54
FIGURE 5 (continued)
S 29: L CC S 31: L CC
and had formed a thick, protective covering over the
exposed cortical tissue. This undoubtedly kept the
observed edema to a minimum.
CHAPTER IV
DISCUSSION
Discussion is organized in terms of the data emer
ging from each of the three main phases of the study:
pretraining in avoidance learning! original learning of
a monocularly mediated discrimination {’ ’first eye” learn
ing); and subsequent monocular learning of the discrim
ination via the previously unused eye ("second eye"
learning). The data from each phase are compared with
relevant findings from the literature— or unpublished
data available to the writer; behavioral or neuroanatom-
ical inferences are then drawn from the concurrences and
discrepancies noted; and, finally, theoretical integra
tion is attempted when the data so warrant. Where insuf
ficient data or non-overlapping methodologies leave a
questionable conclusion, suggestions for additional
research are made. The discussion will conclude with
remarks about the merits of the guinea pig as an exper
imental animal in the study of brain-behavior relation
ships .
56
57
Pretraining Avoidance Learning
A strong species difference between the guinea pig
and the rat is suggested by those data obtained during
avoidance pretraining which reflect number of trials to
criterion. The sham-operated guinea pigs yielded a
mean of 48.79 trials to criterion, while the mean of 10
similarly operated and trained, visually pigmented rats
was 21.20 and of 10 intact albino rats 17.20 (Levinson
and Sheridan, in preparation). After finding that these
means differed reliably via analysis of variance (F
(2, 31) = 2.19, p { .001), pairs of groups were tested
by means of t_ tests. The mean of the sham-operated
guinea pigs is reliably higher than the mean of sham-
operated hooded rats (t_ (22) = 4.5* P {.001), but the
latter mean does not differ appreciably from the mean
of the albino rats (t^ (18) = 1.31* P {.20). The
obvious discrepancy between the guinea pig and the rat
may stem from differing reactions of the two species to
aversive stimulation. In the presence of aversive
stimuli, or cues heralding such stimuli, the guinea pig
is more apt than the rat to abandon flight responses
for freezing or immobilizing behavior. Since avoidance
training requires the subject to learn first to escape
and then to leave an area prior to onset of aversive
stimulation, an animal that freezes is going to be more
58 ;
hindered than one that reacts aggressively. Thus the
observed difference in avoidance learning may arise
from differences in the "emotionality" of the two species, j
j
The inferred emotionality, to be confirmed, would have
to be independently measured by more reliable indices,
e.g., frequency counts of fecal boli, activity in an
open field, or psychophysiological measures such as
change in heart rate and respiration. One could argue,
granting presence of emotive differences, that pre-
experimental handling would be important in behavioral
research involving guinea pigs. Rinck (1968) has shown
that pre-handling of the rat greatly affects its perform
ance in the Thompson-Bryant apparatus, and if the guinea
pig is emotionally more labile than the rat, as the
present data suggest, the effects of handling may not
only be important but should be investigated in their j
own right in the species. I
i
The inferior performance of the guinea pigs on !
pretraining avoidance might have been, at least in part, j
an artifact of the apparatus in which they were trained. j
i
In the two rat studies by Levinson and Sheridan mentioned
earlier, the Thompson-Bryant apparatus was equipped to
present an auditory cue (a pure tone of 440 Hz) for ten
seconds immediately preceding the onset of shock in both
the startbox and runway; in the present study, such cueing
59
was not employed. Whether the guinea pig would benefit
from such a warning signal is conjectural: the animal
might utilize it to avoid shock, but the signal might
also induce freezing behavior and thereby inhibit
avoidance learning.
Turning now to the corpus-callosally-sectioned
guinea pigs: the reliable deficit shown by these
animals on the pretraining avoidance task, as compared
to their sham counterparts, was not surprising, as this
phenomenon has been observed in both the albino rat
(Sheridan, 1963) and hooded rat (Levinson and Sheridan,
in preparation). In the case of the hooded rat, the
callosally-sectioned animals yielded a mean of 37.10
trials to criterion as opposed to a mean of 21.20 for
the sham-operated animals. This difference was reliable
(;b (18) = 2.18, <^.05). Stated as a ratio, the group
of callosally-sectloned hooded rats required 1.73 times
as many mean trials to reach the avoidance criterion as
the sham-operated counterparts. For the guinea pig, the
ratio of mean trials to criterion for callosally-
sectioned animals (X = 87.00) to that of sham-operated
animals (X = 48.79) is I.78. The similarity of these
ratios suggests that although the guinea pig experienced
more overall difficulty than the hooded rat in learning
the avoidance task, the behavior of both species was
60
disrupted to a near-equal degree by the callosum-section
ing treatment.
The performance deficit from the surgical treatment
could have been due to damage to the corpus callosum
itself, to damage of the clngulate cortex, to damage of
other structures, or to some combination of these factors.
Perhaps rodents in general are dependent on intact fore-
brain commissures or cingulate cortex (or both) for behav
ior involving cueing, orientation, or the initiation of
responses.
Within the last decade, numerous studies on the
function of the cingulate cortex and related structures
have been performed; the results in the main support the
hypothesis that the anterior cingulate cortex partici
pates in the acquisition of active avoidance tasks and
that damage to the area leads to a corresponding deficit.
Some representative studies in this area were done by
Peretz (i960), and Thomas and Slotnick (1962, 1963); in
each study, bilateral cingulate lesions were introduced
in rats and deficient avoidance learning was thereafter
observed. McCleary (1961) also reported the deficit to
occur in the cat. Thus one might expect that damage to
the anterior cingulate cortex leads to similar deficits
in the guinea pig. Thomas and Slotnick (1962, 1963)
produced their lesions both via surgical sectioning,
and electrolytically, and reported deficits in their
animals' learning of a conditioned avoidance response,
i
this possibly due to an enhancement of freezing behavior.
In the present study, as well as in the previously re
ported study with hooded rats, the cingulate damage was
unilateral and more restricted, and the animals also
had the benefit of one intact hemisphere as no occlusion
was used during pretraining. Nonetheless, the cingulate
"scraping" which accompanies sectioning of the corpus
callosum could be the underlying cause of avoidance !
deficits shown by these animals in the Thompson-Bryant
apparatus.
Recently, several studies have suggested that the
active avoidance deficits shown by animals with cingulate ;
|
lesions might be due to damage to structures other than j
the cingulate. Lubar and his colleagues (Lubar, Perachlo,j
|
and Kavanaugh, 1966j Lubar, Shostal, and Perachio, 1967) j
i
placed small bilateral lesions in the striate cortex of ;
]
the cat and obtained avoidance deficits similar to those }
f
obtained with cingulate lesions; however, no deficits !
in visual discrimination were observed. It was hypoth
esized that the avoidance deficit was caused by damage
to the optic radiations from the lateral geniculate
nucleus underlying the cingulate cortex, rather than
by damage to the cingulate cortex itself.
As stated earlier, perhaps sectioning of the corpus
callosum contributes to the observed deficits during
pretraining. It is believed, however, that the evidence
points to damage of the cingulate gyrus as the chief
disruptive factor and one that will have to be dealt
with more rigorously in neuropsychological studies of
corpus callosal functioning. Surgery should include a
control procedure which produces the levels of cingulate
damage found in callosally-sectioned animals. One pos
sible method Is as follows: after removal of the bone
flap and cutting of the dura, the callosum-sectioning
needle Is lowered at the midline to a depth 2-3 mm
below the level of the cortex (i.e., but above the corpus
callosum) and passed from the anterior to the posterior
limits of the opening in the skull. The addition of
this procedure to the sham-operation would add a strongly
needed control element to experiments involving callosal
sectioning.
Original (1 1 first-eye") learning j
|
Acquisition data in the form of average trials to j
criterion are presented for several species and types of j
discriminations in Appendix B, as are (where data are j
j
available) homolateral control means, second eye acquisi
tion means, and mean interocular decrements. These data
are drawn from several published studies and the present
63
study, and reflect similarities and differences between
the species. Using combined (and weighted) mean trials
to criterion for callosally-sectioned and sham-operated
subjects, it is seen that the guinea pig is able to
acquire a tilt discrimination about as rapidly as the
hooded rat (135.61 vs. 130.40). However, on the shape
discrimination the guinea pig reaches criterion more
rapidly (278.98 vs. 351.90). These findings suggest
that the guinea pig has achieved a level of visual CNS
development equal and possibly superior to that of the
pigmented rat. They also suggest that there are two
distinct mechanisms for encoding shape and tilt informa
tion, as Deutsch (i960) has hypothesized. Hubei and
Wiesel (1959), in their microelectrode studies on the
receptive fields of neurons in the visual system of
the cat, described "complex" cortical cells in Area 17
which respond maximally to the orientation or axis of
a slit or bar of light. In 1965, Hubei and Wiesel
described "hypercomplex" cells in Areas 18 and 19 which
give a maximal response to two boundaries (or edges) of
light at right angles to each other; the response varies
as a function of the angle. The frequency of firing of
these cells probably serves to encode information concern
ing angles, which is basic to shape perception. If it
is assumed that these phenomena also occur to some degree
64
in the rodent brain, one could hypothesize that the mech
anism for encoding information based upon shape is more
highly developed in the guinea pig than in the rat.
Another possible explanation of the "shape-and-tilt"
discrepancy is generated from the earlier discussion of
differences in the emotionality of guinea pigs and rats
in regard to relative difficulty in learning avoidance
tasks. When actual discrimination training begins, the
animal is faced with a variety of new and possibly
aversive conditions, such as, e.g., being monocularly
occluded, being wrapped in a towel (whereby occlusion
is accomplished), etc. In all likelihood, the most
aversive situation which the animals encounter is a
locked door in the entryway with an electrified grid
immediately preceding it. (During pretraining, both
doors are unlocked and neither doorway grid is electri
fied. ) Generally, animals establish a position prefer
ence during pretraining which they subsequently must
extinguish in order to acquire a discrimination involving
randomly positional changes of the "correct cue" across
trials. The position responding will lead an animal to
the incorrect door approximately 50 percent of the time,
and to Inevitable shock on these "Incorrect" trials.
During the first and second day of training, many animals
persist in attempting to go through the incorrect door,
65
and as a result they receive much more shock than in the
latter stages both of pretraining and of discrimination
training. The guinea pig may likely be hindered to a
greater degree than the rat by the additional shock and
subsequently may require more trials to extinguish fear
responses as the present data suggest, perhaps as many
as 50-100 more trials. Thus vdiile the rat is learning
the discrimination itself, the guinea pig may still be
overcoming fear responses. It is believed that once the
guinea pig begins attending to the stimulus cues, he
learns more rapidly than the rat; thus with easier dis
criminations the two species appear to be learning at
the same rate, whereas with difficult tasks, the superior
ability of the guinea pig becomes evident. This hypoth
esis can be tested by either training animals to learn
a simple discrimination (such as luminous flux) prior
to pattern or shape, or by Including a procedure in pre
training wherein the animals are given a number of trials
with the door on the preferred side locked and the grids
in front of it electrified. If the hypothesis is correct,
the addition of either of these procedures (if successful,
in extinguishing non-adaptive responses), will lead to
superior performance by the guinea pig on any discrimin
ation task.
66
That the corpus-callosally-sectioned guinea pigs
were able to learn to discriminate shape further limits
the generality of the findings of Bianki and Morozova
(1964), It will be recalled that these investigators
were unable to train callosally-sectioned albino rats
to distinguish geometric shapes; their findings implied
that integrity of the corpus callosum is essential for
the discrimination of shape in the rat. Related to
these findings is a report by Hubei and Wiesel (1967) in
which they described the receptive fields of various
cells in the cat corpus callosum. These cells are
similar to those found in Areas 17 and 18-19 (complex,
hypercomplex), however, their receptive fields overlap
the midline. Therefore it appears that the corpus
callosum links cells whose fields are close together
but on opposite sides of the vertical meridian (or
straddling it). If hypercomplex cells encode shape
and contour, and callosal cells transfer this information
across the midline, it is conceivable that sectioning
the corpus callosum could disrupt the acquisition of
shape discriminations, as does ablation of the striate
regions. However, the previously reported finding that
callosally-sectioned hooded rats have learned to dis
criminate shape, coupled with the results of the present
study, refutes the hypothesis that integrity of the
corpus- callosum is necessary for the discrimination of j
shapes. One might ask, then, why do the outcomes of ;
j
these two studies differ from the outcome of the Biankl
and Morozova (1964) experiment? There are many potential I
sources of variance, as discussed by Levinson and Sheridan!
(1969); these Include differences in species, apparatus,
and methodology (e.g., albino rats, binocular discrim
ination tasks, and an apparatus similar to an operant
chamber with alimentary reinforcement and rear projected
stimuli were used by Bianki and Morozova while animals
with pigmented eyes, monocular discrimination tasks,
and an apparatus involving shock motivation and patterned
doors were used by Levinson and Sheridan, as well as in
the present study). Any one or a combination of these
factors could have been responsible for the differences
i
in results. It is clear, however, that there are at j
least some situations wherein callosally-sectioned rats,
i
and guinea pigs, are able to acquire a shape dlscrimin- j
at ion. i
1
I
!
The sizable but statistically unreliable enhance
ment of original eye acquisition of the tilt and shape
tasks shown by callosally-sectioned hooded rats (Levin
son and Sheridan, 1969) was also shown by callosally-
sectioned guinea pigs. As this phenomenon was of doubt
ful reliability in the hooded rat, it was merely reported
and not discussed. That it also occurred in the guinea
pig, however, indicates that it might be a generalizable
effect of callosum-sectioning, and therefore discussion
is warranted.
This is an appropriate point to view the finding
that animals with lesions of the corpus callosum acquire
discriminations more rapidly than animals without such
lesions in light of the speculation that much of the
Interaction between the hemispheres is inhibitory in
nature, as postulated by Jung (1962). It is not un
reasonable to suppose that sectioning of the forebrain
commissures relieves each hemisphere from Inhibitory
effects of the other (i.e., removes interhemispherie
inhibition)j the net result would be increased excita
tion which in turn might result in enhanced rates of
acquisition. The effect here is probably similar to
that obtained in the studies on the effects of unilateral
striate ablations in the rat. In these experiments
(Creel and Sheridan, 19665 Boles and Sheridan, 1969),
animals learning via an eye whose contralateral fibers
projected to an undamaged hemisphere utilized fewer
trials to reach a criterion of 18/20 correct than did
animals with no damage to either hemisphere. The en
hanced acquisition rate resulting from callosum-section
ing is not nearly as great as that resulting from
unilateral striate ablation; however, it does appear to j
reflect the same underlying neurological event(s)—
disruption of inter-hemispheric inhibition.
Although the relief-of-inhibition hypothesis is
tenable, the observed enhancement of acquisition rates
shown by callosally-sectioned animals could be an artifact
of their inferior performance on the pretraining avoidance
measure. As stated earlier, the avoidance deficit has been
observed in callosally-sectioned albino and hooded rats,
and now in the guinea pig. Because these animals receive
reliably more trials during pretraining than their sham-
operated counterparts, they are probably less "naive" at
the outset of discrimination training, and therefore
require fewer trials to reach criterion via the original
eye. (These savings, and their relation to pretraining,
are reflected in Figure 4.} The possibility that the
observed enhancement is an artifact of differences on
the avoidance phase is a strong one, and it vividly
illustrates the necessity of achieving equivalence of
the experimental and control groups on the pretraining
measure.
Examination of the performance of the various species
on homolateral reacquisition reveals that the guinea pig
showed slightly less stability than the hooded or albino
rat (see Appendix B). This instability may stem from a
70
genetically disposed emotional lability of the guinea pig
which was hypothesized to account for the differences in
observed pretraining avoidance performances, guinea pig
and rat vis-a-vis. Evidence of the guinea pig's lability
on this measure was only slight, and therefore the deficit
probably had no effect on subsequent acquisition via the
second eye. It is also noted that the callosally-sectioned
guinea pigs showed somewhat more stability than the sham-
operated controls; however, it is doubtful that this dif
ference affected performance on the second eye.
Acquisition mediated via the opposite
eye (^ ’second-eye" acquisition).
The sham-operated guinea pigs exhibited a very high
degree of interocular transfer on both tilt and shape
discriminations. The small decrements on tilt (4.14 per
cent) and shape (2.97 percent) appear to be due to the
slight instability of learning which was mentioned with
regard to homolateral reacquisition performances. A close j
look at Table 3 reveals that for the shape discrimination j
the sham-operated animals required almost the same mean j
j
number of trials to reach criterion on second-eye aequlsi- |
tion as on homolateral reaquisition (4.71 vs. 4.74). The
sham-operated animals learning the tilt discrimination
required on the average more trials to reach criterion
on second-eye acquisition than on homolateral reacquisition
71
(9.1^ vs. O.33); however, this discrepancy seems to be
due mainly to the inferior performance of S 30 on second-
eye learning (35 trials to criterion). If these consider
ations are taken into account, It may be stated that the
normal guinea pig shows near-perfect Interocular transfer
of both easy and difficult discriminations, as does the
pigmented rat, while the intact albino rat (Sheridan,
1965) and rabbit (Bianki, 1959; van Hof, 1970) show incom
plete transfer of pattern and brightness tasks. The high
levels of transfer exhibited by the guinea pig could be
due to Ipsilateral Input via uncrossed visual fibers (as
appears to be the case In the hooded rat), or to a high
level of callosal function, or to a combination of the
two.
Some insight Into the neurological mechanisms under
lying the near-perfect transfer observed in normal guinea
pigs is gained by an examination of the performance of the
callosally-sectioned guinea pigs on second-eye learning.
The behavior of the animals Immediately following the
occluder shift was strikingly different from that of the
sham-operated animals--or even callosally-sectioned hooded
rats— under similar conditions; it was similar to that
usually observed during the early stages of discrimination
learning. Several animals (particularly S 7) exhibited
behavioral patterns almost identical to those exhibited
on the first day of discrimination training on the original;
i
eye (e.g., persistent position responding)— it was as if
these animals had never received any training at all. Many
callosally-sectioned guinea pigs changed their original
position preference following the occluder shift, which
resulted in a subsequent loss of orientation. Nk> sham-
operated animal exhibited such behavioral signs of con
fusion, and it was immediately apparent that the corpus
callosal sectioning treatment had disrupted interhemis-
pheric communication to a great degree. Analysis of
second-eye acquisition scores and computation of inter
ocular decrements revealed that this disruption resulted
in losses in transfer ranging from 25 percent to 50 per
cent, more than twice the decrement observed in albino
and hooded rats. It appears, therefore, that the corpus
callosum is probably better developed in the guinea pig
than in the rat. Yet the intermediate transfer values
obtained in the callosally-sectioned guinea pigs implicate
the ipsilateral fiber system to a greater degree than
expected.
There are several studies in the literature which,
although not totally in agreement, partially resolve this
problem. Three of these have already been cited (i.e.,
Polyak, 1957i Hess, 1958; Creel, 1969). To review briefly,
two of the studies on the histological composition of the
73 j
I
visual system of the guinea pig are in conflict. Polyak
estimated that more than 99 percent of the visual fibers
decussate at the optic chiasm, as is the case in the albino
rat. Thus, if he is correct, sectioning of the corpus
callosum should reduce the percentage of transfer in the
guinea pig to the low levels observed in the albino rat.
Hess, however, estimated that at least 25 percent of the
visual fibers in the guinea pig project to the ipsilateral
hemisphere. If this estimate is correct, one would expect
to obtain high levels transfer with or without sectioning
of the corpus callosum, similar to the fairly high levels
of transfer obtained in callosally-sectioned hooded rats
which are seemingly due largely to Ipsilateral input from
the 5-10 percent non-decussated fiber system. Some recent
research by Creel (1969, 1970) supports Polyak's model.
As mentioned earlier, Creel (1969) reported that both the
albino and visually pigmented strains of guinea pigs
showed negligible ipsilateral visual evoked responses
(VERs) in contrast to strong contralateral VERs. He
stated, however, that the ipsilateral VERs may be processed
at loci other than the locus (Area 17) from which he was
recording.
More recently, Creel (1970, in preparation),enu-
i
cleated guinea pigs unilaterally and recorded both ipsi- !
lateral and contralateral VERs from cortical areas ranging j
from lambda (A -2.0) to about bregma (A +12.0) and later- j
ally from L 1.0 to L 8.0. (These coordinates were taken I
from Luparello's stereotaxic atlas of the guinea pig fore- j
brain, 1967.) From the Ipsilateral hemisphere, primary
VERs were only obtained in an area of approximately 2 i
square millimeters, with its center at L 5.0, A +1.0.
On the contralateral side, however, large VERs were
i
recorded at all cortical sites, and even the smallest
of these was still larger than the VERs recorded on the
ipsilateral side. These evoked potential data reflect
the sparse ipsilateral fiber system as reported by Polyakj j
upon examining the meager VERs recorded from the small
locus on the ipsilateral side, it is difficult to envision ;
a percentage of non-decussated fibers as large as that
estimated by Hess. Polyak's model, therefore, would appear1
to be the more accurate of the two. If this be the case,
however, one would expect total loss of transfer upon
i
sectioning of the corpus callosum. How, then, does one j
explain the intermediate transfer values (50-75 percent) j
observed in callosally-sectioned guinea pigs? j
i
The transfer observed could be due to availability
of visual information in spite of occlusion. However, as
1
reported In the results section, binocularly occluded
animals performed at chance levels, even after reaching
criterion on acquisition via the second eye. This measure,
'coupled with previous tests of the guinea pig occluder
(Petre and Sheridan, 19665 Levinson and Sheridan, 1967a), j
probably precludes the possibility that the observed
transfer in the callosally-sectioned guinea pigs is due j
to occluder leakage.
A more likely explanation of the intermediate values
arises from the data obtained by Levinson and Sheridan
(1967b) on reversal and Irrelevant second-eye learning
in albino rats. The relevant finding of this research
was that acquisition of a problem via the second eye is
greatly facilitated by acquisition of any discrimination
via the first eye. By the time an animal begins training
on the second eye he has acquired general information
about the apparatus, cues, and task itself; the animal
is not totally naive on the second eye and therefore shows ;
facilitation. Consider the case of callosally-sectioned j
animals: even if there is complete blockage of correct
cue information from one hemisphere to the other, the
animals will still appear to be exhibiting some degree
of interocular transfer, as their second-eye acquisition i
i
scores will be lower than the first-eye acquisition scores,j
due to the "nonspecific learning" phenomenon just described
Perhaps sectioning of the corpus callosum in the guinea
pig does reduce transfer to a low level, most of the
apparent transfer obtained being an artifact of nonspecific
learning, which the animals are retaining when performing
with the second eye. It is difficult to say how much of
the transfer observed in the callosum-sectioned animals I
in the present study is due to carry-over of nonspecific ;
learning; however, Sheridan (1970, manuscript in prepar- ;
t
ation), in a review of the research on interocular transfer;
in the rodent, states that approximately 60 percent of the :
learning during acquisition of pattern discriminations in
the Thompson-Bryant apparatus is general (nonspecific)
learning. He also reports evidence indicating that virtu
ally all of the general information acquired via one eye
transfers to learning via the other eye. Theoretically,
then, a callosum-sectioned animal could show up to 60
percent savings on acquisition via the second eye even
with complete blockage of interocular transfer of the
correct (and specific) cue information. Considering the
50-75 percent total observed transfer shown by the callo
sally-sectioned guinea pigs: if most of this transfer is 1
due to carry-over of nonspecific learning and is parcelled ;
out, the remaining transfer adduceable to the specific
component amounts to 0-15 percent. This range of values !
concurs with the values predicted by Polyak's model; to
1
I
this model, however, should be added a highly functional
corpus callosum, based upon the near-perfect transfer !
i
exhibited by the sham-operated guinea pigs. j
The hypothesis of nonspecific factors is obviously !
i
I
viable enough to warrant empirical testing. One approach
77 |
j
would Include binocular training of the animals on an j
irrelevant (e.g., luminous flux) discrimination immediately;
I
i
following pretraining. After meeting criterion on this
task, the animals would undergo the normal monocular
training procedures used in studies of interocular trans
fer. The addition of the binocular training phase would
hypothetically equate the two eyes for nonspecific learn
ing, and thus savings on second-eye acquisition would more
accurately reflect the degree of transfer of correct cue
information via the corpus callosum.
Sheridan (1965) observed very little transfer in
callosally-sectioned albino rats on a tilt discrimination; !
it appears, therefore, that they did not benefit from non
specific learning obtained during acquisition via the
original eye. These animals, however, underwent deeper
sectioning of midline structures than did the hooded rats
or guinea pigs, and subsequently, transfer of hypothesized j
i
general factors from one hemisphere to the other could havel
been blocked. More recently, Hottman (1970) measured j
i
interocular transfer of a black-white discrimination in
albino rats which underwent either a sham operation,
sectioning of the corpus callosum only, or sectioning of
the corpus callosum along with sectioning of the posterior
commissure and superior colliculi. Both the sham-operated j
and callosally-sectioned groups of animals exhibited
transfer of the task (the callosally-sectioned group show- j
ing less transfer), while the deep-sectioned group showed
no transfer. Hottman reported that the animals in the
deep-sectioned group had also suffered complete incidental
sectioning of the hippocampal and habenular commissures.
It is not known whether midline sectioning of structures
such as the posterior and hippocampal commissures would
further reduce transfer in the guinea pig to the levels
observed in the albino rat (perhaps, e.g., by blocking
transfer of nonspecific information) ; this possibility
awaits empirical investigation.
It is doubtful that acquisition via the second eye
was appreciably affected by the differences on the pre-
training avoidance task reported earlier. Nonetheless,
since the scores on original eye acquisition might be
influenced by pretraining differences, the Interocular
decrement percentage would also be affected, as It is
calculated by dividing the number of trials to criterion j
r
on second-eye acquisition by the number of trials to
criterion on original-eye acquisition. Thus equivalence j
of groups on pretraining is necessary for accurate measure- j
i
ment of interocular transfer as well as relative ability
i
In discrimination learning. j
The guinea pig; as a subject for I
neuropsychological research;
concluding "statement's j
j
The finding that guinea pigs performed as well as j
|
pigmented rats on original (first-eye) acquisition of the i
tilt and shape discriminations suggests that they are
particularly suitable subjects for research on visually
mediated behavior. The observed superior learning ability
of the experimentally sophisticated guinea pig must be
contrasted, however, with its marked tendency toward
freezing behavior in response to aversive stimuli, par
ticularly when experimentally naive. It is believed,
therefore, that behavioral research utilizing guinea pig
subjects will have to include pre-experimental handling
as a rigorously pursued element of investigation.
For reasons discussed earlier, it appears highly
likely that Polyak's (1957) estimate of a meager per
centage of non-decussating optic fibers occurring in the
guinea pig is accurate. Since a large proportion of the
fibers (99 percent plus) probably cross at the chiasm, it
would appear that all that is necessary to transform the
guinea pig into an elegant split-brain preparation is to
section its corpus callosum. (In the cat, for example,
one must also section the optic chiasm— necessitated
because 30 percent of its optic fibers do not cross.) The
high percentage of decussating optic fibers in the guinea
pig also qualifies it for research on interhemispheric
inhibition as inferred, e.g., via the technique of uni
lateral striate cortical ablation. Further clarification
of the functioning of the visual and callosal fiber systems;
in the guinea pig should arise from studies similar to
those performed by Sheridan and Shrout (1966) and Creel and!
Sheridan (1966) on the effectiveness of the crossed and
uncrossed visual fiber systems in the rat, as well as
from empirical testing of the nonspecific learning hypoth
esis.
In general, the housing and feeding requirements for
guinea pigs are similar to those for rats. Guinea pigs
require somewhat more water, and their diet must be sup
plemented with Vitamin C. The gestation period for the
guinea pig is 57-62 days, as opposed to 21 days for the
rat; while rat pups are virtually helpless for two weeks
after birth, guinea pigs less than one hour post-partem
are hearty enough to participate as subjects and therefore
can be used for behavioral assays of development of CNS
function (see Petre and Sheridan, 1966).
The present study was performed with the following
objectives in mind: first, to assess the merits of the
guinea pig as a subject for neuropsychological research;
second, to gain insight into the optic and neural mechan
isms underlying interocular transfer in guinea pigs; and
81
third, to test the generalizability of extant data on
interhemispheric interaction obtained from pigmented and
albino rats. These objectives have to some extent been
achieved, but much future research could follow which
would shed light on comparative and developmental (as
well as physiological) correlates of visually mediated
behavior.
CHAPTER V
SUMMARY
Monocular acquisition and interocular transfer of
an active avoidance task involving the discrimination of
(a) stripes differing in orientation (tilt), or (b) a
triangle and a square (shape), were measured in guinea
pigs which had undergone either surgical sectioning of
the corpus callosum or control surgery involving making
of a bone flap and slitting of dura. A reliable deficit
in learning an active avoidance task during pretraining
was observed in callosally-sectioned animals (t(26)=2.321,
P <".03), possibly due to cingulate damage which invariably
accompanied sectioning of the callosum. However, commis
surotomy did not impair acquisition of either discrimin
ation. It was found that guinea pigs were able to learn
the tilt discrimination within approximately 150 trials,
and the shape discrimination within about 350 trials.
These data are similar to published data obtained from
other species.
Although sham-operated animals exhibited interocular
decrements ranging from 1-3 percent, callosally-sectioned
animals showed decrements ranging from 25-50 percent,*
82
83
these were reliably greater than those seen in the controls
(F(1,23) = 13.15, £ K ■005). Yet the observed decrements
for callosally-sectioned animals are intermediate to the
values predicted by anatomical studies on the crossed and
uncrossed visual fiber systems in the guinea pig. It was
hypothesized that these intermediate values might be due
to facilitation on second-eye acquisition resulting from
the carry-over of nonspecific learning on the original
eye. Sectioning of the corpus callosum resulted in a
larger reduction in transfer In the guinea pig than that
which was previously reported for the hooded or albino rat;
this finding is interpreted as Indicating a greater depend
ence in this species on intact callosal fibers.
A discussion of the behavioral data and neuroana-
tomical implications, as well as data from the literature,
provided answers to several important questions:
1. How does the guinea pig compare with the rat in
terms of learning capabilities? The guinea pig exhibited
more difficulty In avoidance training than did the rat,
but acquired discrimination tasks as rapidly or more
rapidly than hooded or albino rats.
2. Does the guinea pig show good transfer of diffi
cult tasks as well as easy ones? The sham-operated guinea
pigs showed near-perfect interocular transfer of both the
tilt (96 percent) and the shape (97 percent) discrimin
ations.
84
3. How large is the interooular decrement in the
guinea pig; Indirectly, how much transfer is accounted for
by the corpus callosum? The callosally-sectioned guinea
pigs showed interocular decrements ranging from 25-50
percent. The corpus callosum may account for more specific
interhemispheric transfer of visual cue information than
suggested by these data, since much transfer of general
{"nonspecific") information may take place via intact
transverse fiber systems.
4. How does callosal functioning in guinea pigs
compare with callosal functioning in other species? The
guinea pig is more dependent on an intact corpus callosum
for interhemispheric communication than is either the
hooded or albino rat, perhaps even as dependent as is the
cat or monkey. Several studies were suggested which could
further clarify the degree and roles of callosal function
ing in the guinea pig.
5. Does callosum-sectioning facilitate acquisition
in guinea pigs? Although the callosally-sectioned guinea
pigs required fewer trials to meet criterion on both tasks,
their performance did not differ reliably from that of the
sham-operated animals.
6. Can cononissurotomized guinea pigs learn to dis
criminate shape? All of the callosally-sectioned guinea
pigs which were assigned to the experimental treatment
involving the discrimination of shape were able to success
fully master this task.
A brief consideration of the merits of the guinea
pig as a subject for neuropsychological research suggests
that this animal has several characteristics which uniquely
qualify it for research on interhemispheric communication
and related phenomena.
1
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APPENDICES
APPENDIX A
Chance stimulus sequences for discrimination
tasks based on Fellows (1967).
94
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r “ ‘ " . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1111111111111111111111111
I) d I) B te M B I) ts
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 II1 1 1 1 1
S & 8 f t » F 2 R R ^ R l H ^ R ? i ? 3 ! 8 ? 3 S « & S J R
1 1 1 1 1 1 1 1 1 1 1 1 1 1 M11 1111111
^ »q»jijasoaiJ*i0«ot»i*4*4«B3»aesas
I h • « < • . » . . s a 9 s n n s is i « i e t :
SXXISSI3X*ISS88g9Sfi88|glK
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
i m i i H i i i i i i i i i i i i K i i i
e< it a t* li n t>
i i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
D i l l l l H I U i t l l l H l l l I I I
1 1 1 1 1 1 m 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
i i i i i i i i i i i i i i i i i i i i i i i u
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
APPENDIX B
Mean trials to criterion on original acquisition,,
homolateral reacquisition, and second-eye acquisi
tion for several species discriminating "tilt"
and "shape" tasks; and resultant mean interocular
decrements.
97
1st Eye Horaolateral 2nd Eye J6
Species - Surgical condition Acquis. Control Acquis, decrement Reference
Tilt Discrimination
Guinea Pig Callosum
119.1^
0.28 43.14 38.54 Present study
Sham
154.83 0.33
9.14 4.14
Hooded Rat Callosum 82.80 3.60 13.40 16.60 Levinson &
Sham 178.00 0.00 2.80 2.20 Sheridan (1989)
Albino Rat* Callosum section 0.00 68.30 Sheridan (1969)
no surgery 0.00 40.50
Shape Discrimination
Guinea Pig Callosum
249.71 1.43 74.43
28.77
Preterite; study
Sham 308.14 4.74
4.71 2.97
Hooded Rat Callosum 335.60 3.80 33.40 8.80 Levinson &
Sham 368.20 0.00 0.00 0.00 Sheridan (1969)
* Median values are presented for the albino rat.
CO

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Creator Levinson, Daniel Mayer (author)

Core Title Monocular Acquisition And Interocular Transfer Of Two Types Of Discriminations In Normal And Corpus Callosally-Sectioned Guinea Pigs

Degree Doctor of Philosophy

Degree Program Psychology

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

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Advisor Galbraith, Gary C. (committee chair), Holmes, John Eric (committee member), Szafran, Jacek (committee member)

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