How does the primate visual system encode three-dimensional motion? The macaque middle temporal area (MT) and the human MT complex (MT+) have well-established sensitivity to two-dimensional frontoparallel motion and static disparity. However, evidence for sensitivity to three-dimensional motion has remained elusive. We found that human MT+ encodes two binocular cues to three-dimensional motion: changing disparities over time and interocular comparisons of retinal velocities. By varying important properties of moving dot displays, we distinguished these three-dimensional motion signals from their constituents, instantaneous binocular disparity and monocular retinal motion. An adaptation experiment confirmed direction selectivity for three-dimensional motion. Our results indicate that MT+ carries critical binocular signals for three-dimensional motion processing, revealing an important and previously overlooked role for this well-studied brain area.
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