Random-dot stereopsis in microstrabismic children: stimulus size matters.

Purpose: Although children with microstrabismus demonstrate stereoscopic abilities when assessed with clinical tests containing visible contours (local stereopsis), severe stereoscopic impairments are reported when using random-dot stereogram (RDS). This differential performance may be associated with the decreasing interocular correlation resulting from the central suppression present in the deviated eye, affecting global stereopsis to a greater extent.

Methods: To test this hypothesis, stereoscopic performance on tasks using contour/local (experiment 1), RDS depth (experiment 2), and RDS shape discrimination (experiment 3) was obtained in nine microstrabismic children and compared with that of control participants. For each task, stereoscopic stimuli of 4 and 12 degrees diameter were used to differentially solicit the contribution of the central suppression area.

Results: Results demonstrated that performance on the local stereopsis task was very similar for both target sizes, measurable for all microstrabismic participants, and comparable to that of control subjects. On the other hand, global stereoacuity using RDS was not measurable for one-third of the microstrabismic participants when small stimuli were used, but stereoperception became possible for large stimuli condition in which the interocular correlation disruption was presumably negligible. Along these lines, global shape stereopsis was found to increase by nearly 50% in the microstrabismic group when large targets were used. In all experiments, only coarse stereopsis could be measured in microstrabismic children, regardless of the stimulus size.

Conclusion: Although our findings suggest that interocular correlation may play a role in deficits of stereopsis in microstrabismus, other altered processes such as abnormal experience, retinal correspondence, and high-level coding are likely to also be involved.