Getting a grip on three-dimensional surface orientation: Binocular vision, cue integration, and computation

We explored how humans use visual information to compute estimates of three-dimensional (3D) surface orientation that can be used to guide motor behavior. First, we investigated how the visual system integrates monocular and binocular information for two different natural tasks, object placement and prehension. Binocular information influenced 3D orientation estimates more when subjects grasped a disc than when they placed an object on the same disc, regardless of whether the tasks were performed separately by different subjects or together by the same subjects in interleaved sessions. We also measured processing speeds since these can affect cue integration, but there were no significant differences between tasks. We concluded that how one uses visual cues for motor control depends on the information demands of the task being performed, whereas how quickly the information is processed appears to be task invariant. Second, we assessed the usefulness of stereopsis across the visual field. Binocular information had a smaller influence relative to monocular information on 3D orientation estimates for stimuli at larger retinal eccentricities and distances from the horopter where stereoacuity is worse than monocular acuity. The results were as predicted by a Bayesian integration scheme in which the cues were weighted according to their relative reliabilities across the visual field. We concluded that stereopsis is of limited use in the periphery and away from the horopter because monocular cues are more reliable in these regions. Third, we evaluated the potential role of orientation disparity as a binocular cue to 3D orientation by simulating a population of binocular visual cortical neurons tuned to orientation disparity and measuring the amount of Fisher information contained in the activity patterns. We concluded that orientation disparity provides an efficient source of information about 3D orientation and that it is plausible that the visual system could have mechanisms that are sensitive to it, although it would be most useful when combined with estimates from position disparity gradients and monocular perspective cues.