Abstract
We have examined the effect of single visual stimulus upon the amplitude and phase of alpha waves by a complex demodulation method. The frequency for demodulation is set with the peak frequency of the subjects' alpha activity. The visual stimuli were generated by LED-mounted goggls in an eyes-closed condition. The data for the analysis are measured at Pz, which is a dominant electrode for event-related potentials. The interstimulus interval (ISI) was 10sec with a constant interval. The subjects were 10 healthy adult students with normal vision. The amplitude of alpha wave decreased gradually after the stimulus, showing a minimum at around 300msec after stimulus. Then it recovered to the same level around 700msec after stimulus with the pre-stimulus amplitude level. It continued to increase showing some rebound, and returned again around 2, 200msec after stimulus. The averaged phase variation is calculated with the reference at stimulus onset as zero. The variation after stimulus tends to be significantly larger than that before stimulus. Averaged phase showed a uniform delay before stimulus and showed acceleration after stimulus. This means that the frequency after stimulus is higher than that before stimulus. Moreover, averaged phase returned to the same slope as pre-stimulus around 2, 550msec after stimulus. According to the results of amplitude and phase variation, visual stimulus effects alpha waves until about 2, 500msec after the stimulus. Phase variation before and after stimulus shows significance from 112 to 678msec after stimulus. This result indicates that there are some relations between event-related potentials and alpha waves.
