Visual evoked potentials to stimulus trains: normative data and application to photosensitive seizures

Visual evoked potentials (VEPs) to intermittent flash trains of various stimulus frequencies (VEP-T) were studied in 10 normal subjects and in 10 patients with photosensitive generalized seizures, 5 on medication and another 5 subjects without medication. In the normal group, early negative peaks were consistently demonstrated at 71 +/- 8 msec for monocular stimuli and 67 +/- 9 msec for binocular stimuli, regardless of different frequencies of stimuli. An oscillatory potential, poststimulus continuation of the oscillatory activity and damping out within 100 msec followed the initial peaks. The RMS values of the response epochs were stable and demonstrated consistent temporal changes at each stimulation frequency. In photoconvulsive seizures, the responses were facilitated with most marked changes over the second and third 125 msec epoch of the VEP-Ts with late slow negative deflections in some cases. These changes were pronounced in patients who were not taking anticonvulsant medication, but were not significantly different than controls in patients whose seizures were controlled with medication. It is suggested that the VEP-T is a more effective technique than single flash stimuli in consistently identifying early significant peaks and demonstrating neuronal excitability changes within the visual system.