Motion perception was tested by requiring adult subjects to view gratings that remained stationary but reversed in contrast several times per second. Subjects viewed monocularly and judged whether the gratings were stationary, or moving in one direction, in successive 3s trials. Subjects who had early-onset strabismus most frequently perceived vertically oriented gratings to be moving nasalward, and horizontally oriented gratings to be moving up or down. Normal subjects and subjects who had late-onset strabismus most frequently perceived the gratings to be stationary. The asymmetries of motion perception in early-onset strabismus imply that the visual motion neurons of cerebral cortex develop properly only if they receive normal binocular input during infancy.