The Visual Response to Alternating Chromatic Stimuli

Abstract

Sequential exposure of two adjacent stimuli fulfilling certain conditions with regard to proximity, duration and form elicits the perception of an object moving between the stimuli locations. Kenkel (1913) designated this form of motion "beta" movement. Substantial evidence indicates that activation of movement-sensitive neurones by alternating stimuli is responsible for the appearance of the phenomenon. Smell alternating stimuli subtending about 2 mins. of arc, presented against a black background, are used to establish the maximum spatial threshold for beta motion. Such spatial thresholds are considered to delineate perceptive fields of movement-sensitive neurones. The perceptive field is the psychophysical equivalent of visual receptive field organisation found in man. Significant threshold variation occurs with stimulus wavelength, suggesting that spectrally opponent cells may act as inputs for movement—sensitive neurones. A direct relationship is established between stimulus duration and the perceptive field size for beta motion. Significant interaction effects between pulse duration and wavelength, resembling the Bezold - Brucke hue shift, are indicated. The influence of both temporal overlap and interstimulus interval upon beta motion, is related to the existence of phase differences between excitation and inhibition. A linear increase in perceptive field size for beta motion is found with increasing eccentricity up to 10. Rod activity is considered responsible for the increased sensitivity to short wavelength stimuli occurring with eccentricity in excess of 4.