Footsteps and inchworms: Illusions show that contrast affects apparent speed

A horizontal grey bar that drifts horizontally across a surround of black a nd white vertical stripes appears to stop and start as it crosses each stri pe. A dark bar appears to slow down on a black stripe, where its edges have low contrast, and to accelerate on a white stripe, where its edges have hi gh contrast. A light-grey bar appears to slow down on a white stripe and to accelerate on a black stripe. If the background luminances at the leading and trailing edges of the moving bar are the same, the bar appears to chang e speed, and if they are different the bar appears to change in length. A p laid surround can induce 2-D illusions that modulate the apparent direction , not just the speed, of moving squares. Thus, the motion salience of a mov ing edge depends critically on its instantaneous contrast against the backg round.