On and off effects in the background EEG activity during one-second photic stimulation.

The changes in alpha band power of the background EEG activity during 1-sec light stimulation were investigated in 10 normal human subjects. The EEG data were band-pass filtered, centering on the alpha range, and the time-dependent power changes were evaluated for their significance. The most significant finding was a light-on power decrease (on-effect) occurring in occipital areas 250--500 msec after onset of the stimulus, observed in all subjects, and reaching values of 72.2 +/- 13.2% decrease, with reference to power in the pre-stimulus interval. In 8 subjects there was also a significant light-off power decrease occuring 250--500 msec after offset of the photic stimulus (average across subjects: 41.8 +/- 24.4%) also in the occipital region. Similar on- and off-effects were observed in central regions, with lesser statistical significance. This pattern of light-on and -off power decreases was consistent and reproducible, with small inter- and intraindividual variability.