Binocular processing of motion: Some unresolved questions

The unresolved questions relating to binocular processing of motion include: Is the perceived speed of the motion in depth (MID) of an approaching object inversely proportional to the time to collision?; What visual information supports judgements of the direction of MID?; What is the relation between binocular and monocular processing in the perception of MID? We review whether the perception of stereomotion in depth of a monocularly visible object is caused entirely by a rate of change of disparity, and conclude that the difference between the horizontal velocities of the object's left and right retinal images makes at most only a small contribution to speed discrimination, but conclusions may be different for detection, perceived speed and directional discrimination. We review laboratory evidence on the relative importance of binocular and monocular information for interceptive action and collision avoidance and conclude that, in addition to the effect of considerable intersubject variability, the relative importance depends on the physical size of the approaching object, its distance and, if nonspherical, its direction of motion and whether it is rotating. We compare attempts to find whether the human visual system contains a mechanism specialized for the speed of cyclopean motion within a frontoparallel plane, and find the question ill-posed.