Alteration of motor cortex excitability in response to intermittent photic stimulation

Objective: To measure the duration of cortical silent period (C-SP) in response to different frequencies of intermittent photic stimulation (IPS). Methods: Ten normal volunteers without a history of seizures were studied. First, using a magnetic stimulator we determined the resting motor threshold (RMT). The C-SP produced by a single magnetic shock 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 with closed eyes. Results: The average duration of C-SP was 135±33.2 ms with no photic stimulation. This did not change significantly when subjects were exposed to IPS at 5, 10, 15, 20, and 30 Hz. The duration of C-SP, however, decreased significantly at higher frequencies. At 50 Hz it was 116.2±43 ms (P=0.003) and at 60 Hz it was 112.7±44.6 ms (P<0.001). Conclusion: Our findings suggest that motor cortex inhibition is altered in response to IPS at 50 and 60 Hz.