Moving in a fog: stimulus contrast affects the perceived speed and direction of motion

Moving cars give the illusion of slowing down in foggy conditions, because low contrast reduces perceived speed. A grey square that drifts horizontally across a surround of black and white vertical stripes appears to stop and start as it crosses each stripe, because its contrast keeps changing. A moving square whose vertical and horizontal edges have different contrast will show illusory distortions in perceived direction. Contrast also affects the apparent amplitude and salience of back-and-forth apparent motion. Also, a line of black and white dots on a gray surround moves in illusory directions, because of a mismatch in the contrast along the dotted line. Thus, motion signals in the early parts of the visual system are profoundly altered by stimulus luminance and contrast. This suggests that motion is coded by the relative firing rates of neural channels tuned to "fast" and "slow" motion.