Comparison of electrical thresholds for evoking movements from sensori-motor areas of the cat cerebral cortex and its relation to motor training

Motor maps and electrical thresholds for evoking movements from motor areas of the cerebral cortex were evaluated in normal cats by using intracortical microstimulation techniques. Stainless steel chambers were implanted over craniotomies in adult cats trained to perform reaching and retrieval movements with their forelimbs. Prehensile motor training was continued and movement performance monitored for about 6–10 weeks during which the cortex was progressively explored with sharp tungsten electrodes inserted into cortical gyri (anterior and posterior sigmoid, and coronal) and the banks of sulci (cruciate, presylvian and coronal). Twice weekly, under light general anaesthesia, 3–4 tracks were made in either hemisphere till about 50 tracks were made in each hemisphere. Mean thresholds for evoking forelimb movements from different cytoarchitectonic areas (4γ, 4δ, 6aγ and 3a) were compared and no consistent or significant differences were observed between the different areas. In the animals (4/6) which used either forelimb to perform the tasks, there were no consistent differences in the mean thresholds for evoking forelimb movements from the two hemispheres. However, in 2 animals, which used their right forelimbs predominantly or exclusively to perform all the tasks, mean thresholds for evoking forelimb movements was significantly higher in areas 4γ and 6aγ of the left hemisphere (compared to the right); no consistent differences in the mean thresholds for evoking hindlimb or facial movements were observed between the two hemispheres. These findings suggest that ICMS thresholds for evoking forelimb movements may be similar in different sensorimotor areas of the cat cerebral cortex, and these thresholds could be influenced by motor training.     

Changes in electrical thresholds for evoking movements from the cat cerebral cortex following lesions of the sensori-motor area

We evaluated motor maps in the cerebral cortex and motor performance in cats before and after lesions of the forelimb representation in the primary motor area. After the lesion there was a reduction in the use of the affected forelimb and loss of accuracy in prehension tasks using the forelimb; some recovery occurred during the mapping study. Electrode tracts and lesion sites were located in cytoarchitectonically identified cortical areas 4gamma, 4delta, 6aalpha, 6agamma, 3a. The lesions were mainly in area 4gamma. In the lesioned hemisphere there were many points around the lesion site (in areas 4gamma and 3a) from which movements could not be evoked. In some areas distant from the lesion site (e.g. area 6agamma) the mean thresholds for evoking forelimb movements were significantly elevated. Mean thresholds for evoking hindlimb and facial movements were not different from before. In the contralateral hemisphere mean thresholds for evoking forelimb, but not hindlimb or facial movements, were significantly elevated in several sensorimotor areas (area 4gamma, 6agamma and 3a). Mean thresholds for evoking forelimb movements appeared to progressively increase during the time of study. Minimal currents required to evoke forelimb movements from the cerebral cortex increase (possibly progressively) following a lesion of the forelimb representation in the primary motor area, affecting many interconnected motor areas in the hemispheres ipsilateral and contralateral to the lesioned site. This increase in thresholds may play a role in the changes in cortical control of the affected and contralateral limbs following brain lesions and explain the increased sense of effort required to produce movements.     

Changes in electrical thresholds for evoking movements from the cat cerebral cortex following lesions of the sensori-motor area

We evaluated motor maps in the cerebral cortex and motor performance in cats before and after lesions of the forelimb representation in the primary motor area. After the lesion there was a reduction in the use of the affected forelimb and loss of accuracy in prehension tasks using the forelimb; some recovery occurred during the mapping study. Electrode tracts and lesion sites were located in cytoarchitectonically identified cortical areas 4γ, 4δ, 6aα, 6aγ, 3a. The lesions were mainly in area 4γ. In the lesioned hemisphere there were many points around the lesion site (in areas 4γ and 3a) from which movements could not be evoked. In some areas distant from the lesion site (e.g. area 6aγ) the mean thresholds for evoking forelimb movements were significantly elevated. Mean thresholds for evoking hindlimb and facial movements were not different from before. In the contralateral hemisphere mean thresholds for evoking forelimb, but not hindlimb or facial movements, were significantly elevated in several sensorimotor areas (area 4γ, 6aγ and 3a). Mean thresholds for evoking forelimb movements appeared to progressively increase during the time of study. Minimal currents required to evoke forelimb movements from the cerebral cortex increase (possibly progressively) following a lesion of the forelimb representation in the primary motor area, affecting many interconnected motor areas in the hemispheres ipsilateral and contralateral to the lesioned site. This increase in thresholds may play a role in the changes in cortical control of the affected and contralateral limbs following brain lesions and explain the increased sense of effort required to produce movements.     

Patterns of spatio-temporal correlations in the neural activity of the cat motor cortex during trained forelimb movements

In order to study how neurons in the primary motor cortex (MI) are dynamically linked together during skilled movement, we recorded simultaneously from many cortical neurons in cats trained to perform a reaching and retrieval task using their forelimbs. Analysis of task-related spike activity in the MI of the hemisphere contralateral to the reaching forelimb (in identified forelimb or hindlimb representations) recorded through chronically implanted microwires, was followed by pairwise evaluation of temporally correlated activity in these neurons during task performance using shuffle corrected cross-correlograms. Over many months of recording, a variety of task-related modulations of neural activities were observed in individual efferent zones. Positively correlated activity (mainly narrow peaks at zero or short latencies) was seen during task performance frequently between neurons recorded within the forelimb representation of MI, rarely within the hindlimb area of MI, and never between forelimb and hindlimb areas. Correlated activity was frequently observed between neurons with different patterns of task-related activity or preferential activity during different task elements (reaching, feeding, etc.), and located in efferent zones with dissimilar representation as defined by intracortical microstimulation. The observed synchronization of action potentials among selected but functionally varied groups of MI neurons possibly reflects dynamic recruitment of network connections between efferent zones during skilled movement.     

Thermostimulation-Induced Modulation of Conditioned Reflex Movement-Related Reactions of Cortical Neurons of the Cat

In awake cats trained to perform a food-procuring conditioned operant reflex (placing movement), we studied impulse reactions of 86 neurons of the motor cortex (field 4) related to realization of the above movements. As conditioning stimuli (CS) initiating the reflex, we used either non-noxious electrocutaneous stimulation (ECS) of the contralateral forelimb or an acoustic stimulus (sound click). Impulsation of cortical neurons was recorded under conditions of (i) isolated presentation of the CS (control), (ii) presentation of the CS (either ECS or acoustic stimulus) combined with thermostimulation (heating with a miniature electric bulb) of the skin of the working forelimb, and (iii) the same, but with stimulation of the resting forelimb. When we recorded spike activity of neurons within the projection motor zone of the resting limb subjected to ESC, alternating thermostimulation of both forelimbs resulted in considerable intensification and an increase in the duration of neuronal responses, especially in cases where thermostimulation was applied to the working limb ipsilateral to the recording site (a two- to threefold increase). When spike reactions were recorded within the motor cortex of the working forelimb, thermostimulation resulted in a considerable increase in the intensity of these reactions and a decrease in their latency, but only when such stimulation was applied to the working forelimb. Thermostimulation of the resting (ipsilateral, subjected to ESC) limb evoked opposite effects (the intensity of neuronal reactions dropped). In both situations, placing movements remained within the control limits. When sound click was used as a distant CS, thermostimulation of the working limb enhanced neuronal responses, increased their duration by 50-100%, and also increased the time of forestalling of the movement initiation by spike neuronal reactions. Thermostimulation of the resting forelimb in this situation also suppressed neuronal reactions. We conclude that foreign stimulations directed toward modifications of the receptor model of the operant reflex experimental situation formed in the animal result in a decrease in the intensity of the spike responses of field-4 neurons and prolongation of the latencies of these responses, while stimulations promoting the inflow of afferent information to the cortical projection of the working limb evoke opposite effects, an increase in the intensity of neuronal spike responses and a decrease in their latencies.     

A Comparison of the Ipsilateral Cortical Projections to the Dorsal and Ventral Subdivisions of the Macaque Premotor Cortex

The cortical connections of the dorsal (PMd) and ventral (PMv) subdivisions of the premotor area (PM, lateral area 6) were studied in four monkeys (Macaca fascicularis) through the use of retrograde tracers. In two animals, tracer was injected ventral to the arcuate sulcus (PMv), in a region from which forelimb movements could be elicited by intracortical microstimulation (ICMS). Tracer injections dorsal to the arcuate sulcus (PMd) were made in two locations. In one animal, tracer was injected caudal to the genu of the arcuate sulcus (in caudal PMd [cPMd], where ICMS was effective in eliciting forelimb movements); in another animal, it was injected rostral to the genu of the arcuate sulcus (in rostral PMd [rPMd], where ICMS was ineffective in eliciting movements). Retrogradely labeled neurons were counted in the ipsilateral hemisphere and located in cytoarchitectonically identified areas of the frontal and parietal lobes. Although both PMv and PMd were found to receive inputs from other motor areas, the prefrontal cortex, and the parietal cortex, there were differences in the topography and the relative strength of projections from these areas.

There were few common inputs to PMv and PMd; only the supplementary eye fields projected to all three areas studied. Interconnections within PMd or PMv appeared to link hindlimb and forelimb representations, and forelimb and face representations; however, connections between PMd and PMv were sparse. Areas cPMd and PMv were found to receive inputs from other motor areas—the primary motor area, the supplementary motor area, and the cingulate motor area—but the topography and strength of projections from these areas varied. Area rPMd was found to receive sparse inputs, if any, from these motor areas. The frontal eye field (area 8a) was found to project to PMv and rPMd, and area 46 was labeled substantially only from rPMd. Parietal projections to PMv were found to originate from a variety of somatosensory and visual areas, including the second somatosensory cortex and related areas in the parietal operculum of the lateral sulcus, as well as areas 5, 7a, and 7b, and the anterior intraparietal area. By contrast, projections to cPMd arose only from area 5. Visual areas 7m and the medial intraparietal area were labeled from rPMd. Relatively more parietal neurons were labeled after tracer injections in PMv than in PMd. Thus, PMv and PMd appear to be parts of separate, parallel networks for movement control.     

Influence of acute cerebellar lesions on somatosensory evoked potentials (SEPs) in cats.

We studied the effect of acute unilateral cerebellar lesions on the cerebello-thalamo-cortical projection in cats. The lesions were classified into two groups according to their extent. In group I the lesion only covered the cerebellar cortex, while in group II both the cerebellar cortex and deep cerebellar nuclei were removed. Early (short-latency) and late (long-latency) waves, evoked by an electrical stimulation of a forelimb, were collected contralateral to the stimulated leg hemisphere. Pre- and postsurgery recordings from primary and non-primary (motor and parietal) cortices were compared. Cerebellar impairment had a strong influence on discharges of all the considered cortical areas. Early non-primary and primary responses increased in group I and remained unchanged in group II. Late somatosensory evoked potentials components were suppressed in both groups. An inhibitory influence of the cerebellar cortex on the thalamo-cortical projection was confirmed. Changes within the primary sensory cortex may suggest an engagement of that area in the compensation process of cerebellar dysfunction shortly after cerebellar lesion. An alteration in the unaffected hemisphere activation indicate that the spino-cerebellar and cerebello-cortical inputs, responsible for somatosensory evoked potentials generation, are regulated through contralateral and ipsilateral pathways. These pathways are unmasked by cerebellar lesion.     

Unitary activity in the cat motor cortex during a conditioned postural adjustment reflex

Unitary activity in the motor cortex (area 4) during a conditioned postural adjustment reflex was investigated in cats. Responses of the overwhelming majority of neurons connected with conditioned-reflex placing movements were activational in type. They consisted of several components and preceded the movements themselves by 50–600 msec. During realization of incorrect responses to presentation of a differential stimulus and of "spontaneous" interstimulus movements, the unitary responses were similar in direction but differed in their lower intensity and, in most cases, they appeared simultaneously with these movements. In the course of extinction both the conditioned-reflex movements and the corresponding unitary responses disappeared simultaneously. The technique of formation of a conditioned postural adjustment reflex suggested in this paper can be used to from natural, well-coordinated forelimb movements in animals in response to conditioned stimulation which are necessary initial components of more complex behavioral motor responses.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 6, pp. 745–753, November–December, 1984.     

Connections of the representational area of the distal section of the forelimb in the rat sensorimotor cortex

Using horseradish peroxidase, studies have been made on the distribution of retrogradely labeled nervous cells in the sensorimotor cortex of rats. The enzyme was injected into electrophysiologically identified zone of representation of the distal part of the forelimb in areas S2 and S1. It was found that this zone in S2 contains afferent connections mainly from representation of the same extremity in S1 and only a few afferents from other areas of S1, S2 and M1 of the same hemisphere. Single labeled neurones were found in areas S2, S1 and M1 of the contralateral hemisphere. Representation of the forelimb in S1 receives mainly cortical afferents from the same region of S1 and from single cells of homologous zones S2 of the same and S1 of the contralateral hemisphere. Connections from S1 to S2 are more numerous than the opposite ones. In contrast to cats and monkeys, in rats afferent cortical fibers to zone S2 pass not only from the third layer, but also from the fifth and sixth layers of the cortex. It is suggested that during progressive development of the neocortex in mammals, the increase in the degree of separation of neurones (which give origin to corticofugal and cortical connections) among different layers of the cortex takes place.     

The distribution of corticocortical, thalamocortical, and callosal inputs on identified motor cortex output neurons: mechanisms for their selective recruitment.

Motor cortex neurons were identified antidromically in anesthetized cats by their axonal projections to one of six targets: (1) somatosensory cortex, (2) opposite motor cortex, (3) red nucleus, (4) lateral reticular nucleus, (5) spinal cord, and (6) ventrolateral thalamus. Three inputs to motor cortex were tested for their influences on the identified cortical efferent neurons. The tested inputs originated from ipsilateral somatosensory cortex, opposite motor cortex, and ventral thalamus. Subthreshold effects of input pathways were detected by monitoring latency variations of antidromic responses. The three afferent sources, when activated by electrical stimulation, were not equally effective on motor cortex neurons. Ipsilateral corticocortical and thalamocortical excitation were found for the majority of neurons; the influenced proportions ranged from 55% to 100%, according to the target of the output neurons. Effects from the opposite hemisphere were found for only 5% to 30% of the neurons in the same projection classes. Many neurons (36 of 81, or 44%) were excited from more than one source, but few (5 of 37, or 14%) were influenced by all three possible sources of input, even in small regions of cortex innervated by all three of the inputs. Among 19 electrode tracks where all three inputs were present, there were only 2 tracks where all the neurons shared the same combination of inputs. Even for neurons in closest anatomical proximity ("clusters"), it was unusual (only 7 of 25 clusters) for all the neurons to have the same input pattern. Among the seven clusters where all the neurons shared the same input pattern, five of the clusters projected to the same target. These variable combinations of inputs to motor cortex neurons support the conclusion that efferent neurons could be recruited selectively from separate cortical layers or from within clusters of nearby neurons, according to the target of their axonal projection.     

Sexual differences in the interhemispheric asymmetry of the transcallosal responses of the associative areas in the cat neocortex

Sex differences of hemisphere asymmetry of homo- and heterotopic transcallosal responses in association cortex of 48 cats (24 male and 24 female) immobilized by tubocurarine have been studied by means of topographic EPs recordings in both hemispheres. In males left hemisphere dominates by the amplitude of homotopic and positive wave of heterotopic EPs and right hemisphere dominates by the amplitude of negative wave of heterotopic sensorimotor cortex EPs. The individual asymmetry of EPs has been observed in sensomotor cortex of females and in parietal cortex of animals of both sex. The interhemispheric asymmetry is expressed distinctly in females than in males. It is concluded that sex dimorphism is present in functional organization of associative system of (callosal and intracortical) connections in cat's neocortex projection and association areas which means its more expressed hemisphere lateralization in males with more expressed interhemispheric asymmetry of functional transcallosal connections in females.     

The Distribution of Corticocortical,Thalamocortical, and Callosal Inputs on Identified Motor Cortex Output Neurons: Mechanisms for Their Selective Recruitment

Motor cortex neurons were identified antidromically in anesthetized cats by their axonal projections to one of six targets: (1) somatosensory cortex, (2) opposite motor cortex, (3) red nucleus, (4) lateral reticular nucleus, (5) spinal cord, and (6) ventrolateral thalamus. Three inputs to motor cortex were tested for their influences on the identified cortical efferent neurons. The tested inputs originated from ipsilateral somatosensory cortex, opposite motor cortex, and ventral thalamus. Subthreshold effects of input pathways were detected by monitoring latency variations of antidromic responses.

The three afferent sources, when activated by electrical stimulation, were not equally effective on motor cortex neurons. Ipsilateral corticocortical and thalamocortical excitation were found for the majority of neurons; the influenced proportions ranged from 55% to 100%, according to the target of the output neurons. Effects from the opposite hemisphere were found for only 5% to 30% of the neurons in the same projection classes.

Many neurons (36 of 81, or 44%) were excited from more than one source, but few (5 of 37, or 14%) were influenced by all three possible sources of input, even in small regions of cortex innervated by all three of the inputs. Among 19 electrode tracks where all three inputs were present, there were only 2 tracks where all the neurons shared the same combination of inputs. Even for neurons in closest anatomical proximity (“clusters”), it was unusual (only 7 of 25 clusters) for all the neurons to have the same input pattern. Among the seven clusters where all the neurons shared the same input pattern, five of the clusters projected to the same target. These variable combinations of inputs to motor cortex neurons support the conclusion that efferent neurons could be recruited selectively from separate cortical layers or from within clusters of nearby neurons, according to the target of their axonal projection.     

Control of breathing and brain activation in human subjects seen by functional magnetic resonance imaging.

Functional magnetic resonance imaging (fMRI) was used to demonstrate the brain activation during transition from unconscious to conscious breathing in seven healthy human subjects. In right-handed volunteers, the activated areas were found in both hemispheres. The medial part of the precentral gyrus (area 4) was constantly activated in the left hemisphere. Additional activated areas were demonstrated in the premotor cortex and in the posterior parietal cortex. The activated cortical sites exhibited analogous distribution in the right hemisphere. In two out of the seven subjects. activated sites were also observed in the cerebellar hemispheres, and in the lentiform and caudate nuclei.     

Disruption of food-getting movements after removal of the motor or premotor zone of the cerebral cortex in cats]

Instrumental food-procuring movements were studied in cats before and after unilateral or bilateral ablation of the motor or premotor cortical area. It is shown that unilateral impairment of the motor area affects the strength and accuracy of movements of the contralateral fore-leg, whereas the ablation of the premotor area leads to a slowing down of movements and breaking of a goal-directed movement into separate components. Bilataral ablation of the motor area irreversibly abolished the instrumental reflex. The ablation of the premotor cortex destroyed the animal's reaction to the sound signal, but food-procuring movements of the fore-legs were disturbed only temporarily. The obtained data are discussed on the basis of the concept that in cats the above cortical areas play different roles in the organization of goal directed behaviour.     

Topographic organization of baboon primary motor cortex: face, hand, forelimb, and shoulder representation

(1) The fine details of the motor organization of the forelimb, face, and tongue representation of the baboon (Papio h. anubis) primary motor cortex were studied in four adult animals, using intracortical microstimulation (ICMS). (2) A total of 293 electrode penetrations were made. ICMS was delivered to 10,052 sites, and of these, 6,186 sites were verified to have been located within the grey matter. Motor effects were evoked from 30% of these sites. (3) The baboon motor cortex is confined, in large part, to the cortical tissue lying along the anterior bank of the central sulcus. When the electrode penetrations were confined to the precentral gyrus, few sites were capable of evoking movement when stimulated by currents of 40 microA or less. (4) The details of the motor maps varied among the four animals; nonetheless, a general topographic organization existed, with the tongue musculature being represented most laterally, followed by a medial progression of the face, digits, wrist, forearm, and shoulder. Within the representation of a given body part, the muscles were organized as a mosaic, wherein the same muscle was multiply represented. (5) A zone of unresponsive cortex was observed to lie consistently between the face and forelimb representation in all four animals. Repeated electrode penetrations within the unresponsive zone failed to elicit muscle contractions even with stimulating currents as high as 80 microA. (6) Our results suggest that the baboon motor cortex is topographically organized; however, embedded within this overall pattern lies a fine-grained mosaic incorporating multiple representations of the same muscle.     

Asymmetry of pectoral flipper use in the orca <Emphasis Type="Italic">Orcinus orca</Emphasis> (Linnaeus, 1758) from the Avachinskii Bay (Eastern Kamchatka)

Asymmetrical forelimb use is characteristic of many mammalian species. However, little is known about this phenomenon in cetaceans. We examined the asymmetry in pectoral flipper use by the wild orca Orcinus orca (Linnaeus, 1758). During observations from motor boats in the Avachinskii Bay (East Kamchatka) we recorded the use of flippers in slapping the water by individually identified resident (fish-eating) orcas. The studied individuals mostly used their right flipper. The revealed right-sided bias presumably reflects the pivotal role of the left brain hemisphere in the control of forelimb movements, which is consistent with the data that have been obtained for other cetacean species.     

Organization of the spatial-temporal relationships of cerebral electrical processes in children during exposure to verbel commands]

An EEG cross-correlation analysis has shown that in children aged four to five years higher sensory analysis of verbal commands and their meaning was reflected in the nature of synchronous interactions between oscillatory processes and their spatial-temporal patterns. At the moment of perception of the command "listen" highly synchronous synphasic relations were recorded between biopotentials in the associative infero-parietal cortex and projection temporal centres of the left hemisphere. Oscillations of the parietal areas preceded the rhythms of the occipital, motor and frontal lobes in the left hemisphere; slow oscillations with a 3 osc/sec frequency predominated, and the intensity of the periodic processes increased. The command "look" evoked a high degree of synchronous synphasic relations of biopotentials in the parietal-occipital cortical parts of both hemispheres; oscillations with 6 osc/sec frequency predominated; their intensity rose; synphasic relations of oscillations in parietal and motor and temporal centres grew more manifest, while the rhythmic activity in the parietal zones preceded the potentials in the frontal lobes of both hemispheres.     

Sympathetic cholinergic nerve contributes to increased muscle blood flow at the onset of voluntary static exercise in conscious cats

We examined whether a sympathetic cholinergic mechanism contributed to increased blood flow of the exercising muscle at the onset of voluntary static exercise in conscious cats. After six cats were operantly conditioned to perform static bar press exercise with a forelimb while maintaining a sitting posture, a Transonic or pulsed Doppler flow probe was implanted on the brachial artery of the exercising forelimb, and catheters were inserted into the left carotid artery and jugular vein. After the baseline brachial blood flow and vascular conductance decreased and became stable in progress of postoperative recovery, the static exercise experiments were started. Brachial blood flow and vascular conductance began to increase simultaneously with the onset of exercise. Their initial increases reached 52 +/- 8% and 40 +/- 6% at 3 s from the exercise onset, respectively. Both a sympathetic ganglionic blocker (hexamethonium bromide) and atropine sulfate or methyl nitrate blunted the increase in brachial vascular conductance at the onset of static exercise, whereas an inhibitor of nitric oxide synthesis (N(omega)-nitro-l-arginine methyl ester) did not alter the increase in brachial vascular resistance. Brachial blood flow and vascular conductance increased during natural grooming behavior with the forelimb in which the flow probe was implanted, whereas they decreased during grooming with the opposite forelimb and during eating behavior. Thus it is likely that the sympathetic cholinergic mechanism is capable of evoking muscle vasodilatation at the onset of voluntary static exercise in conscious cats.     

Role of different projection areas of the motor cortex in reorganization of the innate head-forelimb coordination in dogs

Dogs were trained to perform the forelimb tonic flexion in order to lift a cup with meat from a bottom of the foodwell and hold it during eating with the head bent down to the cup. It is known that conditioning of the instrumental reaction is based on reorganization of the innate head-forelimb coordination into the opposite one. In untrained dogs, the forelimb flexion is accompanied by the anticipatory lifting of the head bent down to the foodwell. The following lowering of the head leads to an extension of the flexed forelimb. Tonic forelimb flexion is possible if the head is in the up position. Simultaneous holding of the flexed forelimb and lowered head providing food reinforcement is achieved only by learning. It was shown earlier that the lesion of the motor cortex contralateral to the "working" forelimb led to a prolonged disturbance of the elaborated coordination and reappearance of the innate coordination. In the present work we studied the influence of local lesions of the projection areas in the motor cortex, such as a "working" forelimb area, bilateral representation of the neck, and the medial part of the motor cortex, on the learned instrumental feeding reaction. It was found that only the lesion of the forelimb but not neck projection led to a disturbance of the learned head-forelimb movement coordination.     

The size of cells providing interhemispheric and intrahemispheric connections in the visual cortex of binocular vision-impaired cats

The size (somatic area) of 658 cells located in layers 2/3 of cortical areas 17, 18 of both hemispheres in intact monocularly deprived and bilateral strabismic cats was measured. These cells were retrogradely labelled after injections of horseradish peroxidase into ocular dominance columns in areas 17, 18. In all groups of cats, the mean somatic area of callosal cells was significantly larger than the mean somatic area of intrahemispheric cells. It was found that the mean somatic area of callosal cells was increased by 26.6% in monocularly deprived cats and by 20.2% in strabismic cats in relation to the mean somatic area of callosal cells in intact cats. In addition, the mean somatic area of intrahemispheric cells in monocularly deprived cats was indistinguishable from the mean somatic area of intrahemispheric cells in strabismic cats and in intact cats. It is concluded that early binocular vision impairments produce enlargement of callosal cells' size in the visual cortex.