Visual Latency as a Function of Stimulus Onset, Offset, and Background Luminance*

Abstract

Test and comparison stimuli were presented at various asynchronies, and subjects judged which changed first. Relative visual latencies between the onset and offset of test and comparison stimuli were obtained in a factorial design. This procedure allowed a direct comparison of on and off visual latencies, using the perceived-order method. The test stimulus subtended 10′ of visual angle and had an average duration of 2.5 s. It was delivered to both the fovea and 4.5° in the periphery. Two luminances were used and the stimulus was presented alone or on a 30′ background disk, the luminance of which was varied. The comparison stimulus was a 2.5-s pulse of light presented to the periphery of the left eye. It was of fixed luminance and subtended 10′ of visual angle. A maxwellian-view optical system was used. Test-stimulus offsets were found to have significantly shorter latencies than test-stimulus onsets, in both the fovea and periphery, and for high and low test-stimulus luminance. The difference between onset and offset latency was not affected by background luminance, indicating that the onset–offset difference was not due to differential light adaptation. Also, the latency of the test stimulus was found to be shorter in the fovea than in the periphery, and shorter for high-luminance stimulation than for low luminance. Latency of the test stimulus increased for high backgrounds in the periphery, but not in the fovea.