Occlusion contributes to temporal processing differences between crossed and uncrossed stereopsis in random-dot displays

A Joint Lateral Motion-Stereo Constraint

Purpose

We developed a stereo task that is based on a motion direction discrimination to examine the role that depth can play in disambiguating motion direction.

Methods

In this study, we quantified normal adults' static and dynamic (i.e., laterally moving) stereoscopic performance using a psychophysical task, where we dichoptically presented randomly arranged, limited lifetime Gabor elements at two depth planes (one plane was at the fixation plane and the other at an uncrossed disparity relative to the fixation plane). Each plane contained half of the elements. For the dynamic condition, all elements were vertically oriented and moved to the left in one plane and to the right in another plane; for the static condition, the elements were horizontally oriented in one plane and vertically oriented in another plane.

Results

For the range of motion speed that we measured (from 0.17°/s to 5.33°/s), we observed clear speed tuning of the stereo sensitivity (P = 3.0 × 10-5). The shape of this tuning did not significantly change with different spatial frequencies. We also found a significant difference in stereo sensitivity between stereopsis with static and laterally moving stimuli (speed = 0.67°/s; P = 0.004). Such difference was not evident when we matched the task between the static and moving stimuli.

Conclusions

We report that lateral motion modulates human global depth perception. This motion/stereo constraint is related to motion velocity not stimulus temporal frequency. We speculate that the processing of motion-based stereopsis of the kind reported here occurs in dorsal extrastriate cortex.

Factors limiting sensitivity to binocular disparity in human vision: Evidence from a noise-masking approach

Our visual system uses the disparity between the images received by the two eyes to judge three-dimensional distance to surfaces. We can measure this ability by having subjects discriminate the disparity of rendered surfaces. We wanted to know the basis of the individual differences in this ability. We tested 53 adults with normal vision using a relative disparity detection task. Targets were wedge-shaped surfaces formed from random dots. These were presented in either crossed or uncrossed disparity relative to a random dot background. The threshold disparity ranged from 24 arc seconds in the most-able subject to 275 arc seconds in the least-able subject. There was a small advantage for detecting crossed-disparity targets. We used the noise-masking paradigm to partition subject performance into two factors. These were the subject's equivalent internal noise and their processing efficiency. The parameters were estimated by fitting the linear amplifier model. We found both factors contributed to the individual differences in stereoacuity. Within subjects, those showing an advantage for one disparity direction had enhanced efficiency for that direction. Some subjects had a higher equivalent internal noise for one direction that was balanced out by an increased efficiency. Our approach provides a more thorough account of the stereo-ability of our subjects compared with measuring thresholds alone. We present a normative set of results that can be compared with clinical populations.

Stereoillusory motion concomitant with lateral head movements

Stationary objects in a stereogram can appear to move when viewed with lateral head movements. This illusory motion can be explained by the motion-distance invariance hypothesis, which states that illusory motion covaries with perceived depth in accordance with the geometric relationship between the position of the stereo stimuli and the head. We examined two predictions based on the hypothesis. In Experiment 1, illusory motion was studied while varying the magnitude of binocular disparity and the magnitude of lateral head movement, holding viewing distance constant. In Experiment 2, illusory motion was studied while varying binocular disparity and viewing distance, holding magnitude of head movement constant. Ancillary measures of perceived depth, perceived viewing distance, and perceived magnitude of lateral head movement were also obtained. The results from the two experiments show that the extent of illusory motion varies as a function of perceived depth, supporting the motion-distance invariance hypothesis. The results also show that the extent of illusory motion is close to that predicted from the geometry in crossed disparity conditions, whereas it is greater than the predicted motion in uncrossed disparity conditions. Furthermore, predictions based on perceptual variables were no more accurate than predictions based on geometry.