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° test spot centered on a background of 20°. Background luminance varied from −1 to 3 log td and retinal eccentricity from 0° to 50°. 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. Type A stimulation resulted in similar time thresholds for increments and decrements or, under some conditions, slightly shorter decrement thresholds. Type B stimulation resulted in similar thresholds for foveal vision. However, with increasing step size, decreasing background luminance, and increasing eccentricity, the time threshold for the decrement progressively exceeded that for the increment (up to 80 msec). This difference is attributed to different rise and fall times of the photoreceptor response as well as to Troxler’s effect.
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