Abstract
Purpose :
The unique anatomy of the fovea is leveraged by the oculomotor system to sample our visual environment with high resolution. Under normal conditions, the preferred retinal locus (PRL) of fixation is in the foveola, but non-foveal PRLs can develop in cases where central vision is lost. We investigate whether a similar shift in the PRL exists in the S-cone pathway of normal subjects, where a small natural scotoma is present due to the absence of S-cones in the foveola.
Methods :
We used an adaptive optics scanning laser ophthalmoscope (AOSLO) to record retinal videos while subjects (n = 8) viewed a tumbling-E stimulus presented under L/M- and S-cone isolating conditions through a custom RGB display channel coaligned with the AOSLO raster. For each condition, the size of the letter E was set to match the subject’s acuity threshold measured in a preliminary experiment. In the main experiment, trials consisted of a 5-second window during which the optotype was presented randomly at 1 of 9 positions in a 3x3 square grid (grid spacing = 0.5°) centered on the imaging raster. Subjects were instructed to direct their gaze at the letter and report its orientation via keypress. Once the response was logged, the stimulus disappeared for 200 ms before moving to a new location, where it was shown until the subject responded again. This process repeated until the 5-second trial concluded. AOSLO videos were recorded during each trial to monitor eye position. For each cone-isolating condition, 60 trials (300 seconds of video) were obtained, corresponding to ~200 stimulus presentations. All trial videos were registered to a seamless retinal map generated using custom software, enabling the determination of the retinal landing points for every stimulus delivery. For each condition, the PRL was defined as the mean stimulated retinal locus during the 233 ms preceding each response.
Results :
Relative to the L/M-cone PRL, the S-cone PRL was shifted superiorly in all subjects by an average of 6.9 arcmins. The magnitude of this displacement was significantly correlated with S-cone-mediated logMAR (r(6)=0.71; p=0.048), with smaller shifts observed in subjects who exhibited better S-cone acuities.
Conclusions :
In general, the S-cone PRL was superiorly displaced from the L/M-cone PRL, although the magnitude of this shift varied between subjects. These results suggest there may be heterogeneity in the foveal topography of S-cones in normal individuals.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.