IMPROVED ACQUISITION OF LEFT-RIGHT RESPONSE
DIFFERENTIATION IN RAT FOLLOWING SECTION OF CORPUS CALLOSUM
Michael Noonan and Seymour Axelrod
Canisius College and The State University of
New York at Buffalo
As published in Behavioural Brain Research,
1991, 46, 135-142.
SUMMARY: Split-brained rats learned a left-right response differentiation in a
water maze significantly faster than rats with sham surgery. It is unlikely that
this superiority resulted from improvement in performance variables since
callosotomized rats did not differ significantly from sham operates in speed of
acquisition of a brightness discrimination in the same apparatus. Callosotomy
likewise had no effect on the acquisition of a water-maze task requiring
consistent unilateral responses. The superiority of the callosotomized animals
in forming the left-right response differentiation supports a hypothesis
implicating the forebrain commissures in left-right confusion.
ACKNOWLEDGEMENTS: This work was supported in part by a
grant to Canisius College from the Charles A. Dana Foundation. The authors thank
and praise Jill Krakowiak, Debra Lipczynski, Michelle Penque and Julie Renda who
served as research assistants. John Nyquist prepared the illustrations and
Sandra Thamer provided histological processing. Doug Weldon provided instructive
comments which were, as always, invaluable.
An animal can be deemed capable of telling left from right
if either of two conditions can be met7. Left-right stimulus discrimination
(LRSD) is demonstrated if the animal can consistently generate non-mirror-image
responses differentially to mirror-image stimuli. Left-right response
differentiation (LRRD) is demonstrated if the animal can differentially
generate mirror-image responses to stimuli which themselves convey no left-right
information. Non-human animals, young humans, and occasional human adults, have
difficulty with both types of task, and hence are often described as left-right
confused. Our goal in the present study was to examine the role played by the
forebrain commissures in such confusion.
Two alternative views of commissural transmission can be
counterposited15,7. The first supposes that the commissures contribute to a
left-right mirror equivalence (and hence confusability) of sensory-motor events,
this contribution perhaps being underlain by homotopic interconnections of the
hemispheres. The second supposes that the commissures, perhaps via their
heterotopic components, maintain left- and right-specific events as distinct.
Despite an abundance of reports dealing with the effects of split-brain
procedures on information flow between the hemispheres, the literature addressed
specifically to the effects of commissural section on behavioral
left-right equivalence is remarkably scant. Indeed, we know of only three lines
of relevant evidence.
Experimental investigations of the role of the commissures
in left-right confusion were included in the classical-conditioning studies on
dogs in Pavlov's laboratory. Krasnogorski, Bykov (both cited in Bykov &
Speranski, 1924, p. 47), and Koupalov (1916) reported that a conditioned
response established to somesthetic stimulation at a locus on one side of the
body was elicitable by stimulation of the homologous contralateral locus. Anrep
(1923) found that such responses generalized to test stimulations of other loci,
the vigor of the response decreasing with increasing distance of the test locus
from the conditioned one, and that this generalization gradient was bilaterally
symmetrical: stimulation of a locus on the side contralateral to training
elicited responses almost identical in vigor to those following stimulation of
the homologous locus on the trained side. The left-right equivalence displayed
by these dogs evidently resulted from callosal interconnections: Bykov and
Speranski (1924) and Bykov (1925) reported that conditioned responses to
stimulation on the one forelimb could not be elicited by stimulation of the
other forelimb in dogs whose corpora callosa had been sectioned; the animals
with separated hemispheres appeared not to "confuse" their left and right body
sides. In callosotomized dogs, but not intact ones, it was possible to establish
a differentiation between homologous body parts by pairing the food with
stimulation on one side and withholding it when the contralateral locus was
Likewise, the avian tectal commissure appears to mediate
an equivalence (and hence confusability) of left-right mirror-image stimuli.
Pigeons trained to peck at an oblique line later pecked preferentially not only
at the trained line but also at its mirror image (Thomas, Klipec & Lyons, 1966).
This effect likely derives from a cross-hemispheric reversal: Mello (1965) found
that birds trained monocularly to peck for food at a 45-deg line pecked most
frequently at a 135-deg line when tested with the untrained eye. Furthermore,
pigeons whose tectal commissures had been severed pecked preferentially only to
the trained-line orientation (Beale, Williams, Webster & Corballis, 1972).
The anterior commissure may also make a contribution.
After re-analyzing Noble's (1968) data on the effects of sectioning various
commissures on the ability of chiasm-sectioned monkeys to discriminate mirror
images, Achim and Corballis (1977) concluded that transmission across the
anterior commissure evidently contributes to the special difficulty of
left-right discrimination: for monkeys in which the severing of the anterior
commissure was included in the split-brain procedure, the discrimination of left
and right was no more difficult than that of up and down.
These lines of evidence are consistent in showing that
commissurotomy reduced or eliminated a left-right equivalence, and so support
the first view mentioned above. Nevertheless, our understanding of the
relationship between commissural integrity and left-right confusion is far from
complete. For one thing, the relevant studies all focused on left-right
stimulus equivalence; none compared animals with intact and severed
commissures on the ability to differentiate left and right responses.
Furthermore, the only work of which we are aware that specifically assessed the
effect of section of the corpus callosum on behavioral left-right
confusion was the Anrep/Bykov research which was carried out on only one
species, the dog.
We know of no study which directly examined the role of
interhemispheric communication in behavioral left-right confusion in the rat.
Yet, there is reason to suspect that the rat may be organized differently from
the dog with respect to left-right equivalence. An effort to produce bilateral
somesthetic generalization gradients in rats similar to those found in Anrep's
dogs failed. Using an operant-conditioning paradigm (bar-press for water),
Axelrod and Kankolenski3 trained intact rats to discriminate between periods
during which pulsatile stimulation was presented to a locus on one side of the
body, and periods during which no stimulation was presented. Although subsequent
test stimulation of other loci revealed systematic generalization gradients on
the side ipsilateral to the conditioned locus in each of three rats, response
rates to test stimulation of contralateral loci were markedly reduced, and there
was only a suggestion of contralateral gradients; i.e., left-right equivalence
could not be unambiguously elicited with this procedure.
Accordingly, in the present work we tested whether the
effects of commissurotomy on left-right response differentiation in the
rat would parallel those found for left-right stimulus discrimination in
the dog, monkey and pigeon. We expected that callosotomy would improve
the animal's ability to behaviorally differentiate left- and right-going
responses. To test whether any obtained effects of commissurotomy could be
accounted for by extraneous performance variables, we also measured the same
subjects' performance on a brightness-discrimination task. Additionally, we
report an experiment which explored the generality of the effects of callosotomy
by assessing its effect on another learned lateralized behavior, namely, the
rats' ability to generate consistently unilateral responses.
PROCEDURES AND RESULTS
Water Maze. We employed two water-mazes12,
which were T-shaped plexiglass tanks, 46 cm deep (Figure 1). The reinforcement
was escape from the water, accomplished by climbing up a ramp which extended
down into the water at the end of the appropriate arm. Each maze arm extended
for 30 cm and then turned 90° back, so that the escape ramp was out of sight to
a rat at the choice point. Water entered continuously (6.3 liter/min at 24° C)
at the floor of the starting box, and flowed out at the floor at the ends of
both arms, being maintained at a depth of 25 cm.
The walls of the maze were white and could be
back-illuminated. When not back-illuminated, the walls as viewed from within the
maze appeared to the human observer as uniform and dark. When back-illuminated,
alternating dark and light 3 cm-wide vertical stripes appeared.
All animal handling and data collection were carried out
by observers blind to the surgical status of the rats (callosotomy or sham), and
recorded on videotapes which were then reviewed by a second blind observer. In
this and related projects, we have also conducted a number of checks to confirm
that our subjects' scores were not functions of confounding variables: when
testing was continued beyond criterion, but by different handlers/observers, in
different mazes, in different rooms, and when oriented differently with respect
to compass heading, the subjects continued to demonstrate reliable LRRD.
Surgery. The split-brain preparation was carried
out by a procedure developed in our own lab. Our procedure utilizes a Z-shaped
callosotomy knife (Fig. 2), made by modifying a dental amalgam-plugger. The
knife was maneuvered under the meninges at a point lateral to the superior
sagittal sinus and then through a sequence of pivots illustrated in Fig. 3 so
that its leading edge could enter the longitudinal fissure to transect the
The anesthetized rat (ketamine [Ketaset] 52 mg/kg combined
with xylazine [Rompun] 2.6 mg/kg) was placed for stability in the head-holder of
a stereotaxic apparatus. A midline scalp incision was made and the underlying
periosteum retracted. With a variable-speed drill, the skull was thinned over an
area extending from 4 mm anterior to the coronal suture to 2 mm anterior to
lambda. The thinned area was approximately 6 mm wide anteriorly and 3 mm
posteriorly, as illustrated in Fig. 4A and 4B. We counterbalanced the side of
this cranial window and subsequent surgical approach across sex and sugical
The rat was then approached from the front (i.e., the rat
and surgeon were positioned face to face). A puncture wound through the adherent
osteo-dural tissue was created approximately 2 mm anterior to the coronal suture
and 2 mm lateral to the sagittal suture using a microknife, care being taken to
avoid any obvious blood vessels (Fig. 4C). A small hook was then used to lift
the osteo-dural tissue at the puncture wound site. The callosotomy knife, held
with its plane parallel to the surface of the brain, was slipped through the
puncture wound between the dura and the pial surface (Fig. 4D), and maneuvered
until its leading segment (a-b in Fig. 3(1)) lay directly under, and parallel
to, the sinus (Fig. 4E). The knife was then rotated along the long axis of its
middle segment (b-c in Fig. 3) so that the leading segment slid between the
cerebral hemispheres down into the callosal tissue (Fig.s 4E and 4F). The knife
was then pivoted along the axis of the leading segment (a-b) so that the knife
came to lie in the midsagittal plane (Fig. 4G). This pivot pressed the middle
segment of the knife against the side of the puncture wound and caused
stretching and distortion of adherent osteo-dural tissues, and a lateral
displacement of the sinus. The knife was then slid posteriorly in the
longitudinal fissure to the coronal suture (Fig 4H), taking care to maintain its
first bend (b in Fig. 3(1)) in contact with the underside of the meninges (that
is, not letting it drop ventrally, deeper into the brain). It was then slid
anteriorly along the same path and dropped ventrally approximately 1 mm as it
passed the coronal suture in an attempt to ensure full sectioning of the genu.
When the leading segment of the knife came to lie directly under the puncture
wound, which in its stretched state lay directly above the longitudinal fissure
anteriorly, the probe was withdrawn directly upward (dorsally).
It was rare for more than a few drops of blood to well up
during the knife's entire maneuvers. When the bleeding was stabilized, and the
skull was cleaned of any residual bone chip debris, the scalp was closed using
11 mm wound clips. The entire surgical procedure, from scalp incision to
closure, lasted approximately 15 min. per rat.
As a sham surgical treatment in the preparation of control
subjects, we carried out an identical procedure using an instrument with a
shortened leading segment. This probe slid under the sinus and within the
longitudinal fissure, but above the corpus callosum.
Post-operatively, the subjects displayed no obvious
behavioral deficits, maintaining normal weight, and demonstrating normal
locomotion and reactivity. A minimum of six weeks postsurgical recovery was
provided before beginning behavioral testing, and in the week preceding testing
each rat was handled twice daily in order to habituate it to human contact.
Histology. Following behavioral testing, the rats
were perfused intracardially with saline followed by formalin. The brains were
sectioned at 8 microns, and every 20th section was stained with cresyl violet.
Subjects. Male and female hooded (Long-Evans) rats
were obtained from Blue-Spruce (Harlan-Sprague-Dawley) at 35 days of age. They
were randomly assigned to undergo either callosotomy or sham-surgery at
approximately 100 days of age, and they underwent LRRD testing at 150 days of
age. The rats were housed singly and maintained on an alternating 12:12
white:red light cycle, with all testing occurring during the red phase. Room
temperature was held at 26° C.
1-A. Left-right Response Differentiation
Procedure. In this test12, when both arms of the
maze were lit, the escape ramp was placed in the right arm, and when the maze
was in the unlit condition the ramp was in the left arm. A "dummy" ramp, which
extended down only to 15 cm above the water surface and thus could not be used
for escape, was placed in the opposite arm so that the escape ramp location
could not be determined by a view from outside the tank. Pseudorandomly
sequenced trials (maze-illuminated/maze-unilluminated) were presented at 4-min
inter-trial intervals. On each trial the rat was placed in the starting box and
allowed to swim until it found the ramp; it was then returned to its home cage
until the next trial. Each rat was tested for 25 trials per day until it reached
the criterion of 10 successive correct first turns at the choice point (or for a
maximum of five successive days). The number of trials taken to reach this
criterion served as the index of left-right confusion (cf. Noonan & Axelrod,
Results. Callosotomy produced a striking (24.6%)
and significant superiority over the control treatment, the callosotomized
subjects (N = 28) requiring a mean of only 49.3 trials to reach
criterion, compared with 65.4 trials for the sham-operated controls (N =
28); see Table 1. The mean scores of the male and female rats were virtually
1-B. Brightness Discrimination
In an effort to assess whether the superiority of the
callosotomized subjects on LRRD was due to variables which might have improved
water-escape performance in general, we tested half of the same subjects using
the same water-maze on a visual discrimination task whose solution did not
depend on the left-right distinction.
Procedure. Three to five weeks following LRRD, the
male rats only underwent a test in which on each trial, only one of the two arms
of the maze was lit, and the other arm unlit. The escape ramp was always to be
found in the lit arm, with the side being varied pseudorandomly from trial to
trial, counterbalanced across left and right. As before, the rats were tested
for 25 trials a day for a maximum of five days or until the rat reached the
criterion of ten successive correct first responses.
Results. The brightness-discrimination scores of
the split-brained rats did not differ reliably from those of the sham operates
Histology. Figure 5A and 5B show representative
sections of callosotomized subjects. Postmortem examination of the prepared
brains revealed that, for the callosotomy group, the proportion of the corpus
callosum sectioned, defined as the number of sections in which the callosum was
severed divided by the total number of sections in which the callosum crossed or
would have crossed the midline, averaged .73 (range .53 - 1.00). There was a
greater tendency for splenial fibers to have been left intact (some fibers being
present in 83% of the brains) than for genu fibers (27%). Among these
callosotomized subjects, neither the LRRD or BD scores of rats with genu--or
splenium--fibers remaining did not differ significantly from those without such
fibers. There was a weak tendency (r = -0.25) for the proportion of
callosal fibers cut to be associated with decreasing (improving) LRRD scores,
but this relationship was not statistically reliable. None of the sham subjects
showed any damage to callosal fibers.
A degree of unilateral damage caused by the cutting probe
straying from the midline was observed in the septum in 66% of the brains (an
example is presented in Fig. 5A), in the fimbria in 43%, in the hippocampus in
62% (an example is given in Fig. 5B), in the thalamus in 23%, and in the tectum
in 37%; however, post-hoc ANOVA's revealed no reliable differences in LRRD or BD
scores between callosotomized subjects with damage in these regions and those
without such damage.
All of the callosotomized brains showed unilateral lesions
of medial neocortex incident to the passage of the surgical knife into the
longitudinal fissure. The brains of the sham subjects (an example is given in
Fig. 5C) also evidenced medial neocortical damage, although the lesions were
somewhat smaller than those suffered by the callosotomy subjects. Thus, the
possibility cannot be ruled out that the differences in performance between our
two groups was due to differences in extent of medial cortex damage. To assess
this possibility, for each callosotomized subject we computed an index of the
amount of medial-cortex damage incorporating both the depth of the lesion and
the number of histological sections in which damage occured. Water-maze
performance was not reliably correlated with this index of cortical damage.
2. Consistent Unilateral Response Test
The question can be raised of the generality across tasks
of the role of the corpus callosum in contributing to left-right confusion. To
index rats' ability to distinguish left and right, Zimmerberg, Strumpf and
Glick18 used a T-maze in which the animal escaped electric shock by entering one
lateral arm. Over successive trials, the animal's entries into the unreinforced
side were interpreted to reflect left-right confusion. We term such tasks
Consistent Unilateral Response Tests (CURT). An improvement in CURT scores
following commissurotomy would suggest a degree of commonality across the two
types of task in the callosal contribution to left-right confusion.
Subjects. The Long-Evans rats used in the preceding
study were mated and their offspring were assigned as subjects in Experiment
Two, randomly except that litter was counterbalanced across treatments. The rats
were housed singly from 28 days of age, underwent surgery at approximately 65
days of age, and underwent water-maze testing at approximately 100 days. Again,
we employed an alternating 12:12 white:red light cycle, with all testing
occurring during the red phase, and room temperature was held at 26° C.
Procedure. In this test, the water maze was
entirely unlit on every trial. On the first day (acquisition), no escape ramp
was present when the animal was first placed into the water-maze. Once the
animal turned into one of the lateral arms, the escape ramp was placed into the
opposite arm, with the rat being left to swim until it reached the ramp; thus,
each rat had a similar experience of an "incorrect" first choice. On subsequent
trials the ramp was placed beforehand in the initially-reinforced arm. Trials
were administered (inter-trial interval = 4 min) either until the rat reached
the criterion of ten consecutive correct responses, or for a maximum of 50
trials. On the following day (retention), the rats were retested to the same
criterion. They were then retrained on the third day (reversal), on a task in
which the ramp was always placed in the arm opposite to the one which had been
correct on the two previous days of testing.
Results. Callosotomy had no effect on CURT scores.
On none of the three testing days did the callosotomized animals differ reliably
from the controls (Table I). Male and female rats did not differ on either the
acquisition or retention tests; however on day 3 (reversal), males took
significantly fewer trials (14.7 ± 6.4) than females (21.8 ± 14.9, F
(1,44) = 4.32, p < .05).
None of the sham subjects evidenced any callosal damage.
In the callosotomy group, the proportion of the corpus callosum sectioned
averaged .94 (range .63 - 1.00). Some splenial fibers were left intact in 39% of
the brains, and genu fibers in 4%; again the presence or absence of these fibers
was not related to water-maze performance. Consequent to efforts to sever a
greater proportion of callosal fibers than in Experiment 1, sub-callosal damage
was more frequently observed: unilateral damage was evident in the septum in 91%
of the brains, in the fimbria in 96%, in the hippocampus in 96%, in the thalamus
in 22%, and in the tectum in 9%. Accordingly, to assess possible contributions
of these incidental lesions to CURT performance, we compared the scores of those
subjects which evidenced no or slight damage to the septum, fimbria or
hippocampus with those showing evidence of somewhat greater damage. No
differences were found for the acquisition or retention tests. For the reversal
test, although there was no relationship between trials-to-criterion and degree
of hippocampal or septal damage, subjects with greater incidental damage to the
fimbria were significantly better than those with lesser fimbria damage (F
(1,19) = 4.36, p = .05); we will offer no interpretation of this
unanticipated and isolated result. No reliable differences were found in any
CURT scores between callosotomized subjects with and without damage in the
thalamic or tectal regions. Again, all of the callosotomized brains showed
unilateral medial neocortical lesions, which were somewhat more extensive than
those seen in the brains of sham operates, and whose size did not relate to
The expected and most common result of damage to the
central nervous system is of course a deficit in function: a compromised brain
is as a rule less capable than an intact one of meeting the needs of the
subject. Nevertheless, functional improvement can occasionally be shown to
result from destructive processes (e.g., 14,10), and in such instances, the
paradox provides an opportunity for an elucidation of the relationship between
the neural substrate and the demands of the task set for the subject.
We interpret the facilitation of LRRD performance by
callosotomy to implicate callosal transmission in left-right confusion. The
neurophysiological effects of either homotopic or heterotopic commissural fibers
are still largely matters of speculation, as are their effects on the flow of
information. At this stage, we can only conclude that whatever their numerous
beneficial effects may be (see 8,9,13), as a whole, the callosal connections, by
allowing lateralized signals to intermix, evidently increase the difficulty of
distinguishing between left and right. Our split-brained subjects, not
experiencing the confusing intermixing of left and right information, were
better able to differentially associate lateralized responses with the
The inference of a contribution of the callosum to a
process by which left- and right-specific neural events are equated can also be
drawn from electrophysiological evidence. Electrical stimulation of the
sensorimotor cortex of one hemisphere produces an evoked response in the
contralateral sensorimotor cortex, a response which is abolished by midline
sectioning of the corpus callosum17.
The absence of a facilitatory effect of callosotomy on
either the brightness-discrimination or the CURT task, both depending upon water
escape in the same maze used for LRRD, makes it unlikely that the facilitatory
effect seen in the LRRD test was due to enhancement of extraneous performance
variables (information processing capacity, swimming skill, escape motivation,
arousal, attentiveness, etc.) not specific to the LRRD test itself.
The failure of the callosotomized subjects to achieve
superior CURT scores when compared with controls might have been due to a "floor
effect". That is, the task might simply have been too easy for intact animals
for any relative improvement to have been demonstrable. On Day 1, for example,
the overall mean trials-to-criterion score was 15.3, including the ten criterial
trials. Nevertheless, it is worth noting that the relative ease our subjects
experienced with this test matches that shown in similar studies7 pp.38-42 in
which non-contingent consistent unilateral responses were demanded. Thus CURT
may be different in kind from LRSD and LRRD, which characteristically prove
difficult for animals. We wonder whether the neural operations underlying the
process of response selection on the CURT task even need to take the left-right
distinction under consideration; it may be possible for one unilateral response
to be "turned on" without the mirror homologous response even being considered.
In any event, we cautiously conclude that callosal transmission makes no
contribution to the processes involved in CURT performance.
In experiments now under way, we are asking whether the
facilitatory effect of callosotomy on left-right response differentiation can be
narrowed down to a specific anatomical division of the corpus callosum, and
whether LRRD performance can be related to individual variation in callosal
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MEAN TRIALS TO CRITERION ± STANDARD
|1A. Left-Right Response Differentiation
||49.3 ± 18.7
||65.4 ± 22.3
|1B. Brightness Discrimination
||60.9 ± 32.2
||58.7 ± 23.4
|2. Consistent Unilateral Response
||15.1 ± 4.6
||15.5 ± 5.9
||12.6 ± 5.9
||15.2 ± 8.5
||20.5 ± 14.2
||16.2 ± 9.1