Abstracts

RATIONALE: Intermittent photic stimulation (IPS) can elicit a driving response at the flash frequency or at higher or lower harmonics. In some subjects, IPS may provoke seizures. To investigate potential neurophysiologic mechanisms for IPS-induced seizures, we studied whether IPS can affect motor cortex excitability. METHODS: Motor cortex excitability was assessed by measuring the cortical silent period (C-SP). Ten normal volunteers, without a personal or family history of seizures, were studied. A transcranial magnetic stimulator was used to determine the resting motor threshold (RMT). The C-SP produced by a single magnetic pulse at 120% of RMT was measured in the maximally contracted abductor pollicis brevis. C-SP was then measured while subjects were exposed to IPS at 5, 10, 15, 20, 30, 50, and 60 Hz. RESULTS: The average duration of C-SP was 135?33.2 msec with no photic stimulation. There was no significant change in C-SP when subjects were exposed to IPS at 5, 10, 15, and 20 Hz. The duration of C-SP did, however, decrease significantly at higher frequencies. At 30 Hz it was 126.6?35 msec (p=0.009); at 50 Hz it was 116.2?43 msec (p=0.001); and at 60 Hz it was 112.7?44.5 msec (p=0.001). CONCLUSIONS: Motor cortex inhibition is altered in response to IPS at 30, 50, and 60 Hz. This finding suggests a potential mechanism by which IPS may provoke seizures in susceptible individuals.