Interhemispheric transmission time and visual eccentricity

Abstract

The effects of cancellation of both interhemispheric callosal transfer and interocular interactions, were studied in early monocularly deprived cats. The main purpose of this study was therefore to prove whether unilateral hemispheric dominance would result under these conditions and to what extent each hemisphere will be functionally independent. Secondly, we have attempted to establish such an experimental model physiologically, on the single cell level. Interhemispheric transfer was surgically canceled by sagittal transection of the corpus callosum. In addition, the ocular projections were separated by sagittal transection of the optic chiasm in the transbuccal approach. This condition had practically induced visual split brain condition in these cats. These manipulations were carried out concurrently with monocular deprivation (SBDK group) which was surgically done by eye closure during the critical period of development of the visual system. Thus, the hemisphere ipsilaterally to the visually deprived eye had developed under conditions of deficient visual experience while the hemisphere ipsilaterally to the normal eye had developed under conditions of unaltered visual experience. A group of cats (SBK) similarly operated but equally binocularly exposed during development was served as controls. In addition, adult cats similarly operated during adulthood either chronically or acutely were studied to evaluate the effects of interhemispheric and interocular separation. Other groups of cats were also studied for comparison, and included sham operated and normal adult cats. At adulthood, electrophysiological studies were done on these cats, in which action potentials were extracellularly recorded from single cells in the visual cortex (area 17–18 boundary) following anesthesia and paralysis. Stimulation was carried out manually and by a computer driven optical system, presenting on a tangent screen light bars at various spatial positions, orientations and directions. Receptive fields were thus mapped for all neurons and their dimensions and eccentricities were measured. The responsiveness, ocular dominance and other parameters were also studied for these cells. The results in the early deprived cats and in their controls, had shown a full separation between the two hemispheres, as reflected in the almost absolute ipsilateral eye responsiveness ( 97.0% cells). In comparison, in the sham operated and in the normal control cats only minor proportions of cells (13.0–18.7%) have been found as ipsilaterally and monocularly driven, showing almost full interhemispheric and interocular interaction. The main difference, however, in the results between the early monocularly deprived cats and their controls is that in the first group the two hemispheres were asymmetric concerning the amount of visual activation and in the second one they were very symmetric. This is shown by the fact that 67.9% of the cells were visually responsive in the normal (right) hemisphere and only 36.2% in the inexperienced (left) hemisphere of the SBDK group, while in the SBK group the proportions were 68.5% and 69.6% in the right and in the left hemispheres, respectively. The visually experienced hemisphere of SBDK cats had also dominated the inexperienced one as reflected in the increased proportion of orientation specific (86.6% in comparison to 71.4%) and direction specific (76.4% in comparison to 44.1%) cells, out of all responsive cells. Other parameters of the receptive field quality have been also affected as expressed in the remarkable increase in the proportion of irregular cells (e.g. cells with diffuse, incomplete and other irregular properties) in the inexperienced in comparison to the fellow hemisphere. Our results in the other groups of split brain adult cats (chronic and acute) confirm the finding that the asymmetry induced in the SBDK cats is the result of the almost differential activation of the two hemispheres during development. It has been concluded that an experience dependent asymmetric cerebral dominance occurs in the brain following biased visual experience and without interhemispheric interaction. This condition may serve as a model of hemispheric dominance in the visual and other sensory systems.