Some effects of commissurotomy on the reinstatement and potentiation of lesion deficits

Some effects of commissurotomy on the reinstatement and potentiation of lesion deficits

135 Behavioural Brain Research, 29 (1988) 135-146 Elsevier BBR 00808 Some effects of commissurotomy on the reinstatement and potentiation of lesion ...

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Behavioural Brain Research, 29 (1988) 135-146 Elsevier BBR 00808

Some effects of commissurotomy on the reinstatement and potentiation of lesion deficits Douglas P. Crowne, Ana Adelstein, Kim A. Dawson and Claudette Richardson Department of Psychology, University of Waterloo, Waterloo, Ont. (Canada) (Received 23 June 1987) (Revised version received 8 January 1988) (Accepted 8 January 1988)

Key words: Parietal cortex; Frontal cortex; Corpus callosum; Neglect; Circling; Lateralization

Two experiments investigated in rats the effects of cutting the corpus callosum after recovery from unilateral cortical lesions that produce transient symptoms of neglect and circling. Side of lesion was also examined. In Expt. I, 60 rats received left or right lesions of parietal, medial frontal, or motor cortex. After one month of testing for visual, auditory and somatosensory responsiveness and for circling, the callosum was cut, and the sequence of measures was repeated. Callosotomy reinstated neglect after recovery from the lesions in the parietal and medial frontal groups, more severely and consistently in the frontal group. Side of lesion made no difference. Circling was predominantly ipsiversive after the cortical lesions, due entirely to the frontal group. Callosum section markedly potentiated contraversivc circling in the left parietal group; right parietal animals showed no preference. This was the only hemisphere difference found. Circling remained ipsiversive in medial frontal animals after callosotomy. These circling biases did not diminish in the postcallosotomy period. Expt. II replicated the circling procedures with 58 animals that were given the same unilateral cortical lesions or were unoperated controls. Callosotomy was performed one month postlesion. Again, left parietal animals circled contraversively, and there was no bias in the right parietal group. A left-right difference was also evident in the motor cortex group, left lesions producing contraversive turning. We confirm the reinstatement of neglect from frontal lesions by callosum section previously found in the monkey and show that it also occurs with parietal lesions. While neglect symptoms do not differ after left or right lesions, circling does: left parietal lesions plus callosotomy produce a marked contraversive tendency that may reflect an elemental spatial lateralization.


In the dawning years of this century, Imamura 24 and Yoshimura 45, working in the laboratory of Sigmund Exner, reported that section of the corpus callosum could reinstate the symptoms of recovered unilateral cortical lesions. In dogs, unilateral lesions of occipital or frontal cortex, the latter including the sigmoid gyrus, locus of the frontal eye fields, produced contralateral visual field defects that cleared in 1-4 weeks. With transection of the corpus callosum, the deficits

reappeared and were likely to be permanent. Visual responsiveness in each hemifield was assessed by the method of sausage perimetry 1°. In one version of this procedure, the dog's attention was attracted centrally by two pieces of meat on long skewers. Then, both morsels were moved to the lateral regions of each visual field, and the side chosen was observed. Crowne et al. 6 reexamined some of these findings in the monkey with a more exactly controlled test of the visual impairment that involved visually guided reaching to briefly illuminated

Correspondence: D.P. Crowne, Department of Psychology, University of Waterloo, Waterloo, Ont. N2L 3G1, Canada. 0166-4328/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)

136 targets at varying degrees of eccentricity. Unilateral lesions of the frontal eye fields resulted in a severe contralateral neglect that recovered gradually over the span of a month. After commissure section (corpus callosum and anterior commissure), the neglect reappeared. Unlike the Imamura and Yoshimura results, there was recovery within two weeks to a month. When threat stimuli were presented bilaterally, however, no recovery was apparent. The more sensitive procedure of testing for extinction to bilateral simultaneous stimulation revealed an enduring defect in the contralateral visual field. The experiments we report here were designed to assess the effects ofcallosum section in another species, the rat, after unilateral lesions of two cortical regions, medial frontal and parietal. Damage to both of these areas results in polysensory symptoms of neglect that recover ~'3 and circling biases that do not 3. We asked whether callosotomy would reinstate recovered neglect and exacerbate the postural-motor-spatial symptom of lateralized circling. We also examined the effect of side of lesion. In humans, symptoms of neglect are strongly associated with right-hemisphere damage and are rarely seen after lefthemisphere injury, even in sinistrals 13'19. Disorders of movement control, including spatially oriented movements, originate from lesions in left hemisphere 11'12"2°'34. In animals, there is important evidence of hemispheric functional asymmetries, involving emotionality, circling, and spatial responsiveness 4,7'~5"42. We sought to determine whether callosotomy might potentiate lateralized sensory or motor effects, as it did with emotionality in the Crowne et al. 4 experiment. EXPERIMENT 1

Materials and Methods Subjects, surgery, and histology Sixty male Long-Evans hooded rats, weighing between 250 and 300 g at the start of the experiment, were assigned to parietal, medial frontal, and motor cortex lesion groups. They were individually housed, had unrestricted access to food and water, and were maintained on a 12-h

light/dark cycle. For a week prior to surgery, each animal was handled dally. Surgery was performed under sodium pentobarbital anaesthesia (50 mg/kg i.p.). The unilateral cortical lesions were accomplished by subpial aspiration under a dissecting microscope. Ten left and 10 right hemisphere lesions were made in each group. The final n was reduced by the deaths of 2 animals, one with a left parietal lesion and one with a right medial frontal lesion. The parietal lesions were intended to remove the area of cortex included in case 12 of Jones and Leavitt 25. Within this region are the posterior corticospinal projections 23 and, more caudally, Krieg's architectonic a r e a 7 3°. The medial frontal lesions, on which we have previously reported ~'5, were directed at the area described by Leonard 32 as the projection field of the lateral dorsomedial nucleus of the thalamus. The target for the motor cortex lesions was the entire dorsolateral precentral region. Four weeks after the cortical lesions, the corpus callosum was sectioned in all animals. This was accomplished by a retractable microknife inserted through midline cerebellum2. At the end of the experiment, the animals were given a lethal barbiturate dose and perfused intracardially with 0.9~o saline followed by 10~o formalin, and the brains were removed. Frozen coronal sections of 33/~m were taken through the lesions and for the entire length of the corpus callosum. Every 5th section was saved and stained with metachromatic thionine. The lesions were traced on standard coronal drawings, from which the reconstructions of Fig. 1 were made. The parietal lesions destroyed the intended region and, as previously reported 3'4'27'2s, resulted in degeneration in the dorsal lateral and lateral posterior nuclei of the thalamus. Typically, there was damage to underlying white matter, but neither the hippocampus nor the basal ganglia were invaded except in 3 cases (2 right parietal and 1 left parietal). The orienting and circling of these animals did not differ from the means of their groups. The medial frontal lesions consistently damaged a strip of adjacent motor cortex; removal of the medial wall to the depth of the corpus callosum was complete. Lesions of motor cortex left a small polar remnant.


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Fig. 2. Midsagittal tracing based on the Paxinos and Watson 38 atlas showing the mean area spared by the corpus callosum transections (stippling). The ventricles are crosshatched.


Fig. 1. Reconstructions of representative parietal (A), medial frontal (B), and motor cortex (C) lesions. The numbers indicate distance from bregma. Crosshatching shows ventricular or lesion-induced cavities, and stippling identifies the degeneration in dorsal lateral and lateral posterior nuclei of the thalamus resulting from the parietal lesions.

Transection of the corpus callosum through the splenium and posterior trunk was seen in all but two cases. In some animals, there was sparing of the mid-trunk and more rarely the genu. The areas spared are shown in Fig. 2. We calculated from the serial sections through the corpus callosum the length in mm of the callosal division in each animal, including only regions between sections that showed complete transection (it was not infrequently the case that ventral fibers were cut and the dorsal half was intact). An analysis of variance on these callosum-section lengths revealed no significant between-group differences. The mean length of the callosal cuts overall was 5.16 mm.

No lateralized damage to the medial walls of the hemispheres was produced by the callosotomies. Because of our planned examination of potential hemisphere differences in lesion deficits, it was important to establish that the lesions on each side were comparable. Each section through the lesions was projected and enlarged, and the perimeter of the lesion was drawn. These outlines were traced on an Apple Graphics Tablet and the area calculated. The area values were summed over the number of sections in which a lesion was observed, and lesion volumes were calculated. A lesion x hemisphere analysis of variance found no differences in size of the left and right parietal and medial frontal lesions. Procedure Orienting responses to visual, auditory, and somatosensory stimulation and circling were measured by techniques we have heretofore described 1'5. Briefly, the tests of orienting were conducted with the rat facing into a small hemispheric enclosure and lightly restrained by hand. The stimuli were presented when the animal was quiet and centred in the enclosure. The initial side of stimulation was chosen randomly. On each of these tests, responses were scored on a 3-point scale (0 = no response or an allesthetic response; 1 = a weak, slow, or poorly-targeted response; 2 -- a strong and well-directed response). A standard sequence was followed, beginning with the visual test. This used a 90-mg sucrose pellet cemented to

138 the end of a coathanger wire wand, which was brought in from the rear until it was just lateral to the vibrissae. Here, the visual probe was slightly moved for 5-10 s. The normal animal typically responds within this interval, turning toward the pellet and sniffing it. The animal with a unilateral lesion responds similarly on its intact side. Neglect of the stimulus is shown by failure to respond, turning toward the non-stimulated side (an allesthetic response), or turning to the side of the stimulus and right past it. Despite its use as an edible stimulus, the visual test depends more on vision than olfaction. The sucrose pellet has little odour, and the rat only orients to it when it is brought within the visual field. Further, other stimuli that are indubitably not olfactory (a small white cardboard disc 26 or briefly flashing lights ~,3s) elicit identical orienting responses. The auditory test consisted of scratching and tapping on the exterior of the enclosure wall, to one side of the animal and then the other. This elicits a head turn toward the side of the sounds, approach and sniffing in the normal animal. There were two somatosensory tests. First, the wooden end of a cotton swab was brought in from the rear, and the perioral vibrissae were gently stroked. Normally, the rat turns toward the stimulated side. The orienting tests were concluded by pressing on the outer toe of each forepaw with the wooden swab stick, a stimulus that elicits vigorous withdrawal and occasional squealing in the intact rat. The measure of circling was a 2-min observation of the direction of rotation of each rat in its home cage, placed atop a small elevated platform. Left and right turns (to the nearest half-turn) were recorded. These orienting and motor tests were administered on the third, fourth and fifth days postoperatively and on corresponding days of the next 3 weeks. Nine rats with parietal lesions were not tested in the fourth week. Because of this omission, our postlesion analyses were restricted to the 3 weeks succeeding surgery. However, we present below the Week 4 results of the remaining parietal-lesion animals and those in the other groups. The tests were resumed two days after callosotomy and continued for 4 weeks. For each animal, a weekly score was calculated on each

measure, totalling the 3 days' scores. The experimenter was blind to the lesions of the animals throughout. Results

The weekly scores on the orienting and circling tests were expressed as ipsilaterall-contrallateral differences divided by the total (I - C/I + C), a version without correction for errors of a statistic used in dichotic listening studies3L These scores were cast in Lesion x Hemisphere x Weeks analyses of variance of each test. As a first step in our analysis of symptom reinstatement by callosotomy, it was incumbent to demonstrate recovery from the cortical lesions before hemisphere bisection. Fig. 3 shows that the orienting deficits of animals with medial frontal lesions continued into the third week. Had they recovered by the following week? Orienting in Week 4 and circling in Weeks 3 and 4 are shown in Table I. The zero orienting difference scores of all groups on each of the orienting tests clearly signify the recovery of contralateral responsiveness. In an analysis of Weeks 3-4 circling, only a lesion main effect was significant. The direction of circling did not reliably change over these two weeks. Orienting tests Fig. 3 presents the postlesion and postcallosum section weekly orienting difference scores on the 4 tests. It shows that the effects of two of the lesions, medial frontal and parietal, declined in severity over weeks, while the motor cortex group displayed no orienting impairment in the first week or thereafter. The analyses of variance gave Lesion × Weeks interactions that were each significant beyond P = 0.001. The F ratios (12, 312) were 28.07 (visual), 15.61 (auditory), 14.70 (vibrissae), and 13.68 (toe pinch). None of the hemisphere main effects or interactions were significant, and inspection of the means did not disclose any coherent pattern of hemisphere differences. The deficits from the parietal and medial frontal lesions, their recovery, and reinstatement were examined in each test by standard sets of planned individual contrasts with Tukey's honestly significant difference procedure.


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Fig. 3. Postlesion and postcallosum section orienting direction on the 4 sensory tests (Vis. = visual; Aud. = auditory; Vib. = vibrissae; Toe = toe pinch) by each lesion/hemisphere group. The orienting difference score is Ipsi. Contra./Ipsi. + Contra. The T above or below each bar is the S.E.M.

Comparisons with the motor cortex lesion group, considered as a control, used Dunnett's test. Visual. The first postlesion week orienting difference scores of the parietal and medial frontal groups showed a strong bias toward exclusively ipsilateral responding, while the scores of the motor cortex group were near zero. At 0¢= 0.05, Week 1 significantly differed from Week 3 in both

the parietal and medial frontal groups, but did not in the motor cortex group. Thus there was substantial recovery of contralateral responses in the animals with deficit-inducing lesions. A small residual in the medial frontal group had gone by Week 4 (see above). Contralateral visual field defects were reinstated by hemisphere bisection as shown by significant Week 5-Week 3 differ-

140 ences in both parietal and medial frontal groups. Recovery again occurred in parietal and medial frontal animals. The Week 5-Week 7 comparison was significant in each case, and by Week 8 the orienting difference scores were zero for all groups. Individual between-group contrasts at = 0.05 showed the parietal and medial frontal groups differing from the motor cortex group in Week 1. In Week 3, the medial frontal-motor cortex groups comparison was significant, and the medial frontal group had a greater deficit than the parietals. The parietal and motor cortex groups were not different. Following callosotomy, both groups differed from the controls; the reinstated contralateral-side deficit in the medial frontal animals was more severe than in parietal animals, and this was also true in Week 6 (Tukey's test). At Week 7, the parietal group did not differ from the motor cortex group; the difference between the medial frontal and motor cortex groups did not reach the critical level. Auditory. In the parietal and medial frontal groups, the pattern of results on this test was quite similar to the visual findings: weak contralateral responding in the first postlesion week, recovery by Week 3, reinstatement after callosum section (Week 5), and a second recovery. At the critical level, the Week 1-Week 3, Week 5-Week 3, and Week 5-Week 7 comparisons were significant in both groups, with no change over weeks in the motor cortex lesion animals. Auditory orienting of the parietal and medial frontal groups differed significantly from the motor cortex group in Week 1. By Week 3, the parietal-motor comparison was not significant; the medial frontal-motor and medial frontal-parietal contrasts were, indicating a remaining deficit only in the medial frontal group. In both Weeks 5 and 6, the auditory scores of the parietal and medial frontal groups differed from the motor cortex group. Postlesion callosotomy produced a stronger ipsilateral auditory bias in medial frontal than in parietal animals that endured through Week 6 (Tukey's test). There were no between-group differences in Week 7. Vibrissae. The initial effect of the parietal and medial frontal lesions, a severe weakening of responses to contralateral stimulation of the

whiskers, had disappeared by the third week (Week 1 orienting difference scores were significantly greater than Week Ys). The reimposition of contralateral neglect by callosum section occurred only in animals with medial frontal lesions. Although Week 5 did not differ from Week 3 in this group, it did from Week 4, and the medial frontal-motor cortex contrast was significant in Week 5 (Dunnett's test). Responsiveness opposite the lesions had returned almost fully by Week 6. Toe pinch. Both parietal and medial frontal lesions considerably diminished contralateral responses on this mildly nociceptive test, recovery occurring by Week 3. Section of the corpus callosum was without effect in the parietal group but briefly reinstated neglect in medial frontal animals. The Week 5-Week 3 comparison in the latter group was significant (Tukey's test), as was the Week 5 difference between the medial frontal and motor cortex groups (Dunnett's test). The process of recovery was complete by Week 7.

Circling Analysis of variance of the circling directionality scores disclosed significant main effects for Lesions (F2,s2 = 21.69, P < 0.001) and for Hemisphere (F1.52 = 14.30, P < 0.001), but neither main effects nor interactions involving weeks. Because there were no regular week-to-week changes, we collapsed this repeated measure into two more appropriate divisions, postlesion and postcallosum section. Fig. 4 shows these circling means. A second analysis of variance resulted in 3 significant main effects: Lesion F2,52 = 22.71, P < 0.001 ; Hemisphere F1,52 = 14.21, P < 0.001 ; and Operations Fi,52 = 5.74, P < 0.02. Fig. 4 makes evident the pattern of these 3 effects. The lesions' main effect arose from predominantly ipsiversive circling by the animals with medial frontal lesions, continuing after callosotomy; contraversive circling by parietal animals, strongly potentiated by callosotomy; and little or no circling preference in the motor cortex lesion group. In the hemisphere effect, animals with right lesions tended to circle to the right, showing the well-known ipsiversive tendency. Those with left lesions also circled rightward,




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Fig. 4. Postlesionand postcallosum section circlingby each lesion/hemispheregroup. Circlingdirectionis expressedlike orienting direction in Fig. 3. The T above or beloweach bar is the S.E.M. away from their lesions. The operations effect resulted from more ipsiversive circling in the postlesion period and a shift toward contraversive circling after callosotomy. From the data in Fig. 4, it was clear that these findings were not uniform across the lesion/hemisphere groups, and so a series of group comparisons (Tukey's HSD) and tests of circling bias were conducted. Comparisons between left and right lesions of each cortical region were made for the postlesion and postcallosum section periods. No side-of-lesion differences appeared in the postlesion period. Following callosotomy, however, circling directionality in animals with left parietal lesions was significantly more contraversive than circling in right parietal lesion animals ( M = - 0 . 6 5 vs M = - 0.13, P < 0.05). No other left-right difference was significant. Comparison of circling directionality in the postlesion and postcallosum section periods within each lesion/hemisphere group showed only one group significantly changing after callosotomy. Animals with left parietal lesions had a postlesion mean of -0.18; after callosum section, the mean in this group was -0.65. Concluding the analysis of circling directionality, we tested each lesion/hemisphere group mean against zero. In the postlesion period, only the circling directionality of the right medial frontal group showed a significant bias ( t = 3.83,

P < 0.00 l). After section of the corpus c allosum, the mean circling directionality of both the left parietal and right medial frontal groups differed significantly from zero (left parietal: t = 5.46, P < 0.001; right medial frontal: t = 3.41, P < 0.01). To summarize, circling in animals with left parietal lesions became strongly contraversive after cutting of the corpus callosum. The circling of left and right parietal lesion groups differed significantly after callosotomy, and the mean of the left parietal group was significantly different from zero. After right medial frontal lesions, circling was significantly toward the side of the lesion and remained so after callosum section. However, the left and right medial frontal lesion groups did not differ significantly. Did the cortical lesions or commissurotomy affect circling irrespective of directionality - - that is, was rotational activity affected? An analysis of variance of total circling (the sum of ipsiversive and contraversive rotations) in the postlesion and postcallostomy periods disclosed no significant effects.

Correlations of length of callosum section and lesion volume with neglect and circling In each of the groups, there was callosal sparing in some animals, and there was variation in the volumes of the lesions. Callosum-section lengths and lesion volumes were correlated with neglect and circling - - length of callosum section with postcallosotomy circling and Week 5 neglect and lesion volume with Week 1 neglect and postlesion circling and with Week 5 neglect and postcallosotomy circling. These correlations were computed within lesion groups for each neglect test and within lesion/hemisphere groups for circling. Among the sensory measures, length of callosum section and lesion volume significantly correlated with directional orienting scores only on the first postcallosum section week of the visual test (rs = 0.65, P < 0.01 and 0.49, P < 0.05 respectively). In a multiple regression of lesion size and length of callosum section on visual orienting scores in that week, only the callosum section coefficient was significant. There was a similar finding on the auditory test. None of the other regressions of lesion size and callosotomy length

142 on Week 5 neglect reached significance. Of the circling correlations, only one, between lesion volume and postcallosotomy circling in the left medial frontal lesion group, was significant (r= 0.71, P < 0 . 0 5 ) . No multiple regression yielded significant coefficients. Coupled with the absence of significant between-groups variation in length of callosum section and lesion volume, the correlational analysis makes clear that variability in surgical treatments played no systematic role.

Discussion Orienting tests Two of the cortical lesions, medial frontal and parietal, produced, as in previous reports 1,3, initially extreme biases toward the damaged side that recovered in 3-4 weeks. The medial frontal lesion resulted in slightly more severe impairments of contralateral responding. With transection of the corpus callosum after complete recovery, the ipsilateral biases reappeared, more consistently and radically in the medial frontal lesion group on each test. Indeed, contralateral somatosensory neglect did not reappear in animals with parietal lesions. Thus, like the frontally damaged monkey6 and dog 24'45, callosum section in the rat with either frontal or parietal lesions re-establishes neglect, and particularly in the case of the frontal lesion does so in visual, auditory, and somatosensory modalities. Lesions of frontal cortex and simultaneous callosotomy also produce a transient neglect6"45. The effect of callosum section must be to disrupt a balance between the two hemispheres reestablished with the return of symmetrical orienting competence. The interruption of input from the undamaged hemisphere renders the damaged one again incompetent until the balance is once more restored. Pritzel and Huston 41 have reviewed evidence of redirection to the damaged side of normally ipsilateral projections within a brief interval after unilateral cortical lesions. Among these renascent pathways are corticotectal fibres that cross to the superior colliculus of the damaged hemisphere. These newly formed connections could, as Pritzel and Huston's data intimate,

mediate recovery and might be responsible for the postcallosotomy restoration of function in the hemisphere with the lesion. Our findings of postlesion neglect, recovery, reinstatement by callosum section, and a second recovery imply that the initial return of function is accomplished by mechanisms (perhaps proliferating cortical fibres) that differ from the regrowth that might provide function after commissure section. Thus, our resuits suggest both parallel pathways in neural regeneration after unilateral cortical injury and a preferential order of recruitment after lesion and commissurotomy. There is a striking human concordance in the effects of callosal interruption. In humans, corpus callosum transection can exacerbate the effects of pre-existent unilateral lesions, but appreciable recovery may occur 37. Unlike humans with neglect syndromes22, we found no evidence of greater severity or persistence of deficit from right hemisphere parietal or frontal lesions in the rat. This contrasts with evidence of cerebral lateralization in animals, some of it cited earlier, in which there are suggestive parallels to lateralized human specializations4, 8,39,40 A possibility remaining for investigation is that the lateralization of some functions in nonhuman species may depend on hemispheric dominance that is more equally distributed between the the two sides of the brain ~5. Alternatively, more sensitive tests of sensory responsiveness may be required, or perhaps mechanisms for the direction of attention to hemispatial fields are simply not lateralized.

Circling Animals with medial frontal lesions showed the well-described ipsiversive circling tendency after their cortical lesions and although those with right lesions exhibited a somewhat stronger bias than animals with left lesions, the difference was not significant. Callosotomy did not affect the circling of rats with the medial frontal lesion. The effects of the parietal lesion were markedly lateralized, but only after section of the corpus callosum. Then, the contraversive turning that we have described before 3 appeared, but significantly only in the left parietal group. In contrast to our earlier report, the parietal lesion itself did not

143 produce differential circling, although in the animals with left hemisphere lesions the direction of circling was contraversive. The neural mechanisms of circling induced by cortical lesions are not as well understood as the mechanisms underlying circling from subcortical lesions, particularly in the nigrostriatal system ~5. Unilateral nigrostriatal lesions result in circling away from the unoperated side that is markedly increased by dopamine agonists such as amphetamine. Intact animals circle as well, after injections of dopaminergic agents and also spontaneously. In many normal animals, this circling occurs in a consistently preferred direction and is related to a hemispheric asymmetry in neostriatal dopamine concentrations 46. Preferential circling apparently reflects a basic spatial-motor competence. Rats without a consistent circling direction cannot learn to associate circling with reward 14 and fail to acquire a left-right discrimination 47. Frontal cortex is implicated in modulating circling asymmetries16. Labeled 2-deoxy-D-glucose uptake is greater in frontal cortex contralateral to the side toward which the animal circles. The asymmetry of circling in animals with parietal lesions after callosotomy is unprecedented, and its basis remains to be demonstrated. Parietal circling is contraversive, in strong contrast to the ipsiversive circling that follows other lesions 1'18'43. We previously suggested that contraversive circling in rats with parietal lesions might reflect the effects of impoverished movement of the contralateral limbs seen in parietaldamaged monkeys 36. More limited and feeble movements on one side would produce a turning bias in locomotion. A more interesting possibility is that contraversive parietal circling might depend on the interruption of corticocortical or corticostriatal connections. For example, disruption of an inhibitory parietal input to medial frontal cortex or to neostriatum could release a contralateral rotating tendency. Projections of the posterior parietal region to medial frontal cortex and to the neostriatum in the rat are well established9,27,44.

Why did parietal contraversive circling occur only in animals with left lesions ? The evidence on circling from nigrostriatal lesions suggests both a

motor and spatial involvement as noted above ~5. The present data recall the proposals by Liepmann 33"34 about the dominance of the left hemisphere in the control of skilled movement and are not inconsistent with Heilman's 2° proposal that 'visuokinesthetic motor engrams' are represented in parietal cortex of the left hemisphere. To understand the contribution of left parietal cortex to spatial-motor behaviour in this animal will require identification of asymmetrical corticocortical or corticosubcortical connections and/or biochemical inequalities between the hemispheres. We have reported that cutting the corpus callosum is necessary to see the full effects on emotionality of right parietal lesions in the rat 4. Our interpretation was that in a brain that is not strongly lateralized, compensatory activity from the intact hemisphere might mask the functional asymmetry. It is also possible that in regions of cortex as in the nigrostriatal system caUosotomy potentiates small hemispheric differences, both behavioural and neurochemicaP v. EXPERIMENT 2

The novelty of the principal circling result, together with the fact that there are sometimes failures to find functional asymmetries in nonhuman species 29, strongly argued for a replication of the circling procedures of Expt. 1. This was the purpose of Expt. 2.

Materials" and Methods Subjects, surgery and histology There were 4 groups in this experiment. Three unilateral lesion groups (parietal, medial frontal and motor) were each allocated 16 rats, 8 receiving right and 8 left lesions. An unoperated control group of 10 animals was also included. The initial body weights of the animals, housing, maintenance and handling, were identical to Expt. 1. Surgery was accomplished by the same procedures as described above. The right parietal and control groups were reduced by the death of one animal in each group. Histological examination of the brains followed

144 the same procedures as in the first experiment, with one additional step. After removal from the skull, the brains remained in 10~o formalin for one week and were then weighed. An analysis of variance on the brain weights of the lesion/hemisphere groups showed only a lesion main effect at P < 0.08. The lesion effect in a second analysis collapsing over hemisphere was significant at P < 0.05. Comparison with the control group by Dunnett's test found the mean brain weights of each lesion group to be lower than the controls at P = 0.05 or less (1-tailed tests). In addition to these analyses of lesion volumes, we again calculated the mean length of callosum section in each group. No significant lesion or hemisphere differences appeared in the analysis of variance. In the entire group of 56 animals, the mean length of section was 4.94 mm, not significantly different from the value of Expt. 1. The lesion and callosotomy reconstructions of Figs. 1 and 2 include the Expt. 2 animals. Procedure The measure of circling in Expt. 1 was exactly replicated. There was, however, a preoperative week of circling observations (3 days), and each group was tested through the fourth postlesion week. The corpus callosum was then cut in all animals, and two days later the 3-day per week measurement of circling resumed, continuing for 4 weeks. Again, weekly total scores were calculated for ipsilateral and contralateral circling from which the circling directionality scores (I-C/I + C) were derived. The 9 control animals were randomly assigned to 'right' (5) and 'left' (4) subgroups. Results

The data were treated by analysis of covariance with preoperative circling as the covariate. Four significant effects appeared. These were Lesions (F3.47 = 5.33, P < 0.003), Hemisphere (F1,47 = 10.07, P < 0.003), Lesion x Operations ((F3,48= 6.72, P<0.001) and Hemisphere × Operations (FL48 = 10.27, P < 0.002). Individual comparisons with Tukey's H S D procedure (~ = 0.05) showed 4 differences. After section of

the corpus callosum, the left parietal lesion group turned contraversively, while right parietal animals showed no preference (M L = - 0 . 4 5 ; M R = 0.03). In right medial frontal lesion animals, there was a significant increase in contraversive circling after callosotomy (Mpostlesio, = 0.07; Mpostcallosotorny--0.55 ). After callosum section, the left motor cortex lesion group circled contraversively while right motor cortex lesion animals did not (ML = - 0.42; M R -- 0.10). Animals randomly assigned to the 'right' control group tended to turn ipsiversively (M = 0.25); the trend among 'left' control animals was contraversive (M -- - 0.16). Compared to a zero circling bias, 3 groups showed significant circling preferences after callosum section: left parietal (t = 2.92, P < 0.01), right medial frontal (t = 3.56, P < 0.001) and left motor (t = 2.66, P < 0.01). The mean directionality scores of the lesion/hemisphere groups are shown in Fig. 5. An analysis of total circling activity gave significant effects for Lesion (F3,47--2.77, P < 0.05) and Lesion x Hemisphere (F3,47 = 3.67, P < 0.02). Comparison of the means produced 4 differences at P < 0.05. Over both postlesion and postcallosotomy periods, animals with medial frontal lesions circled more than control animals. Overall, right medial frontal animals circled more than right parietals, left parietals circled more


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Fig. 5. Postlesion and postcallosum section circling by each lesion/hemisphere group. The plotted values are adjusted means from the covariance analysis, Circling direction is expressed as in Fig. 4. The T above or below each bar is the S.E,M.

145 than 'right' controls, and animals with right medial frontal lesions circled more than 'left' controls. These few differences do not constitute a coherent pattern of non-directional circling activity. Discussion

The data of Expt. 2 provide a close replication of the parietal circling results of Expt. 1 : a strong contraversive bias after section of the corpus callosum in animals with left hemisphere lesions and no postcallosotomy bias in animals with right lesions. This experiment did not find the usual post-medial frontal lesion ipsiversive circling tendency, but it did appear after callosum section, particularly in the right-damaged animals. Further, the combined postlesion-postcallosotomy circling of the entire group of medial frontal animals was significantly directional toward the lesion side (t = 2.41, P < 0.05). Unlike the first experiment, the motor cortex lesion groups showed the same circling direction difference as the parietal groups. In view of the fact that the motor cortex lesions of Expt. 2 did not differ from Expt. l's (Lesion P = 0.16, n.s.; Hemisphere P = 0.80, n.s.), a possibility to consider is that this resemblance may have resulted from interruption of motor commands originating in left parietal cortex and transmitted to motor cortex for execution as in Liepmann's 33 schema (see Heilman and Roth? l) or, conceivably, either lesion could result in disconnection from the basal ganglia as suggested earlier. Taken together, the results of Expts. 1 and 2 point to a spatial-motor asymmetry resulting from parietal neocortical lesions and subsequent hemisphere bisection. They imply a dominant role for left parietal cortex in the processes that underlie the asymmetry, and they strongly argue for experiments investigating both anatomical differences between the hemispheres and other behavioral consequences of lateralized parietal lesions.


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