The Influence of Light Stimulation on Subcortical Potentials Evoked by Single Flashes in Photosensitive Papio papio

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Abstract
Visual evoked potentials (VEPs) from the frontorolandic (FR) cortex and from subcortical nuclei (colliculli superioris, pulvinar, corpori geniculati lateralis, centrum medianum, ventralis lateralis) and from pontine reticular formation were analyzed in Papio papio monkeys rendered photosensitive by a subconvulsant dose of allyglycine. The VEPs induced by single flashes were compared statistically with those induced by flashes preceded by trains of intermittent light stimulation (ILS). This latter mode of stimulation provoked the appearance of paroxysmal VEPs (PVEPs) in the FR cortex with the same morphology as the spikes and waves induced in this area by the ILS. The aim of our research was to provide evidence for the possible implication of the subcortical structures which we have studied in the elaboration of PVEPs and thus of spikes and waves. The VEPs recorded at the thalamic and pontine levels were modified when PVEPs were present. These modifications varied according to the site, but subcortical VEPs were never paroxysmal. In structures with visual functions (colliculi superioris, corpori geniculati lateralis), the VEPs were modified by the ILS, but showed more marked changes when PVEPs were present. Thus, these structures may contribute to the genesis of PVEPs. In the other structures (centrum medianum, ventralis lateralis, and pontine reticular formation), modifications of the VEPs occurred only when PVEPs were present. Thus, these structures would be only secondarily involved. We also present preliminary results concerning the effects of lesioning the pulvinar and the ventralis lateralis on the susceptibility of the FR cortex to produce spikes and waves during ILS. Since this susceptibility returned fully a week after a bilateral lesion of the pulvinar, this nucleus does not appear to exercise a direct action in the elaboration of FR PVEPs. A lesion of the ventralis diminished the epileptogenic susceptibility of the FR cortex without blocking it completely. This effect probably arises because the nucleus ventralis lateralis projects to a cortical area (area 4) where PVEPs and spikes and waves predominate.