Binocular three-dimensional motion detection: contributions of lateral motion and stereomotion

When an object moves along a trajectory in three-dimensional (3-D) space, there are potentially two orthogonal components that could be used to detect its motion: stereomotion resulting from the difference or disparity between the images in the right and left eyes, and lateral motion from the sum or average of the image motions in the right and left eyes. Using a suprathreshold search task for a target moving amid 3-D distractors, we found a range of 3-D trajectories for which increasing the stereomotion component did not improve detection. However, with larger stereomotion components, performance improved. The addition of random-motion noise to only the lateral motion component adversely affected the detection of both lateral motion and stereomotion. These data suggest that the visual system uses the average of the monocular image motions for the detection of a range of 3-D trajectories. In addition, a mechanism sensitive to the changing disparity may also be used but only for a very restricted range of 3-D motions.