We examined the time course of light adaptation in the visual motion system. Subjects judged the direction of a two-frame apparent-motion display, with the two frames separated by a 50-ms interstimulus interval of the same mean luminance. The phase of the first frame was randomly determined on each trial. The grating presented in the second frame was phase shifted either leftward or rightward by π/2 with respect to the grating in the first frame. At some variable point during the first frame, the mean luminance of the pattern increased or decreased by 1–3 log units. Mean luminance levels varied from scotopic or low mesopic to photopic levels. We found that the perceived direction of motion depended jointly on the luminance level of the first frame grating and the time at which the shift in average luminance occurs. When the average luminance increases from scotopic or mesopic to photopic levels at least 0.5 s before the offset of the first frame, motion in the 3π/2 direction is perceived. When average luminance decreases to low mesopic or scotopic levels, motion in the π/2 direction is perceived if the change occurs 1.0 s or more before first frame offset, depending on the size of the luminance step. Thus light adaptation in the visual motion system is essentially complete within 1 s. This suggests a rapid change in the shape (biphasic or monophasic) of the temporal impulse response functions that feed into a first-order motion mechanism.
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