Effects of adapting luminance and stimulus contrast on the temporal and spatial limits of short-range motion

Perception of short-range motion was studied as a function of adapting luminance and of stimulus contrast in computer simulations and in psychophysical experiments. The stimuli were random dots plotted on an oscilloscope in two sequential frames, separated by an inter-stimulus interval (ISI). Both in the simulations and in the empirical studies, we estimated the maximum spatial displacement (dmax) between corresponding dots in the successive frames, and the maximum ISI (tmax) at which coherent motion was perceived 80% of the time. Conceptually, the research was based on a rectified elaborated Reichardt detector with spatial and temporal filters at its inputs. Extant psychophysical and neurophysiological data were employed to provide estimates of the effects of luminance and contrast on the spatial and temporal characteristics of the input filters. Results showed a strong effect of adapting luminance on both the spatial and temporal limits of motion perception: decrements in luminance produced marked increments in both dmax and tmax. However, neither dmax nor tmax was affected by changes in stimulus contrast. These outcomes are entirely consonant with expectations based on the rectified elaborated Reichardt detector. Alternative explanations of the psychophysical results are discussed.