Interictal afterdischarge in focal penicillin epilepsy: block by thalamic cooling

Interictal spikes recorded from a penicillin focus in the precruciate cortex of urethane-anesthetized cats were followed by brief afterdischarge oscillations that occurred at a delay of 170 to 220 ms from the interictal spike and consisted of as many as five cycles at 16 to 22/s. The origin of this afterdischarge was investigated by cooling different subcortical sites and thus blocking them reversibly. At none of the sites did cooling result in a block of the interictal spike whereas cooling of the ventrolateral nucleus of the thalamus resulted in block of the afterdischarge. Other subcortical sites had either no or less reliable effects on it. We conclude that afterdischarge but not the interictal spike depends on thalamic input, either as a generator of the rhythm or as a trigger for a cortically maintained oscillation.