Depth Perception in Strabismus with Perspective Cues

Article: Common eyesight myth demystified
Source: University of St. Andrews (U.K.), via Medical Xpress
Published: January 28, 2021

Colloquially called an eye turn, strabismus is the misalignment of the eyes from the straight forward (orthotropic) position, with one eye turned, for example, outward (temporally) in exotropia or inward (nasally) in esotropia, which can be constant or intermittent. Because the deviation affects normal binocular vision, it is commonly thought that strabismus limits stereopsis or depth perception. Scientists in the U.K. argue that much of what is known about strabismus (in the U.K.) is based on conjecture or on studies that measured motor tasks at near distances. They instead investigated the perception of relative depth from the cue of linear perspective in strabismus using pictorial images and found that "contrary to popular opinion, there is little impact on a major aspect of 3-D vision for a person with a strabismic vision." The senior author of the study explains, “These results support a theory that the brain constructs at least three distinct types of representation that support different competencies in 3-D vision: the perception of distances of objects near the viewer (in personal space), the perception of object distances at walking distances, and the perception of 3-D object shape/layout (relative depth). Our results, combined with a few other studies, suggest that individuals with strabismus may be impaired only in the first component (perception of near distances) but have largely intact perception in the other two.” In other words, although strabismics do experience deficit in depth perception, that limitation applies mostly to near vision, with depth perception of farther distances still largely functional. The study, however, is limited by its very small sample size of 32 individuals, 24 of whom had normal stereovision. A quantitative measure of heterotropia was also not reported in the 8 participants with "no or limited stereovision."

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Zlatkute G, Sagnay de la Bastida VC and Vishwanath D. "Unimpaired perception of relative depth from perspective cues in strabismus." Royal Society Open Science.  7:200955. 23 December 2020. https://doi.org/10.1098/rsos.200955