Necessary conditions for the perception of motion in depth

This study investigated the relation between the perception of motion in depth and ocular vergence movements for a single foveally viewed dot, and for a 30 deg X 30 deg pattern of many dots. When the target's disparity was changed, it appeared to move in depth relative to stationary reference marks, but removing the reference marks completely abolished the sensation of motion in depth for the multi-dot target and left only a weak sensation of motion in depth for the single dot target. However, it is not the case that motion-in-depth sensation per se depends on the presence of reference marks; motion in depth generated by changing-size stimulation was unaffected by removing reference marks. Possible explanations for the loss of motion-in-depth sensation include ocular vergence exactly tracked stimulus motion; vergence changes and disparity changes, though unequal, produced equal and opposite motion-in-depth signals; vergence changes, though producing no motion-in-depth signals, suppressed the signals produced by disparity changes; motion-in-depth sensation requires relative motion. Explanation is rejected because vergence tracking errors were large. Explanation is rejected because vergence changes do not in themselves induce a sensation of motion in depth. Explanation is rejected because motion-in-depth threshold is not affected by vergence changes. Conclusions are as follows. For a single-dot target, visual sensitivity to motion in depth is much higher for changes in relative retinal disparity than for changes in absolute retinal disparity, while for a multi-dot target any residual sensitivity is abolished by an interaction between neighboring coherently moving dots. The authors suggest that the relative velocity elements proposed to explain sensitivity to changing size feed the stereomotion mechanism also.