Abstract
Contrast affects apparent speed. When two bars, one light gray and the other dark gray, moved side by side at constant speed across vertical black and white stripes, the dark bar appeared to accelerate on the white stripes (where it had high contrast) and to slow down on the black stripes (low contrast). The light bar did the opposite, so the bars appeared to move in alternation like walking feet. Contrast can also affect perceived direction. A dark gray square and a light gray square drifted obliquely across a plaid made from crossed square-wave gratings. Each square crossed at the same instant a dark vertical stripe and a light horizontal stripe. This made the dark square appear to slow down horizontally and speed up vertically, so it appeared to veer toward the vertical. The light square appeared to veer toward the horizontal. So the two squares appeared to follow zigzag paths.