Abstract: Binocular perception of motion in depth was studied. The results showed that binocular velocity difference may be of use in detecting the speed of motion in depth,while binocular velocity ratio is of use in discriminating the direction.The perceived speed in depth is proportional to velocity difference between the left and right retinal images,while the perceived direction is uniquely related to velocity ratio.We propose that human visual system might contain binocular mechanisms which are responsible for recovering from two-dimensional retinal information about three-dimensional motion.The behaviour of binocular speed detection and the 0.2°acuity of directional discrimination can be quantitatively understood on the basis of these mechanisms.The theoretical model quite satisfactorily agrees with the experimental findings and everyday visual experience.

Key words: Motion in depth, Binocular, Speed, Direction