Effects of photopic annulus luminance level on reaction time and on the latency of evoked cortical potential responses to target flashes.

Correlated experiments on visual reaction time (RT) and visual evoked cortical potentials (VECP's) were performed to assess the effects of illumination level of steady annulus surrounds on the latency of response to centrally presented target flashes (1 and 3 degrees in diameter) over a wide range of retinal illuminances. The latencies of the electrophysiological (VECP) responses are more variable than those of the psychophysical RT responses. For both classes of response, the visual latency for a given target flash luminance is systematically increased (inhibited) by a spatially contiguous annulus with a retinal illuminance that equals or exceeds that of the target. The inhibitory effect of a given annulus increases progressively as the target retinal illuminance is lowered below that of the annulus. The corresponding RT and VECP curves have the same shape, thus suggesting that the motor component of the RT responses is not differentially influenced by variations in target illumination. The inhibitory effects are characteristically greater for the 3 degrees target than for the 1 degree target. Lateral neural inhibition and stray-light models for these data are considered.