Contrast-reversed binocular dot-pairs in random-dot stereograms for depth perception in central visual field: Probing the dynamics of feedforward-feedback processes in visual inference

Abstract

In a random-dot stereogram (RDS), the spatial disparities between the interocularly corresponding black and white random dots determine the depths of object surfaces. If a black dot in one monocular image corresponds to a white dot in the other, disparity-tuned neurons in primary visual cortex (V1) respond as if their preferred disparities become non-preferred and vice versa, reversing the disparity sign reported to higher visual areas. Reversed depth is perceptible in the peripheral but not the central visual field. This study demonstrates that, in central vision, adding contrast-reversed dots to a noisy RDS (containing the normal contrast-matched dots) can augment or degrade depth perception. Augmentation occurs when the reversed depth signals are congruent with the normal depth signals to report the same disparity sign, and occurs regardless of the viewing duration. Degradation occurs when the reversed and normal depth signals are incongruent with each other and when the RDS is viewed briefly. These phenomena reflect the Feedforward-Feedback-Verify-and-reWeight (FFVW) process for visual inference in central vision, and are consistent with the central-peripheral dichotomy that central vision has a stronger top-down feedback from higher to lower brain areas to disambiguate noisy and ambiguous inputs from V1. When a RDS is viewed too briefly for feedback, augmentation and degradation work by adding the reversed depth signals from contrast-reversed dots to the feedforward, normal, depth signals. With a sufficiently long viewing duration, the feedback vetoes incongruent reversed depth signals and amends or completes the imperfect, but congruent, reversed depth signals by analysis-by-synthesis computation.

The central-peripheral dichotomy and metacontrast masking

“…The perceptual impression was qualitatively different from that of seeing too many contour fragments crowded together like in visual crowding. In depth perception of random-dot stereograms in central vision, it has been demonstrated that stimulus components (from dichoptically contrast-reversed dots) that are normally vetoed (and thus invisible) can nevertheless enhance or sometimes degrade another perceptual outcome arising from other stimulus components (from dichoptically contrast-matched dots) ( Zhaoping, 2021 ). This manifests a complex interaction between the feedforward and feedback processes.…”

“…This faster deterioration suggests, according to the CPD, that top-down feedback is likely involved to achieve this hyperacuity feat. Indeed, at fovea, this acuity worsens with shorter viewing durations ( Westheimer and McKee, 1977 ), presumably because a shorter viewing hinders or prevents the feedback process to function (as suggested by an example of depth perception at fovea ( Zhaoping, 2021 )), and, if so, the CPD predicts that, at a more peripheral location, hyperacuity should suffer less from a shorter viewing duration. Many other visual discrimination tasks, on which human performance deteriorates with visual field eccentricity faster than suggested by a reduced V1 cortical magnification factor ( Strasburger et al, 2011 ), could be examined analogously in this light.…”

EEG response in humans for frequency-tagged anticorrelated random-dot stereograms: Increased coherency and alpha oscillations

“…In order for a dACRDS to provide any sort of sensation of depth, special measures need to be taken: for example, for it to produce reversed depth at all, the dot density must be below 2% (Cumming et al, 1998 ); reversed depth perception can also occur when a dACRDS disk is shown surrounded by a dCRDS annulus in the central visual field (Aoki et al, 2017 ), but this reverse perception of depth also depends on the size of the gap between the two visual stimuli (Asher and Hibbard, 2018 ). Further, depth perception can be reversed when the temporal aspects of vision are exploited: when dACRDS mixed with zero-disparity dCRDS dot noise has been demonstrated to modulate (augment or degrade) depth perception, when it is flashed very briefly in the central visual field and the correlation turns during this short time (Zhaoping, 2021 ). Moving away toward the peripheral visual field where the spatial resolution drops considerably, dACRDS has also been demonstrated to elicit reversed depth perception when the stimulus is intentionally projected into this area (Zhaoping and Ackermann, 2018 ).…”

Robust natural depth for anticorrelated random dot stereogram for edge stimuli, but minimal reversed depth for embedded circular stimuli, irrespective of eccentricity

“…When stimuli contained just a step edge in depth, the majority (67%) of participants who perceived depth in CRDS also perceived depth in the natural rather than reversed direction for ACRDS [ 17 ]. Finally, reversed depth in ACRDS has been reported to be more common when stimuli are presented in the periphery of the retinal image, rather than the centre [ 18 , 19 ].…”

“…Brief presentations will tend to enhance the responses of the transient stereoscopic system, which may be important in the perception of depth from ACRDS [ 21 ]. Longer durations will support the effects of top-down feedback, and thus the reduction of perceived depth in ACRDS [ 19 , 20 ].…”