The deafferented visual cortex and interhemispheric relationships: a physiological approach.

A condition of asymmetrical activation in the visual system, induced by unilateral optic tract section (OTX) was examined in nine adult cats, four of which had been reared after early onset of monocular deprivation (MD-OTX) during the critical developmental period. Their results were compared to those of monocularly deprived (MD) and normal control cats. Extracellular single-unit recordings from areas 17, 18 and their border were performed at various intervals after OTX to study the effects of this isolation of one hemisphere from direct geniculo-cortical input. Electrophysiological results from the isolated hemispheres of both OTX and MD-OTX cats revealed that contralateral input via the corpus callosum was virtually undetectable in the majority of cats. Only in the cats in which the deprivation was ipsilateral to the OTX, 3.5% of the cells exhibited visual activity in the isolated hemisphere. In the intact hemisphere of the OTX group, binocularity was unaffected overall in comparison to the normal control animals, except for a slight decrease within the 0-4 degrees region from the vertical meridian. In the MD-OTX group, the cats with onset of deprivation prior to natural eye opening possessed an increase in deprived eye responsiveness compared to the control MD cats. No such an increase was seen in later onset of deprivation (3-4 weeks following natural eye opening) in the MD-OTX cats. Overall, visual responsiveness was reduced in all of the OTX and MD-OTX cats, with a return towards normal control values seen only in one animal with extended recovery (6 months). Orientation and direction selectivity were dramatically decreased in the OTX and MD-OTX cats. As the majority of receptive fields mapped from the OTX cats were in the ipsilateral visual field to the section, the remaining small number of receptive fields in the contralateral "blind" visual fields adds further support for a nasotemporal overlap in the retina of the cat. The conclusion from these results is that an asymmetrical level of direct geniculo-cortical input in the visual system of the adult cat yields a physiological bidirectional inactivation of the callosal pathway for the transfer of visual information. Thus, activation of the callosal pathway connecting the cortical visual areas has been postulated to be dependent upon simultaneous, reciprocal interaction between the two hemispheres.