Contrast facilitation in motion detection: evidence for a Reichardt detector in human vision.

We measured contrast thresholds for detecting the direction of movement of a grating in a two-frame sequence with a quadrature shift between frames. The threshold contrast for the first frame was determined for a range of contrasts of the second frame. As the contrast of the second frame was increased, the threshold contrast of the first frame initially fell, and then remained constant at a value that was approximately 0.3 log units below the threshold contrast when the two frames had the same contrast (the yoked threshold). We show that this motion facilitation effect is a specific prediction of a Reichardt detector, with additive noise before the multiplication site. When the order of the two frames was reversed, the motion facilitation effect was found for contrast of the second frame, but at sufficiently high contrasts of the first frame, masking was found. The temporal order asymmetry is not found at near yoked-threshold contrasts of the first frame. The Motion Facilitation Effect was smaller than the contrast facilitation in a classical contrast discrimination experiment.