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).


CONCLUSIONS
Our findings suggest that motor cortex inhibition is altered in response to IPS at 50 and 60 Hz.
Entezari-Taher
M
M
Department of Neurology, University of Utah, 50 North Medical Drive, Salt Lake City, UT 84132, USA. me.taher@m.cc.utah.edu
Dean
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D000328:Adult; D020195:Epilepsy, Reflex / Q000503:physiopathology; D005260:Female; D006801:Humans; D008280:Magnetics; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D010775:Photic Stimulation