[A testing device for binocular depth perception].

The eyes receive slightly different views of the world, and the differences between their images (binocular disparity) are used to see depth. Several authors have suggested how the brain could exploit this information for three-dimensional (3D) motion perception, but here we consider a simpler strategy. Visual direction is the angle between the direction of an object and the direction that an observer faces. Here we describe human behavioral experiments in which observers use visual direction, rather than binocular information, to estimate an object's 3D motion even though this causes them to make systematic errors. This suggests that recent models of binocular 3D motion perception may not reflect the strategies that human observers actually use.

Depth Perception↗

PubMed2005

Anticipating the three-dimensional consequences of eye movements.

Rapid eye movements called saccades give rise to sudden, enormous changes in optic information arriving at the eye; how the world nonetheless appears stable is known as the problem of spatial constancy. One consequence of saccades is that the directions of all visible points shift uniformly; directional or 2D constancy, the fact that we do not perceive this change, has received extensive study for over a century. The problems raised by 3D consequences of saccades, on the other hand, have been neglected. When the eye rotates in space, the 3D orientation of all stationary surfaces undergoes an equal-and-opposite rotation with respect to the eye. When presented with a an optic simulation of a saccade but with the eyes still, observers readily perceive this depth rotation of surfaces; when simultaneously performing the corresponding saccade, the 3D orientations of surfaces are perceived as stable, a phenomenon I propose calling 3D spatial constancy. In experiments presented here, observers viewed ambiguous 3D rotations immediately before, during, or after a saccade. The results show that before the eyes begin to move the brain anticipates the 3D consequences of saccades, preferring to perceive the rotation opposite to the impending eye movement. Further, the anticipation is absent when observers fixate while experiencing optically simulated saccades, and therefore must be evoked by extraretinal signals. Such anticipation could provide a mechanism for 3D spatial constancy and transsaccadic integration of depth information.

Depth Perception↗

PubMed2005

Stereothresholds for moving line stimuli for a range of velocities.

This study examined the influence of lateral target motion on the stereothresholds for bright vertical lines at a range of velocities. Stimuli were presented for 200 ms with horizontal velocities from 0 to 12 deg/s. Observers' horizontal eye movements were recorded on additional trials, and confirmed that the velocity of retinal image motion closely matched the velocity of the stimulus. In three auxiliary experiments, stereothresholds were measured (1) after equating the detectability of targets that moved at different velocities, (2) for moving and stationary stimuli with durations between 20 and 200 ms, and (3) for stationary stimuli presented at eccentricities of 0.6 and 1.2 deg. The results indicate that stereothresholds are unaffected by velocities up to approximately 2 deg/s, but worsen in proportion to the velocity at higher speeds. The results of our auxiliary experiments demonstrate that the increase in stereothresholds during image motion cannot be attributed primarily to a reduction in the detectability of the stimulus, a decrease in the effective exposure duration, or non-foveal viewing. We conclude that the elevation of stereo thresholds during lateral motion is consistent with a shift in the sensitivity of the visual system toward lower spatial frequencies as a result of motion blur.

Depth Perception↗