Fast perception of binocular disparity.

Is depth perception from binocular disparities-stereopsis-slow or fast? Many of the temporal properties of stereopsis are known. For example, rapidly changing disparities are perceptually difficult to track, which suggests that stereopsis is generally slow. But, remarkably, this basic question has not yet been addressed. We compared speed-accuracy trade-off functions between 2 forced-choice discriminations: 1 based on stereoscopic depth and 1 based on luminance. Unexpectedly, both speed-accuracy trade-off functions deviated from chance levels of accuracy at the same response time-approximately 200 ms-with stereo accuracy increasing, on average, more slowly than luminance accuracy after this initial delay. Thus, the initial processing of disparity for perceived depth took no longer than the initial processing of luminance for perceived brightness. This finding, that binocular disparities are available early during visual processing, means that depth is perceived quickly, and, intriguingly, that disparities may be more important for everyday visual function than previously thought.