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Perisaccadic Stereo Depth with Zero Retinal Disparity

Zhi-Lei Zhang1, Christopher R.L. Cantor1 and Clifton M. Schor

When an object is viewed binocularly, unequal perspective projections of the two eyes’ half images (binocular disparity) provide a cue for the sensation of stereo depth. For almost 200 years, binocular disparity has remained synonymous with retinal disparity [1], which is computed by subtracting the distance of each half image from its respective fovea [2]. However, binocular disparity could also be coded in headcentric instead of retinal coordinates, by combining eye position and retinal image position in each eye and representing disparity as differences between visual directions of half images relative to the head [3]. Although these two disparity-coding schemes suggest very different neural mechanisms, both offer identical predictions for stereopsis in almost every viewing condition, making it difficult to empirically distinguish between them. We designed a novel stimulus that uses perisaccadic spatial distortion [4] to generate inconsistency between headcentric and retinal disparity. Foveal half images flashed asynchronously just before a horizontal saccade have zero retinal disparity, yet they produce a sensation of depth consistent with a nonzero headcentric disparity. Furthermore, this headcentric disparity can cancel and reverse the perceived depth stimulated with nonzero retinal disparity. This is the first demonstration that a coding scheme other than retinal disparity has a role in human stereopsis.

Perisaccadic Stereo Depth with Zero Retinal Disparity. Current Biology. 2010 Jul 13;20(13):1176-1181. doi:10.1016/j.cub.2010.04.060

Perisaccadic Stereo Depth with Zero Retinal Disparity

Zhi-Lei Zhang1, Christopher R.L. Cantor1 and Clifton M. Schor

Visual directions of foveal targets flashed just prior to the onset of a saccade are misperceived as shifted in the direction of the eye movement. We examined the effects of luminance level and temporal interactions on the amplitude of these perisaccadic spatial distortions (PSDs). PSDs were larger for both single and sequentially double-flashed stimuli with low than high luminance levels, and there was a reduction of PSDs for low luminance targets flashed immediately before the saccade. Significant temporal interactions were suggested by PSDs for a pair of sequentially presented flashes (ISI = 50 ms) that could not be predicted from the single-flash distortions: PSD increased for the first flash and decreased for the second compared to the single-flash distortions. We also found that when the flash pair was presented near saccade onset, the perceived distortion of the earlier flash overtook that of the later flash, even though the late flash occurred closer in time to the saccade. To explain these effects, we propose that stimulus-dependent nonlinearities (contrast gain control and saccadic suppression) influence the duration of the temporal impulse response of both single- and double-flashed stimuli.

Zhang Z-L, Cantor CR, Schor CM. (2008) Effects of luminance and saccadic suppression on perisaccadic spatial distortions. Journal of Vision. 2008 Dec 18;8(14):22.1-18.

FMRI of vergence control centers in the human brainstem

Jim Maxwell, Zhilei Zhang, Kai Schreiber & Clifton M Schor

Relative to functional imaging of cortical structures, imaging of brainstem tissue is complicated by physiological artifacts due to respiration and circulation. We will characterize the relative strength of these artifacts and design an optimized protocol for data acquisition and analysis tailored to these characteristics. We will then use this protocol to localize the brainstem nuclei that are involved in the oculomotor response to vertical and torsional disparities by controlling vertical and torsional vergence.

FMRI of Depth Perception and Disparity, Using Random Dot Slant Metamers

Zhilei Zhang, Kai Schreiber & Clifton M Schor

FMRI studies of the processing of retinal disparities have commonly confounded disparity processing with the percept produced by changed disparity. We will disambiguate MR signals associated with perceptual changes stimulated by combinations of horizontal and vertical disparity using metameric stimuli that allow us to independently control disparities and depth percepts.

FMRI of Binocular Matching Strategies

Zhilei Zhang, Kai Schreiber & Clifton M Schor

How and at what sites are local features in each eye matched to form disparities when stimulus features are similar or identical to each other, as in a random-dot stereogram, when there are many possible binocular matches? Several rules have been suggested to constrain low-level matches to help solve the correspondence problem, such as uniqueness, smoothness, a disparity gradient limit etc. While there have been extensive psychophysical studies of the matching problem, little is known about where in the visual pathways these constraints are applied. In this aim, we will focus on the question of how early and at what stages the binocular correspondence problem is solved.

FMRI of Depth Scaling with Vergence

Kai Schreiber, Zhilei Zhang & Clifton M Schor

The same retinal input will produce different perceived stereo-slants when the vergence state of the eyes is different, a perceptual phenomenon known as disparity scaling (Rogers and Bradshaw, 1995). Disparity scaling can be predicted from the projection geometry of the visual scene. Where in the visual system do eye movement information and disparity information converge to form a coherent depth percept?

Mathematical Theory of Binocular Disparity

Kai Schreiber & Clifton M. Schor

Binocular disparity is commonly approximated by a two dimensional retinal displacement vector decomposed into horizontal and vertical components. These components have been investigated separately. Mathematically,the foundation of these concepts is not sound, because of the influence of eye movements, and a conceptual complication arising from the notion of retinal correspondence, relative to which retinal disparity has to be measured. This project will work out the basic mathematical properties of retinal disparities and the proper treatment for practical purposes, as well as categorize the use of disparity coordinate systems in the psychophysical literature.

Schreiber, K. M., Tweed, D. B., &Schor, C. M. (2006). The extended horopter: Quantifying retinal correspondence across changes of 3D eye position. Journal of Vision, 6(1), 64-74.

Pulse-step models of control strategies for dynamic ocular accommodation and disaccommodation

Clifton M. Schor and Shrikant R. Bharadwaj

Abstract: Dynamic properties and control strategies of step responses by accommodation and disaccommodation differ from one another. Peak velocity of accommodation increases with response magnitude, while peak velocity and peak acceleration of disaccommodation increase with starting position. These dynamic properties can be modeled as control strategies that use independent acceleration-pulse and velocity-step components that are integrated respectively into phasic-velocity signals that control movement and tonic-position signals that control magnitude. Accommodation is initiated toward its final destination by an acceleration-pulse whose width increases with response magnitude to increase peak velocity. Disaccommodation is initiated toward a default destination (the far point) by an acceleration-pulse whose height increases with dioptric distance of the starting position to increase peak velocity and peak acceleration. Both responses are completed and maintained by tonic-position signals whose amplitudes are proportional to the final destination. Mismatched amplitudes of phasic-velocity and tonic-position signals in disaccommodation produce unstable step responses.

Clifton M. Schor and Shrikant R. Bharadwaj (2006)  Pulse-step models of control strategies for dynamic ocular accommodation and disaccommodation. Vision Research, Volume 46, Issues 1-2, January 2006, Pages 242-258.

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