The role that binocular vision and stereopsis have in evaluating fundus features

We report two experiments that address the role of stereopsis in viewing the fundus.

Thirteen observers viewed stereo slides obtained by lateral displacement of a fundus camera. We quantified the capacity of seven clinicians (normal stereopsis) to resolve depth in a model eye viewed monocularly and binocularly. We also compared the ability of the same seven and six additional clinicians (with reduced stereopsis) to determine the relative position of the cup shown in normal and reverse disparity as well as answering a questionnaire. We determined optimal performance based on a schematic eye.

Monocular viewing gives better than expected results with binocularity being of greatest benefit for small disparities. Clinicians perform much worse in stereoscopic judgments than predicted by calculation. Few clinicians report making stereo depth judgments in practice. All amblyopic observers report using hyperacuity cues.

This study shows that binocularity and stereopsis are beneficial for judging small disparities under limited circumstances. The level of stereopsis achieved in practice fails to approach the expected limit. We believe that this constriction arises from a known limit to stereo acuity associated with extended fields.