SUMMARY Thirty six patients were studied with a classical photoparoxysmal EEG response to intermittent photic stimulation (IPS). Clinical observations and complaints reported by patients during standardised IPS were recorded and compared with historical data. Twenty seven patients experienced impaired consciousness or showed motor phenomena such as involuntary opening of the eyes or jerking on one or both sides of the body. Twenty five patients had sensations such as pain in the eyes, jerking etc. There existed no relation between the duration of the evoked discharges (0 5-3 seconds) and observed signs or complaints. In 11 instances the clinical features found during IPS were not reported in a history taken from the patient and relatives. In the case of spontaneous epileptiforn EEG discharges it is generally accepted that there is a relation between the duration of the discharge and the occurrence of observable clinical features. If generalised spike-wave discharges last less than 3 seconds, they often pass unnoticed by the patient and the observer. " 12 However, it is not known whether photically-induced epileptiform activity differs in this respect from spontaneous discharges. In a previous study of 38 photosensitive patients we found that 39% reported various forms of ocular discomfort (sore eyes, headache, etc.) induced by potentially epileptogenic stimuli, such as disco lighting and the sun shining through trees or on water.13 It is unknown whether subjective complaints during IPS' "1 have any relation to clinical features during IPS or to visually-induced seizures in daily life. We have therefore prospectively examined an unselected group of photosensitive patients, with particular attention to subjective and objective clinical events during IPS.

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