Human visual responses to sinusoidally modulated light

Abstract

Sinusoidally modulated light, even at low percentages of modulation, gives rise to occipital responses, sometimes large. They may reproduce rather well the input sine wave in shape and in amplitude, especially between 9 and 15 c/sec for diffuse large field illumination and above 35 c/sec for strong illumination with small fields. The responses between 9 and 15 c/sec may be partly described by assuming a linear filter action. Resonance phenomena are observed, in which different types of stimulation yield compatible results. Responses and spontaneous activity seem to be additive to a considerable degree, as are the responses to stimulation of each eye. In the 9–15 c/sec frequency range, where the responses exhibit mainly the fundamental frequency, cancellation effects are found with out-of-phase stimulation of both eyes. That the large distortions found may be caused in early stages of the visual process, and are essential, is discussed. Responses have been recorded at many frequencies when the light was seen as fused by the subjects. Although subjective flicker threshold curves bear no clear relation to either the size or shape of the responses, it is hoped that the recordings of evoked responses with sinusoidally modulated light, combined with the techniques presented by Clynes et al. (1964), will be a valuable supplement to the current methods of investigation.