Photic epilepsy evoked by square-wave pulses. Report of a case studied by changing stimulus intensity and duration.

A spike and wave discharge of photic-evoked myoclonic epilepsy was investigated employing square-wave pulses of a stimulator having mechanical shutter, and the following conclusions were reached. The amplitude of the spike and wave complex of photogenic epilepsy depends on the intensity of photic stimulation when this is investigated in the range from 2.5 X 10(3) ergs/cm2 sec to its 4.31 log units high. A spike prior to the slow wave appears as the stimulus intensity increases. The implicit time is shortened as the intensity increases. Prolongation of the stimulus duration makes the ascending phase of the slow wave steep, and an off-response appeared when stimuli of 100 msec or longer were applied. Clinically, a biphasic myoclonic convulsion of limbs is clearly observed in on- and off-phases when stimuli of 300 msec or longer are applied. The responses become prominent when frequency increases from 0.5 to 3.0/sec.