Speed discrimination of motion-in-depth using binocular cues

Although it is well known that motion-in-depth can be detected using binocular cues, it is not known whether those cues can be used to judge the speed of an object moving in depth. There are at least two possible binocular cues that could be used by the visual system to calculate three dimensional (3-D) speed: the rate of change of binocular disparity, or a comparison of the speeds of motion in the two eyes. We tested which of these cues is used to discriminate the speed of motion-in-depth. First, speed discrimination was measured for a dot moving away from the observer in depth (along the z-axis) and for a random dot stereogram in which a central square moved away from the observer in depth. These stimuli contained both disparity and monocular motion cues. Speed discrimination thresholds were as good for 3-D motion as for monocular sideways motion. Second, a dynamic random dot stereogram (in which the random dot pattern was replaced by a new dot pattern every frame) was used to remove consistent monocular cues. 3-D speed discrimination was now very poor, suggesting that the rate of change of disparity is not a good cue for 3-D speed. Finally, we tested whether observers were able to use the monocular motion cue from one eye to perform the speed discrimination task, or whether there had to be a comparison of the two eyes' monocular cues. By adding a small x-axis velocity component (with random direction) to the z-axis motion, it was possible to disrupt the monocular motion signals without altering the speed of the motion in 3-D. This manipulation did not disrupt the observers' performance, suggesting that monocular speed cues were not being used independently but that there was a comparison of monocular motion signals from the two eyes.