THE CORTICAL RESPONSES EVOKED BY DOUBLE PHOTIC STIMULI IN VARIOUS STATES OF MAN

It was previously very difficult to obtain cerebral evoked potentials from the human scalp because the amplitude of these responses is small compared with that of the background EEG. However, the effort to obtain this response has been successful (DAWSON 1949, 1954, COBB & DAWSON 1960). This success. has been attained by averaging the EEG response synchronously with respect to each stimulation. Currently, averaging computers, designed specifically for this purpose, have made it easy to record the response and many reports regarding the evoked potential obtained by computers have appeared (for example see BARLOW 1960 and HRBEK & MAREŠ 1964). Visually evoked potentials in man show shapes of greater variety when the eyes are open than when the eyes are closed. Generally, in the eye-closed state the response to photic stimulation is composed of an initial response and subsequent alpha-like waves. These alpha-like waves have frequencies similar to that of the background. EEG (BARLOW 1960), while in the eye-opened state the response has no such rhythmic waves. The present experiment was designed to determine further the nature of these alpha-like waves resulting from photic stimulation. Theresults suggested that these alpha-like waves may contribute to the related. phenomenon of photic driving of the EEG.