Luminance-dependent visual latency for the Hess effect, the Pulfrich effect, and simple reaction time.

The Hess effect, a monocular, latency-based illusion in which the more intensely illuminated of a pair of laterally-moving targets appears displaced ahead of the other, was studied with white targets on black background at scotopic and photopic levels. The magnitude of the Hess effect was about the same as that of the Pulfrich effect under similar photopic conditions. Reaction time declined more rapidly with increasing stimulus intensity than would have been predicted either by the Hess or the Pulfrich effect. Theoretical time-constants, computed for the Hess effect by assuming a first-order diffusion process followed by a more central non-linearity (temporal centroid), ranged between 0.145 and 0.165 sec for three observers.