Moving objects appear to slow down at low contrasts

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 contrasts will show illusory distortions in perceived direction. Contrast also affects the apparent amplitude and salience of back-and-forth apparent motion. Finally, a line of black and white dots on a grey surround moves in illusory directions, because of a mismatch in the contrasts along and across 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, with contrast-dependence being a motion analog of the Bezold-Brucke hue shift.