Transient impairment of visual perception induced by single interictal occipital spikes.

We have previously reported that focal occipital interictal epileptiform discharges (spikes) cause transiently prolonged reaction time (RT) and increased nonperception of stimuli, especially in the visual field contralateral to the spike. One subject with very frequent spikes was capable of carrying out a visual recognition task along with the RT task. During central fixation, computer-generated random digits were flashed for 150 ms at random locations on a screen. Some stimuli were delivered during spikes, by means of an amplitude-threshold trigger, whereas control stimuli were delivered at random times between spikes. Following each stimulus, the subject had to press a button for RT and then report the digit perceived. There was a statistically significant increase in nonresponse rate (nonperception) during spikes compared to controls, and this effect was maximal contralateral to the spike. Moreover, among the responses, perceptual accuracy (correct vs incorrect) was significantly impaired during spikes, again predominantly in the visual field contralateral to the spike. Thus, not all focal interictal spikes are necessarily "subclinical;" at least some induce a transient cortical dysfunction of the same kind as produced more enduringly from a structural lesion in the same location. These findings may have clinical relevance in patients, especially children, with very frequent epileptiform discharges and higher cortical dysfunction.