Abstract
Purpose :
Up to now, the processes that drive the formation of a preferred retinal locus of fixation (PRL) in healthy and diseased eyes are unclear. Recent studies have shown that neither the PRL is at the location of peak cone density, nor is the newly formed PRL in diseased retinas at the location of best visual acuity. By probing visual sensitivity with adaptive optics corrected cone-targeted stimuli, we here tested the hypothesis that the PRL may be related to a retinal area of highest visual sensitivity.
Methods :
Visual sensitivity to small spot (0.3 arcmin diffraction limited FWHM, 543 nm) light increments at and around the PRL was tested with adaptive optics scanning laser ophthalmoscope-based microstimulation in healthy participants (n = 3, age = 33.0 ± 6.2 years). Individual PRLs were determined by tracking the location of a small visual stimulus on the retina during fixation. Stimulus locations for sensitivity testing were selected on a retinal image, targeting an individual cone at the PRL and at 8 foveal locations, with 0.1° and 0.2° distance from the PRL along the horizontal and vertical axis. By employing real-time retinal image stabilization and tracking, each position was tested 3 times with an adaptive staircase to find a mean sensitivity threshold per location. Sensitivity was mapped to retinal features (PRL and cone density metrics) offline after an experiment.
Results :
Across all examinations, 50% of the light delivered to the test location covered a mean area of 0.6 ± 0.1 arcmin diameter. In two out of three participants, sensitivity was significantly highest at the test location closest to the PRL (p<0.05, Wilcoxon), while the 2nd highest sensitivity (1.5 dB lower) was 0.2 degree away from the PRL in both participants. In all participants, no significant correlation between cone density at the test location and sensitivity was found (p>0.14). Individual differences in maximum cone density [c/deg2] between participants (~23%) were also not correlated with differences of highest sensitivity values (~3%, p=0.76).
Conclusions :
Differences in foveal sensitivity to cone-sized light stimuli do not correlate with the variation of cone density at test locations. The relationship of PRL and retinal location of peak sensitivity remains unclear: while the two locations seem to match in some participants, other locations as far as 0.2 deg away from the PRL with equal sensitivity were found in all participants.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.