Multiunitary activity analysis of cortical and subcortical structures in paroxysmal discharges and grand mal seizures in photosensitive baboons.

Cortical and subcortical multiunitary activities (MUA) and EEG were simultaneously recorded in baboons made photosensitive by a subconvulsant dose of DL-allylglycine. Intermittent light stimulation (ILS) trains induced in these animals fronto-rolandic (FR) paroxysmal discharges (PDs, constituted as spikes and waves) and grand mal seizures. During the induction of FR PDs by ILS trains, the visual structures (occipital cortex, colliculi superioris, pulvinar) showed a significant MUA increase which was not related to the PD spike or wave but correlated with the flashes. The first structure showing bursts of MUA that frequently preceded the PD appearance was the FR cortex. When PDs appeared, the bursts were related to the spikes of PDs and were followed by an inhibition during the slow wave. The pontine and mesencephalic reticular formations and the facial nuclei were activated in bursts after the FR PDs had reached a certain amplitude. The thalamic nuclei ventralis lateralis, centrum medianum and lateralis posterior were activated only later, when the FR PDs had reached an even greater amplitude. It is suggested that the activation of visual structures is necessary for FR PD appearance. The secondary pontine and mesencephalic activation could reinforce that of the FR cortex and then the thalamus, and could determine the myoclonus observed in unparalysed animals. When the ILS is continued, grand mal seizures appear. The onset of the seizures could be linked to the loss of FR cortical control of the subcortical structures. The resulting reticular activation would be responsible for the vasomotor modifications which constitute the first clinical signs of a seizure.