Coarse Stereopsis and Eye Alignment in Strabismus

Depth order judgements are supported by both fused (fine) and diplopic (coarse) binocular disparities. There is evidence that they are subserved by distinct mechanisms, for instance, coarse stereopsis matures earlier than fine and is spared in some types of amblyopia whereas fine stereopsis is disrupted. We hypothesize that coarse stereopsis may be used during development to achieve binocular fusion, align the eyes, and promote fine stereopsis. Here we assess this possibility in children receiving eye alignment surgery for strabismus. We tested 14 children pre-surgery, then 3 and 12 months post-surgery. Stereoscopic stimuli were viewed through liquid crystal shutter glasses. Depth-order discrimination was measured for greyscale cartoon characters, relative to a zero-disparity reference frame. We tested 5 disparities characterized as fine (0.17, 0.67 degrees) or coarse (2.0, 2.5, 3.0 degrees) based on previous work. Aged-matched norms from a previous study with the same stimuli and test paradigm were used to identify atypical performance (>2 SD) in strabismic children. Fusion and interocular suppression were measured using the Worth-4-Dot test and a dichoptic eye chart, respectively. Eye alignment (prism dioptres) was measured with a prism cover test. We found that better eye alignment post-surgery was associated with higher coarse disparity accuracy (r= -0.7, p= .02), but not fine (r= 0.2, p= .6), pre-surgery. Pre-surgery performance was atypical for fine disparities for all participants, but typical for coarse disparities for 86% of participants. Fine stereopsis remained atypical for all except 1 participant at 3 months post-surgery, but improved beyond test-retest variability in 73% of participants (all except 1 with typical coarse stereopsis) by 12 months post-surgery. Five participants (all exotropia) achieved typical fine stereopsis, and three of these exhibited typical sensory fusion and interocular suppression. Our results support the hypothesis that coarse stereopsis facilitates eye alignment and the subsequent development of high-resolution stereoscopic mechanisms.