Objects move through the environment in three dimensions, and primates are very adept at interacting with a dynamic 3D world, both with respect to moving objects and as they move their own bodies. The goal of this proposed research is to understand how neural circuits in the primate brain exploit binocular information to represent the direction of objects moving through 3D space. Our particular focus is understanding how the brain goes from differential patterns of motion falling upon the two retinae to inferences about velocities of real objects in the 3D environment. In order to understand how this critical transformation from an ?ocular? to an ?environmental? representation is performed by the brain, we propose the following suite of psychophysical and electrophysiological investigation. Aim 1: Test whether perceptual sensitivity re?ects ocular or environmental velocity It is critical to assess whether human perception of 3D direction re?ects the velocity of objects in the environment, or remains tied to the velocities that fall upon the retinae. We will use the characteristic patterns of perceptual sensitivity (bolstered by our geometric understanding of how motions in the environment relate to velocities projected upon the retina) to test whether perception remains tied to ocular velocities, or whether it correctly estimates (and uses) viewing distance to express sensitivity in environmental terms, independent of exactly where in space and depth the moving object is. We will also continue to generalize classical psychophysics to a novel continuous tracking task that we developed in the last cycle. Aim 2: Assess 3D velocity tuning across the dorsal stream (MT, MST, VIP) of awake fixating primates In the prior work supported by this grant, we showed that many neurons in area MT are tuned for differential velocities in the two eyes, consistent with 3D direction-selectivity. We now propose to characterize these responses in a broader array of brain areas (MT, MST, VIP) and to test whether these responses re?ect tuning to differential ocular velocity or sensitivity to environmental direction. Aim 3: Test for a selective causal roles of dorsal stream areas in binocular 3D motion perception This aim extends the behavioral tracking paradigm from Aim 1 that will allow us to link human and monkey perceptual sensitivity to 3D direction. We will then employ reversible neural inactivations and stereoscopic stimulus manipulations within this paradigm to test for selective causal roles of dorsal visual areas in 3D motion perception.