Consistency of the PRL Across Vergence Demand as a Measure of Objective Fixation Disparity

The preferred retinal locus of fixation (PRL) is the exact region of the retina where humans place images of fixated stimuli. The PRL is not necessarily at the center of the fovea (the location of peak cone density). However, its location is remarkably stable across days and tasks. It remains unknown if or how the PRL changes under binocular viewing conditions. Subjects reportedly over- or under-converge on targets as a function of vergence demand, a phenomenon known as fixation disparity. This implies that the PRL changes with vergence. However, many of these studies found varying amounts of fixation disparity and relied on eye tracking measurements taken from the anterior segments of the eye to infer the exact retinal position of fixated stimuli. The present study aims to examine the location of the PRL on the retinal surface directly and for both eyes simultaneously through the use of a binocular tracking scanning laser ophthalmoscope. Binocular PRLs for a 15-arcmin-sized fixation stimulus were measured in five emmetropic subjects for three different vergence demands (0°, 4° and 8°). A monocular PRL was also measured for comparison. While previous work would suggest that PRLs would change with vergence demand, our measured PRLs showed only slight variations from one condition to another, with no systematic deviation that would indicate an over- or under-convergence on the target. One or both eyes of some subjects would occasionally deviate from the target in depth but would realign quickly. The deviations were brief and were likely associated with breaks in fusion. Our results indicate that the PRLs of either eye do not change systematically under binocular conditions. This expands on previous findings on the stability of the PRL and shows that it remains consistent not only across days and tasks, but also under binocular conditions.