• Clifton Schor.
• *OD, PhD, FAAO School of Optometry, University of California, Berkeley, Berkeley, California.
• Optom Vis Sci. 2015 May 1; 92 (5): 544-50.

PurposeA head-centric disparity model of anomalous binocular correspondence (ABC) in strabismus provides a framework that captures several associated perceptual-motor characteristics that are unexplained by the retino-centric model (anomalous retinal correspondence) of Von Graefe and Burian. The head-centric model elaborates on the anomalous-projection model of Verhoeff and Brock, originally described by Wells, late in the 18th century, which proposes that three-dimensional space perception is based on information obtained separately from the two eyes in ABC, without binocular retinal correspondence. Binocular parallax angles formed by the two eyes' monocular head-centric directions provide sufficient information to estimate distance but not enough to stimulate diplopia without a reference for zero disparity.Methods (Model Description)The retino-centric model computes binocular disparity from differences between retino-centric directions specified by the two eyes, with each eye's direction referenced to its own primary visual direction. The head-centric analog to retinal disparity is binocular parallax that could provide distance information but not a stimulus for diplopia. Diplopia is computed from differences between binocular parallax angles subtended by object points and a reference for zero disparity, that is, the head-centric Horopter, which adjusts for viewing distance, independently of convergence of the eyes.Results (Clinical Observations)Several perceptual-motor phenomena associated with anomalous correspondence demonstrate two sensory fusion mechanisms in ABC that involve registered vergence signals and changes in the horopter, independent of vergence.ConclusionsIn ABC, the subjective-squint angle is unaffected by registered vergence movements. Binocular sensory fusion is obtained via the head-centric model by adjusting the diameter of the head-centric horopter, independent of the vergence angle, from the fixation distance to the distance of another reference point. By altering the reference viewing distance for zero disparity, the sign and magnitude of disparity stimuli for fusion and diplopia are changed, thereby enabling the perception of a fused fixation target and the appreciation of physiological diplopia in strabismus.

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