Relation of Occipital Spikes evoked by Intermittent Photic Stimulation to Visual Evoked Responses in Photosensitive Epilepsy

Abstract

PATIENTS suffering from photosensitive epilepsy show generalized discharges in their electroencephalogram (EEG) during intermittent photic stimulation. In some patients, these discharges are preceded by occipital spikes, suggesting a focal onset. Hishikawa et al.¹ have reported that the occipital spikes obtained in photo-sensitive epileptics have the same latency as some components of the visual evoked responses (VER), and suggested, that this might explain the origin of the occipital spikes in photosensitive epilepsy. The spikes were, however, obtained at flash rates of 10–16 s⁻¹ (that is, the interval between the flashes was limited to a range of 63–100 ms) and the comparison of the occipital spikes was made not with the VER of the individual patient, but with VERs of a normal population. Further, the latencies for positive and negative components of the VER, as given by the authors, show a very wide range and in fact overlap. The relatively small interval between the flashes and the wide range of the VER components could result in a fortuitous relationship between occipital spikes and VER components. It has also been reported by Rodin et al.² that spikes preceding generalized seizures, induced by ‘Megimide’ during intermittent photic stimulation, were related to normal VERs and consisted of a marked exaggeration of the secondary VER components. Their report, however, is concerned with normal animals (cats); and no details of the photic stimuli and the spikes are given. For these reasons we felt that we should re-examine the problem. The existence of a relationship between abnormal and “normal” responses would be critical to any theory attempting to explain the genesis of the occipital “epileptogenic” spikes.