Abstract
Purpose :
The visual input is constantly moving, even during fixation as a result of ocular drift. Drift profoundly impacts fine spatial vision, to the point that an individual's visual acuity can be predicted by the characteristics of this motion. The impact of drift, however, has primarily been assessed with simple isolated stimuli. Yet, the foveal input is often complex and crowded. While crowding is usually studied in the visual periphery and is regarded as a cortical phenomenon, we posit that crowding in the foveola is influenced by ocular drift. Further, although it is well established that the magnitude of crowding increases with eccentricity, it remains unknown if this increase starts already in the 1° foveola. Here, we examine how crowding changes with eccentricity in this region, and how it is influenced by ocular drift.
Methods :
Acuity (N=13 normally sighted) was measured in a 4AFC task. Digits, in Pelli font, were viewed monocularly in isolation or with flankers, and were presented at different foveal and extrafoveal eccentricities. Stimuli widths ranged from 0.4'-4.5' and were adjusted using an adaptive procedure. Eye movements were measured with high-precision using digital Dual Purkinje Image eye-tracking. To limit visual stimulation around the desired foveal eccentricity, we used a state-of-the-art gaze-contingent display system allowing for more accurate gaze localization and retinal stabilization.
Results :
Results show that crowding increases with eccentricity already within the foveola; acuity in the presence of flankers decreases as a function of foveal eccentricity, with a decrease of 32% at the center of gaze, and an additional 24% drop as early as 10’ away. Further, drift modulated the strength of crowding; patterns generating slightly larger offsets between the preferred locus of fixation and the target resulted in larger crowding effects, but didn’t influence acuity. Finally, crowding increases with eccentricity at a rate three times slower in the foveola than extrafoveally.
Conclusions :
Our findings reveal that visual crowding doesn’t affect the whole foveola equally; its strength significantly increases even with arcminute changes in eccentricity and changes in the pattern of ocular drift. These results indicate that crowding at the foveal scale is not only cortical, but also an oculomotor phenomenon.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.