Rhythmic after-activity to flashes in relation to the background alpha which precedes and follows the photic stimuli

Abstract

1. 1. Occipital EEG responses to slowly repeated, aperiodic light flashes were studied in normal subjects. The amplitude and time course of the evoked potential after-activity were compared with those of the flash-evoked changes in background alpha. We obtained the latter by averaging the filtered (7–14 c/sec), rectified, and smoothed envelope of the ongoing EEG with respect to the light stimuli. 2. 2. Rhythmic after-activity and alpha blocking roughly paralleled each other in time, but there were no systematic relationships between the latency, duration or amplitude of two measures. Inasmuch as the onset of after-activity showed no consistent relation to the beginning of alpha return, we conclude the after-activity cannot be interpreted as a computer average of background alpha returning in constant phase relation with the stimulus. 3. 3. Averaging was performed with respect to the amplitude of the background alpha when the stimuli occurred. The rhythmic after-activity was of greater amplitude and longer duration when the stimulus fell when the background alpha was of relatively large amplitude. The after-activity must have varied with minute-to-minute or even second-to-second changes in alpha amplitude since the individual responses making up the large alpha and small alpha averages were distributed quite uniformly throughout each minute of the recording session. The alpha blocking or alpha envelope response was also greater during times of large alpha amplitude, but alpha envelope and after-activity responses varied independently from subject to subject, from day to day, or within sessions. 4. 4. Our findings support the idea of a close functional relationship between the neural systems giving rise to the alpha rhythm and evoked after-activity, whether or not these systems are physiologically identical.