Failure of motion discrimination at high contrasts: evidence for saturation

The ability of human observers to discriminate the direction of motion of a briefly-presented, slowly moving, 1 c/deg sinusoidal grating varies non-monotonically with the contrast of the grating. At low contrasts, performance improves with increasing contrast, but it reaches a peak between 95% and 100% correct at a contrast of 0.02-0.05. With further increases in contrast performance declines, reaching chance levels at a contrast of about 0.4. Detection of the same stimulus improves with increasing contrast to 100% correct and stays there. This behaviour would be expected if the visual signal which determines direction-of-motion is given by the difference between the responses of paired direction-selective filters tuned to opposite directions of motion and if the responses of these paired filters saturate at modest contrasts.