Purpose: Perception necessarily entails combining separate sensory estimates into a single coherent whole. The perception of three-dimensional (3D) motion, for instance, can rely on two binocular cues: one related to the change in binocular disparity over time (CD) and the other related to interocular velocity differences (IOVD). Although previous work has shown that neither cue is strictly necessary for the perception of 3D motion, observers are able to judge 3D motion in displays in which one or the other cue has been eliminated, it is unclear whether or how the two cues are combined in situations in which both are present.
Methods: We tested the visual performance of a sample of 81 individuals (Mage = 20.34, 49 females) in four main conditions that measured, respectively, static stereoacuity, CD, IOVD, and combined CD+IOVD sensitivity.
Results: We show that the sensitivity to the two binocular cues to 3D motion varies substantially across observers (CD: Md' = 1.01, SDd' = 1.1; IOVD: Md' = 1.16, SDd' = 1.03). Furthermore, sensitivity to the two cues was independent across observers (r[48] = 0.12, P = 0.42). Importantly, however, observed CD+IOVD performance was well-predicted based on the assumption that each observer combines the two cues in a statistically optimal fashion (r[79] = 0.75, P < 0.001).
Conclusions: Our findings provide an explanation for the previously puzzling variability found in 3D perception across observers and laboratories, with some results suggesting that motion-in-depth percepts are largely determined by changes in binocular disparity, whereas others indicate that interocular velocity differences are key. Our results underline the existence of two complementary binocular mechanisms underlying 3D motion perception, with observers relying on these two mechanisms to different extents depending on their individual sensitivity.