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Pregnancy induced an intrahemispheric redistribution of transcallosal flows mainly in the projection areas of the cat cortex. Functional interhemispheric asymmetry (FIA) of transcallosal evoked responses (TEPs) was determined by interhemispheric distribution of negative-positive responses in non-pregnant cats, whereas in pregnant ones it was determined by the asymmetry of positive-negative TEPs. The right hemisphere played the major part in the changes of the TEPs' amplitude parameters, and the left one--in temporal parameters.

In 38 anesthetised cat of both sexes, homotopic transcallosal responses (HTR) were studied in somatosensory, visual, auditory and temporal cortex of the brain. High penetration in cases of interhemispheric asymmetry was more specific for males and found mostly in cortical projection areas. For the functional interhemispheric asymmetry of the HTR amplitude-temporal parameters a high degree of expressiveness was characteristic in female cats. Possible reasons of these phenomena are discussed.

In 38 anesthetized cats, homotopic transcallosal responses (TCR) were studied in somatosensory, visual, auditory, sensomotor and temporal areas. The TCR were found to be functionally asymmetrical. Some specific features of the TCR asymmetry were revealed in males and females. The asymmetry was of the same direction in sensomotor and somatosensory, visual and temporal areas, whereas it was opposite in its direction in visual and auditory areas. The changes of the TCR asymmetry seem to be due to formation of a new system of functional interzonal intra- and interhemispheric connexions.

Interzonal transcallosal EPs occurring in response to stimulation of visual or auditory cortex of opposite hemisphere, were recorded in the cat sensomotor cortex. Visual-motor EPs had mainly an initial positivity whereas audio-motor those were mainly initially negative. Latencies of the former EPs were longer. Negative--positive visual-motor EPs had a greater amplitude than their synphasic audio-motor ones. Initially positive EPs had a greater amplitude in stimulation of A1 rather than 19. The EPs revealed an interhemisphere asymmetry: audio-motor those were greater in the left hemisphere mainly in females. The specifics of the asymmetry seems to be due to an interzonal asymmetry of transcallosal flows of projectional and associative cortical areas.

Studies of the topography of homotopic transcommissural evoked potentials in 85 symmetrical points of the cat motor cortex, prior to and after section of the corpus callosum, revealed the functional areas as follows: acommissural (53% of the total area), purely callosal (23%), mixed (22%), and purely extracallosal (2%). The summation and reciprocal interrelationships between the callosal and extracion flows spreading over the extracallosal system.

In 10 immobilized cats, the interaction between transcallosal potentials and responses to stimulation of the n. ischiadicus was studied in 85 points of the sensomotor cortex. The interaction effect of these responses is determined by their configuration in a given point as well as by time interval between them. The interaction has an obvious spatial-temporal character. Facilitating influences localized mainly in the area of overlapping of EP focuses whereas peripheral areas of the cortex were under a greater depressing effect of the transcallosal flow. The intermediate areas were subject alternately to facilitating and depressing influences. The above effects of interaction seem to reveal a modulating function of transcallosal flow which actualizes the concentration and contrasting of the excitation spreading over ascending projections.

Comparative study of the transcallosal EPs in the cat parietal and sensomotor cortical areas revealed that the parietal EPs were less stable and more uniform than the sensomotor ones. Distribution of the EPs over the sensomotor cortex's surface is more regular as compared with the parietal area. Focus of the transcallosal EPs maximal activity coincided usually with the parietal point symmetrical to stimulation. An individual form of functional interhemisphere asymmetry was revealed for the parietal cortical potentials. An inversion of positive-negative transcallosal EPs was shown to exist in the parietal area's IV layer. The data obtained seem to corroborate the opinion of different functional purposes of the areas under study which can be revealed by the interhemisphere interaction between these areas as well.

Interregional transcallosal responses to the stimulation of auditory cortex occurred throughout the surface of parietal cortex in immobilized cats and were either positive-negative or negative-positive. The former EPs had a longer latency and a greater total amplitude of both components. The negative-positive EPs disappeared after section of the callosal body whereas the positive-negative EPs were but insignificantly altered. The potentials were characterized by a functional interhemisphere asymmetry. The right hemisphere dominated by the average amplitude of the negative phase of negative-positive responses. The amplitude of early positive component dominated over the right hemisphere in males and over the left one in females. Delayed negative wave was greater over the right hemisphere in both sexes. The delayed components of the EPs had a significantly shorter latency of the peak in the dominating hemisphere.

In acute experiments on anesthetized cats, variability of the unit activity in the field 18 of the visual cortex was studied. Both the spontaneous activity and the evoked activity by series of different light stimuli, were analysed. The data obtained show that callosotomy does not affect the variability of the spontaneous unit activity in the parastriate cortex. However, it does decrease the variability of the unit activity qualitative response (excitation--inhibition) to stimulation. The callosotomy increases variability of the predominant input (either ipsi--or contralateral) in regard to the mean spontaneous activity prior to and after a series of stimuli and decreases it in regard to the mean spontaneous unit activity for the whole period of study.

The direct cortical responses and transcallosal potentials were recorded in 170 symmetrical points of the cat motor cortex during the electrical stimulation of one hemisphere. The area of the direct responses was larger than that of transcallosal responses. The latter was concentrated in the center of the motor area, while the direct responses were located in peripheral areas. Thresholds for the direct responses were lower than those for the transcallosal responses. Variation of the strength of the stimulation led to the changes of localization of the EPs in both hemispheres. Callosotomy abolished or decreased the transcallosal potentials and increased or decreased the direct cortical potentials. This suggests the principle of the common pathway for the callosal system activity.

The interaction between transcallosal potentials and responses to stimulation of the contralateral sciatic nerve was studied in symmetrical areas of the cat sensorimotor cortex. The interacting responses and the resulting EPs were characterized by a functional interhemisphere asymmetry of an individual form. In transcallosal conditioning with 10-ms interstimuli intervals, the positive components of the EPs prevailed in the left hemisphere. The characteristics of the interhemisphere asymmetry were determined by the animal sex: the spatial--temporal interaction of the flows had different features in different hemispheres, was characterized by unequal markedness of asymmetry of the EP components as well as by opposite character of the asymmetry in animals of different sex. The characteristics of the EP interhemisphere asymmetry seem to be determined by an interaction of dominant foci occurring under these conditions in each of the hemispheres.

In acute experiments on anesthetized cats, neuronal activity in the parastriate cortex was studied. Character of distribution of the firing rate mean frequency and the total energy of firing during mono- and binocular stimulation prior to (89 neurons) and after (67 neurons) callosotomy, were estimated. The spontaneous neuronal activity prior to callosotomy was found to be much higher than after it. The light stimulation in intact corpus callosum enlarged the range of frequencies of the neuronal activity. During the stimulation, activity of the majority of neurons was focused in a narrow frequency range, i.e. the synchronization occurred. The total energy of neuronal reactions in intact corpus callosum was higher than after callosotomy. The relative reactivity of the neurons was increased after callosotomy.

In immobilized cats, transcollosal (TCR) and direct cortical responses (DCR) from 170 points of the motor cortex, were recorded. The TCR maximal activity focus (MAF) is shifted in relation to the point symmetrical to stimulation (PSS) in oral--caudal direction. Ablation of the cortex around the stimulating electrode is followed by disappearance of TCR in MAF and of PSS and DCR beyond the ablation limits. Ablation of PSS entails diminishing of TCR in MAF. Transection of the corpus callosum eliminates TCR in MAF and PSS. The MAF shift seems to be due to a relatively strengthened convergence of several excitation flows in PSS, the flows spreading over the callosal fibers and intrahemisphere projections of both hemispheres.