TIME THRESHOLDS FOR INCREMENTS AND DECREMENTS IN LUMINANCE

Time thresholds, i. e. , the minimal durations necessary to just detect a change in brightness, were measured for light increments and decrements of a 1 degree test spot centered on a background of 20 degree . Background luminance varied from minus -1 to 3 log td and retinal eccentricity from 0 degree to 50 degree . Step size ranged from 0. 04 to 1. 5 log units and was the same in absolute units for both directions. Two types of stimuli were used: Type A, in which increments and decrements emerge from the same uniform background, and Type B, in which increments are the same as in Type A but decrements consist of a brief interruption of the test spot. The difference observed between the two is attributed to different rise and fall times of the photoreceptor response as well as to Troxler's effect.