Exploring depth and distance perception in strabismus

Abstract

Strabismus can be defined as the inability to coordinate the eye muscles that ordinarily fixate the two eyes on a single target, resulting in lack of a single binocular vision. Strabismus as a disorder has been known since the times of Hippocrates. However, the exact extent of the limitations in depth and distance perception in strabismic individuals with limited or no binocular stereovision in comparison to typically developed binocular individuals remains unclear. This thesis aimed to explore differences in depth perception between strabismic and non-strabismic individuals by examining qualitative aspects of depth perception, egocentric distance perception, and relative depth perception. These aspects were examined by conducting two experiments for each. The first two experiments revealed that feeling of immersive and tangible depth (a.k.a. stereopsis) is not uniquely linked to binocular disparities and may be experienced by individuals with varying levels of binocular stereovision under monocular aperture viewing (evoking monocular stereopsis). The next two experiments explored egocentric distance perception by measuring familiar object size judgments (as a proxy for distance perception) under monocular, binocular, and stereoscopic viewing. All subjects made similar size judgments under all viewing conditions. The last two experiments showed that observers with no/limited stereovision do not have deficits in the perception of relative depth from perspective cues. They showed similar levels of susceptibility and capacity to make depth judgements from the perspective cue to those of stereonormal individuals. The results of this thesis add systematic insight into understanding the way individuals with strabismus perceive depth and distance in comparison to typically developed binocular individuals. Overall, it suggests that there are more similarities between these two groups in the perception of 3D space than suggested by anecdotal reports and conjecture. This emphasizes the need for further systematic exploration to determine the specific limitations strabismics face whilst performing everyday tasks.