Contrast discrimination: invariant to spatial parameters.

Contrast discrimination thresholds were measured for 2-dimensional difference-of-Gaussian stimuli using a spatial 2-alternative forced-choice method. The stimulus diameter was varied from 0.4 to 3 deg and the distance between the centres of stimuli from 1 to 12 deg. When the two stimuli were presented on opposite sides of the fixation point on the horizontal meridian the discrimination thresholds were invariant to stimulus size and distance. When the stimulus configuration was vertical and moved to the temporal visual field, the discrimination thresholds were found to depend slightly on the inter-stimulus distance. A high-contrast bar placed between the stimuli did not influence the discrimination sensitivity.