Poor visibility of motion in depth is due to early motion averaging.

Under a variety of conditions, motion in depth from binocular cues is harder to detect than lateral motion in the frontoparallel plane. This is surprising, as the nasal-temporal motion in the left eye associated with motion in depth is easily detectable, as is the nasal-temporal motion in the right eye. It is only when the two motions are combined in binocular viewing that detection can become difficult. We previously suggested that the visibility of motion-in-depth is low because early stereomotion detectors average left and right retinal motions. For motion in depth, a neural averaging process would produce a motion signal close to zero. Here we tested the averaging hypothesis further. Specifically we asked, could the reduced visibility observed in previous experiments be associated with depth and layout in the stimuli, rather than motion averaging? We used anti-correlated random dot stereograms to show that, despite no depth being perceived, it is still harder to detect motion when it is presented in opposite directions in the two eyes than when motion is presented in the same direction in the two eyes. This suggests that the motion in depth signal is lost due to early motion averaging, rather than due to the presence of noise from the perceived depth patterns in the stimulus.