Asymmetries and errors in perception of depth from disparity suggest a multicomponent model of disparity processing

Abstract

In three experiments, asymmetries between the processing of crossed and uncrossed disparities were investigated. The target was a luminance-defined circle concentric to a fixation mark, viewed stereoscopically on a computer monitor for 105msec, Fifteen disparities were presented according to the method of constant stimuli. Observers indicated the apparent direction of target depth relative to fixation. All experiments measured both the accuracy and latency of this response. Experiment 1 showed fewer errors and shorter reaction times for identifying crossed disparities. Experiments 2 and 3 replicated Experiment 1 and also showed that observers may often perceive a target in the direction opposite that prescribed by the disparity information. Wepropose that the asymmetries and reversals result from differences in computation of sign, not of magnitude. This notion is consistent with a scheme of continuous disparity tuning and accounts for such asymmetries and errors without positing disparity pooling mechanisms.A fundamental issue in the study ofstereopsis concerns the manner in which retinal disparities are processed by the visual system. The two most popular schemes involve either a small number of relatively independent channels (or pools ofdisparity detectors), or a large number ofdistinct yet continuously distributed disparity detectors.The first scheme was proposed by Richards (1971) on the basis of experiments that revealed anomalous individual differences, or "stereoanornalies," in disparity processing. Richards found that over half of the observers he tested were insensitive to either crossed or uncrossed disparities in local stereograms. In addition, he observed that, for any observer, two very different disparities of the same sign could signal the same depth percept. Some observers even exhibited an apparent reversal of the disparity sign information: The entire range ofdisparities (crossed and uncrossed) elicited depth in only one direction. Based on these findings, Richards posited at least two distinct and independent pools of integrated disparity detectors (crossed and uncrossed). That is, Richards presumed that, since some observers appeared insensitive to the entire range of disparities of a particular direction, the disparity detectors for that direction were pooled into a single (malfunctioning) mechanism. Moreover, the pheThis work was supported by NIH/NEI EY 10303 and AFOSR F49620-93-I-0307. We wish to thank S. B. Stevenson for his helpful comments regarding the results and R. P.O'Shea, R. Patterson, and two anonymous referees for their helpful comments regarding an earlier

Deficient stereopsis in the normal population revisited: why current clinical stereo tests may not be adequate

“…Our judgement of depth magnitude and its associated sign may be processed at different stages alone the cortical pathway and may be subject to different limitations. This is very much along the lines of a proposal made by Landers and Cormack 8 , who also reported a large percentage of stereo anomalous observers in the population for determining the sign but not the magnitude of the depth. They suggested that while there may be a continuous representation of disparity within the visual cortex, attentive readout of disparity may be a multicomponent serial process, having the following stages; starting from detection of disparite areas (local correlation), magnitude of disparity (present clinical stereo tests and the stimulus used in this present study), detection on the basis of disparity polarity 1 ) and finally discrimination of depth polarity.…”

How to use individual differences to isolate functional organization, biology, and utility of visual functions; with illustrative proposals for stereopsis

“…Manning, et al, 1987) limits, tolerances for polarity reversal , and rates of development (Birch, Gwiazda, & Held, 1982), for various such mechanisms. However, such findings have also been interpreted as consistent with a model that does not posit distinct disparity pooling mechanisms (Landers & Cormack, 1997). Given the limitations for using each of these methods to assess the distinctness of perceptual mechanisms, an additional approach seems warranted.…”

Sensory uncertainty leads to systematic misperception of the direction of motion in depth

“…In the second manipulation, we varied the target's position in depth relative to fixation. The shift of target position in depth does not affect the sensory uncertainty associated with the two retinal images, but we reasoned that it should impact sensory uncertainty due to reduced sensitivity to stimulus properties, such as binocular disparity away from the plane of fixation (Westheimer & Tanzman, 1956;Blakemore, 1970;Schumer & Julesz, 1984;Landers & Cormack, 1997). We further reasoned that if both sensory noise manipulations have similar consequences from a perceptual inference perspective, they should have similar impacts on behavioral performance.…”