A Distinct Mechanism of Temporal Integration for Motion through Depth

Temporal integration of visual motion has been studied extensively within the frontoparallel plane (i.e., 2D). However, the majority of motion occurs within a 3D environment, and it is unknown whether the principles from 2D motion processing generalize to more realistic 3D motion. We therefore characterized and compared temporal integration underlying 2D (left/right) and 3D (toward/away) direction discrimination in human observers, varying motion coherence across a range of viewing durations. The resulting discrimination-versus-duration functions followed three stages, as follows: (1) a steep improvement during the first ∼150 ms, likely reflecting early sensory processing; (2) a subsequent, more gradual benefit of increasing duration over several hundreds of milliseconds, consistent with some form of temporal integration underlying decision formation; and (3) a final stage in which performance ceased to improve with duration over ∼1 s, which is consistent with an upper limit on integration. As previously found, improvements in 2D direction discrimination with time were consistent with near-perfect integration. In contrast, 3D motion sensitivity was lower overall and exhibited a substantial departure from perfect integration. These results confirm that there are overall differences in sensitivity for 2D and 3D motion that are consistent with a sensory difference between binocular and dichoptic sensory mechanisms. They also reveal a difference at the integration stage, in which 3D motion is not accumulated as perfectly as in the 2D motion model system.