Changes in amplitude of the EEG induced by a photic stimulus.

Occipital EEG responses to a single photic flash were studied by using a complex demodulation technique. Individual EEGs were first digitally filtered by making use of the Ormsby band-pass filter. Envelopes of the alpha (8-12 c/sec) and of the low frequency (3-7 c/sec) activities were obtained in terms of the demodulation calculation. Both individual and averaged data of raw, filtered, and demodulated EEGs were utilized for the present study. Two typical kinds of EEG responses were observed. One showed clearly the blocking as well as the enhancement of the alpha activity. The alpha activity first exhibited a reduction in amplitude in the range 50-200 msec after the stimulus (the minimum amplitude at about 150 msec). Then an evocation, rather than merely a recovery, of the alpha was observed in the range 500-800 msec (maximum amplitude at about 650 msec). The other typical example showed the low frequency evoked potential prominently. This response appeared almost locked to the stimulus and showed roughly a bi-phasic pattern in the time range where the alpha blocking took place (30-200 msec). Phases of the evoked low frequency component were almost locked to the stimulus but no distinct phase coherence was observed for the enhanced alpha wavetrains. Detection by demodulation of the phase change of the alpha was critical for a single EEG. It is pointed out that changes of the alpha must be studied on individual EEGs.