ISI produces reverse apparent motion.

Abstract

A moving random-dot stimulus was presented in two sequential frames separated by an interstimulus interval (ISI) during which the field was spatially uniform with luminance equal to either the average luminance of the stimulus field (grey) or that of the black dots (black). In Experiment 1, black ISIs did not affect perception of motion direction but grey ISIs produced motion in the direction opposite to the physical displacement (reverse motion). In Experiment 2, the contrast of the stimulus was reversed simultaneously with the displacement of the random-dot fields so that reverse motion would be seen with no ISI [Anstis & Rogers, Vision Research, 15, 957, 1975]. In this condition, grey ISIs reversed the reverse motion to produce a veridical perception. Finally, in Experiment 3, we examined whether the negative image that follows the stimulus offset was the source of the reversal in motion direction. A gradual offset of the stimulus necessarily reduces the amplitude of the negative response at stimulus offset and also reduced the frequency of seeing reverse motion, suggesting that the apparent reversal of motion direction with ISI can be attributed to the negative phase of a biphasic impulse response function. A simulation of the temporal response to the displacements of random-dot fields demonstrated that the negative phase of a biphasic impulse response function is sufficient to produce the reverse motion. We therefore claim that there is a significant biphasic temporal response function that precedes the analysis of motion in the visual system. This indicates that the overall temporal response function of the visual system is the result of a cascade of functions from early through late stages and that only a portion of the overall temporal response function can be attributed to stages involved in motion analysis.