An Effect of Stimulus Colour on Average Steady-state Potentials evoked in Man

SCALP potentials evoked by sinusoidally modulated light have been measured for various monochromatic stimuli of equal retinal illuminations. The correlation apparatus made use of the orthogonality of sinusoids to extract from the electroencephalogram the sinusoidal evoked component at stimulus frequency (synchronous component), and to measure its amplitude and phase1,2. It was found that if the light stimulus were viewed with good voluntary fixation, and if its frequency and peak intensity were accurately maintained constant, an average steady-state régime occurred from about 12–20 sec after stimulus onset until at least 70 sec, during which the running average of the amplitude and phase of the synchronous component remained constant1. Data for one subject are described.