EFFECTS OF FLASH FREQUENCY AND REPETITION OF INTERMITTENT PHOTIC-STIMULATION ON PHOTOPAROXYSMAL RESPONSES

The protocol used for intermittent photic stimulation (TPS) may determ ine the likelihood of evoking a photoparoxysmal response (PPR). One-hu ndred and thirty-five electroencephalograms (EEGs) presenting PPRs, fr om 125 patients were studied in order to identify the most effective s timulation frequency to evoke a PPR and the effects of repetition of T PS on the occurrence of a PPR. Two stimulation protocols were used: pr otocol I (starting at 18 Hz and then testing at 2, 6, 8, 10, 15, 20, 3 0, 40, 50, 60 Hz) and protocol II (stimulating at 2, 6, 8, 10,15, 18, 20, 30, 40, 50, 60 Hz). Protocol I was used for patients not known to be photosensitive whereas protocol II was used for patients known to b e photosensitive before recording. Both latency and PPR grade for freq uencies which evoked PPR were measured in all records. The most epilep togenic frequencies (those evoking grade 4 PPRs at the shortest latenc y) were within the range 15-18 Hz for both protocols. In the records w here the IFS was repeated at the same frequency, the PPR latency and g rade seen during the first and second stimulation trial were studied i n order to establish habituation or potentiation of responses. Repetit ion of IFS at the same frequency induced habituation more often than p otentiation, but only if trials were repeated consecutively which sugg ests that habituation is frequency specific and trials repeated during EEG recordings to confirm photosensitivity to a particular frequency should be separated in time or be non-consecutive. Five patients studi ed with protocol I (10.6%) showed a grade 4 PPR only during the initia l trial at 18 Hz. Thus, as a general screening procedure for testing f or photosensitivity commencing stimulation at 18 flashes/s appears to be justified. The combination of two different protocols delivered to patients with and without a history of photosensitivity appears to ach ieve a sensible compromise having a high likelihood of demonstrating p hotosensitivity with a minimum risk of precipitating seizures.