Post-critical period plasticity of callosal transfer to visual cortex cells of cats following early conditioning of monocular deprivation and late optic chiasm transection

We studied whether plasticity-induced callosal transfer exists after the critical period for sensitivity of visual cortex cells in kittens postnatally monocularly deprived and in which interocular competition was cancelled by chiasm transection during adulthood. Callosal transfer was studied acutely (n = 3 cats) and chronically (n = 7) following the chiasm transection (OCAMD). For comparison, adult cats in which chiasm transection only was performed (OCA) were also studied acutely (n = 3) and chronically (n = 9). The results were also compared to cats in which monocular deprivation and chiasm transection were simultaneously performed (OCKMD) during development (n = 6) and to normal control cats (n = 18). Unit recording was extracellularly carried out in visual cortex areas 17 and 18 and their boundary region, where the corpus callosum is represented. When no interocular competition was allowed between the non-deprived and the deprived eye via the thalamocortical direct visual pathways on cortical cells, such as in the OCKMD cats, the absolute majority of the cells were ipsilaterally driven, regardless of which hemisphere was studied. Only a minor proportion (4.1%) of the cells had some contralateral input from the non-deprived eye in the hemisphere ipsilateral to the deprived eye, indicating almost no interhemispheric callosal transfer. A slight increase in the proportion of cells callosally driven from the non-deprived eye (9.8%), was found in this hemisphere in cats in which interocular competition was allowed via the direct visual pathways prior to its cancellation by chiasm transection (OCAMD), if studied acutely after the chiasm transection. A remarkable increase in callosal transfer was found in this hemisphere under chronic conditions. The proportions of these cells were 32.9% and 38.2% in the medium (3 weeks-3 months) and in the long chronic (6-22 months) groups of OCAMD cats, respectively. When no monocular deprivation, and hence interocular competition, was involved, such as in the OCA acute cats, a negligible proportion of cells (none in the left hemisphere and 5.1% in the right one), were callosally driven. A similar proportion of callosally driven cells were found in the OCA cats with medium (3 weeks-1.5 months; left hemisphere: 1.4%, right: 4.8%) and slightly more (left: 11.2%, right: 9.3%) in the cats with the longest postoperative period (6.5-32 months). Consistently, no cell with callosal input was found in the hemisphere contralateral to the deprived eye in the OCAMD cats while such cells, albeit few, were found in all OCA subgroups. It was concluded that preconditioning of cortical cells by monocular deprivation induces and facilitates the interhemispheric transfer of the resulting interocular competition. The non-deprived eye input governs that callosal transfer, occupying cortical cells in the 'inexperienced' hemisphere. However, for the callosal transfer of the interocular competition to take place, by and large, the presence of the whole complement of the thalamocortical visual pathways is needed, at least for a certain limited period. If this condition is fulfilled, then the callosal transfer can be remarkably enhanced, even in the adult cat, after the termination of the plasticity period, providing a sufficiently long interval (following the chiasm transection) is given for recovery.