Detecting binocular 3D motion in static 3D noise: no effect of viewing distance

Comparing motion induction in lateral motion and motion in depth

Induced motion, the apparent motion of an object when a nearby object moves, has been shown to occur in a variety of different conditions, including motion in depth. Here we explore whether similar patterns of induced motion result from induction in a lateral direction (frontoparallel motion) or induction in depth. We measured the magnitude of induced motion in a stationary target for: (a) binocularly viewed lateral motion of a pair of inducers, where the angular motion is in the same direction for the two eyes, and (b) binocularly viewed motion in depth of inducers, where the angular motions in the two eyes are opposite to each other, but the same magnitude as for the lateral motion. We found that induced motion is of similar magnitude for the two viewing conditions. This suggests a common mechanism for motion induction by both lateral motion and motion in depth, and is consistent with the idea that the visual signals responsible for induced motion are established before angular information is scaled to obtain metric motion in depth.

Seeing motion in depth using inter-ocular velocity differences

An object moving in depth produces retinal images that change in position over time by different amounts in the two eyes. This allows stereoscopic perception of motion in depth to be based on either one or both of two different visual signals: inter-ocular velocity differences, and binocular disparity change over time. Disparity change over time can produce the perception of motion in depth. However, demonstrating the same for inter-ocular velocity differences has proved elusive because of the difficulty of isolating this cue from disparity change (the inverse can easily be done). No physiological data are available, and existing psychophysical data are inconclusive as to whether inter-ocular velocity differences are used in primate vision. Here, we use motion adaptation to assess the contribution of inter-ocular velocity differences to the perception of motion in depth. If inter-ocular velocity differences contribute to motion in depth, we would expect that discriminability of direction of motion in depth should be improved after adaptation to frontoparallel motion. This is because an inter-ocular velocity difference is a comparison between two monocular frontoparallel motion signals, and because frontoparallel speed discrimination improves after motion adaptation. We show that adapting to frontoparallel motion does improve both frontoparallel speed discrimination and motion-in-depth direction discrimination. No improvement would be expected if only disparity change over time contributes to motion in depth. Furthermore, we found that frontoparallel motion adaptation diminishes discrimination of both speed and direction of motion in depth in dynamic random dot stereograms, in which changing disparity is the only cue available. The results provide strong evidence that inter-ocular velocity differences contribute to the perception of motion in depth and thus that the human visual system contains mechanisms for detecting differences in velocity between the two eyes' retinal images.

Minimum displacement thresholds for binocular three-dimensional motion

Thresholds for the detection of motion in depth in the median plane of the head are substantially poorer than those for motion in the frontoparallel plane. This suggests the existence of two independent mechanisms for their detection. Any three-dimensional (3-D) motion can be decomposed into components of motion in the frontoparallel plane and in the median plane of the head. Can human performance for the detection and discrimination of other 3-D motions be predicted by a combination of responses from the two independent mechanisms? Minimum displacement thresholds (d(min)) for the detection of 3-D motion and the direction discrimination of 3-D motion were measured for a wide range of 3-D directions. d(min) data were modelled in terms of the probability summation of a pair of independent motion mechanisms, one responding to motion in the median plane of the head, the second to motion in the frontoparallel plane. Detection of 3-D motion was well predicted by probability summation across a range of 3-D directions. Direction discrimination of 3-D motion was similarly well fit by the probability summation model for multiframe motion displays for some observers. However for two-frame motion displays, direction discrimination for 3-D motion was best fit by a model using only a motion mechanism in the frontoparallel plane. Detection and direction discrimination thresholds for 3-D motion can therefore be explained on the basis of one or two mechanisms, sensitive to motion in the frontoparallel plane and in the median plane of the head.