The Preferred Retinal Loci When the Eyes Converge
By Eric Craypo
Significance
This study sought to investigate an incompatibility between two findings about human fixation. Dr. Roorda explains the study: "First, earlier experiments from our lab found that the eye's direction of gaze is extremely precise and repeatable and remains the same between tasks and between days. However, there is also a conventional understanding among clinicians that the eyes do not converge fully when looking at a near object -- a phenomena called exotropia. To resolve this discrepancy, VS student Norick Bowers deployed our eye tracking technology into a binocular system in Susana Chung's lab. Turns out that we were both right. The eyes do not converge fully on near targets, but the deviation is very small. We found that the nondominant eye tends to underconverge by about 5 arc minutes for a near 40 cm viewing distance."
Abstract
The preferred retinal locus (PRL) is the position on the retina to which humans direct stimuli during fixation. In healthy normal eyes, it has been shown to be very stable across time and between different tasks. Previous measurements of the PRL have been made under monocular viewing conditions. The current study examines where the PRLs in the two eyes’ retinas are when subjects fixate binocularly and whether they shift when the demand for the eyes to converge is changed. Our apparatus allows us to see exactly where binocular stimuli fell on the two retinas during binocular fixation. Thus, our technique bypasses some of the issues involved in measuring binocular alignment with subjective techniques and previous objective techniques that use conventional eye trackers. These results show that PRLs shift slightly but systematically as the demand for convergence increases. The shifts cause under-convergence (also called exo fixation disparity) for near targets. They are not large enough to cause a break in binocular fusion. The fixation disparity we observed with increasing vergence demand is similar to fixation disparity observed in previous reports.
Eso and Exo Fixation Disparity
In the image shown here, the upper drawing shows the classic sketches of fixation and fixation disparity. The lower panels show the relative shifts in where the images project onto the retina in the presence of fixation disparity.
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Related Information
Martin Banks
Susana Chung
Austin Roorda