We measured the ability to fuse dichoptic images of a horizontal line alone or in the presence of a textured background with different vertical disparity. Nonius-line measurements of vertical vergence were also obtained. Diplopia thresholds and vertical vergence gains were much higher in response to an isolated vertically disparate line than to one with a zero vertical-disparity background. The effect of the background was maximum when it was coplanar with the target and decreased with increasing relative horizontal disparity. We conclude that vertical disparities are integrated over a restricted range of horizontal disparities to drive vertical vergence. 2000 Elsevier Science Ltd. All rights reserved.

The results of several experiments demonstrate that the estimated magnitude of perceived slant of large stereoscopic surfaces increases with the duration of the presentation. In these experiments, subjects were free to make eye movements. A possible explanation for the increase is that the visual system needs to scan the stimulus with eye movements (which take time) before it can make a reliable estimate of slant. We investigated the influence of large scanning eye movements on stereoscopic slant estimation of large surfaces. Six subjects estimated the magnitude of slant about the vertical or horizontal axis induced by large-field stereograms of which one half-image was transformed by horizontal scale, horizontal shear, vertical scale, vertical shear, divergence or rotation relative to the other half-image. The experiment was blocked in three sessions. Each session was devoted to one of the following fixation strategies: central fixation, peripheral (20 deg) fixation and active scanning of the stimulus. The presentation duration in each of the sessions was 0.5, 2 or 8 s. Estimations were done with and without a visual reference. The magnitudes of estimated slant and the perceptual biases were not significantly influenced by the three fixation strategies. Thus, our results provide no support for the hypothesis that the time used for the execution of large scanning eye movements explains the build-up of estimated slant with the duration of the stimulus presentation.

We measured the percept of changing depth from changing disparity in stereograms composed of random-dot textures that were either persistent or dynamically changed on every frame (a dynamic random-dot stereogram). Disparity was changed between frames to depict a surface undergoing smooth temporal changes in simulated slant. Matched depth was greater with dynamic random-dot stereograms than with persistent random-dot stereograms. These results confirm and extend earlier observations at depth threshold. We posit an explanation based on cue conflict between stereopsis and monocular depth cues. © 2000 Elsevier

Observers viewed large dichoptic patterns undergoing smooth temporal modulations or step changes in simulated slant or inclination under various conditions of disparity -- perspective cue conflict and concordance. After presentation of each test surface, subjects adjusted a comparison surface to match the perceived slant or inclination of the test surface. Addition of conflicting perspective to disparity affected slant and inclination perception more for brief than for long presentations. Perspective had more influence for smooth temporal changes than for step changes in slant or inclination and for surfaces presented in isolation rather than with a zero disparity frame. These results indicate that conflicting perspective information plays a dominant role in determining the temporal properties of perceived slant and inclination.

There are a variety of stereoscopic after-effects in which exposure to a stimulus with a particular slant or curvature affects the perceived slant or curvature of a subsequently presented stimulus. These after-effects have been explained as a consequence of fatigue (a decrease in responsiveness) among neural mechanisms that are tuned to particular disparities or patterns of disparity. In fact, a given disparity pattern is consistent with numerous slants or curvatures; to determine slant or curvature, the visual system must take the viewing distance into account. We took advantage of this property to examine whether the mechanisms underlying the stereoscopic curvature after-effect are tuned to particular disparity patterns or to some other property such as surface curvature. The results clearly support the second hypothesis. Thus, 3D after-effects appear to be caused by adaptation among mechanisms specifying surface shape rather than among mechanisms signaling the disparity pattern. © 2001 Elsevier

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