Focal spike-induced cerebral dysfunction is related to the after-coming slow wave

By means of a computerized system of spike detection, presentation of visual stimuli, and registration of reaction times (RTs), we have shown previously that focal posterior interictal spikes cause transiently prolonged RT and increased nonperception and misperception of stimuli, especially contralateral to the spike. We report here the temporal profile of this phenomenon, ascertained by systematically increasing a delay between spike and stimulus-flash, until the latter was well beyond the limits of the entire spike-wave complex. In each of two subjects, the spike effect began just before the spike and ended with the termination of the after-coming slow wave. For stimuli with identical delays following spikes, RTs were significantly prolonged if the end of the slow wave overlapped the stimulus, and not otherwise. In one subject, who had an amplitude dissociation between spikes and after-coming slow waves, the prolongation of RT was associated with the larger waves, regardless of spike amplitude. These findings suggest that the after-coming slow wave (surround hyperpolarization) transiently disrupts aspects of cortical functioning, in addition to whatever effect the spike itself may have. Focal spike-wave-induced cortical dysfunction may be relevant to a variety of interictal cognitive disorders.