Sensory latency and reaction time: dependence on contrast polarity and early linearity in human vision

Relations between luminance contrast and reaction time were studied for foveal vision over a three-decade range of background luminance. On each background, the contrast equivalence relation between negative and positive contrast flashes conformed almost exactly to the result expected if equal luminance steps of opposite sign produce equal visual effects. The same result held for threshold detection for flashes of variable duration. Analysis of these data suggests that reaction time is triggered by the early, rising phase of an internal response and that the effective stimulus energy that triggers the response is only moderately suprathreshold. On all backgrounds the sensory latency for reaction time (L) was described reasonably well by the relation L = bS-0.67, where b is constant and S is the absolute value of the luminance step. This implies that reaction time is largely independent of contrast polarity and the background luminance. Parallels between the present results and recent intracellular work suggest that the contrast equivalence relation for reaction time is largely shaped by early linear mechanisms in cones.