Asymmetries in fine spatial vision and cone density within the foveola

Vision is characterized by well-known asymmetries in the visual periphery; sensitivity is higher along the horizontal compared to the vertical meridian. These asymmetries stem in part from differences in retinal photoreceptors densities. The extent of these asymmetries is reduced with decreasing eccentricity, suggesting that they may be absent in the central 1 deg fovea. Yet, cone density is characterized by asymmetries even within the foveola. Thanks to high-precision eyetracking and a system for gaze-contingent display control, here we investigated whether perceptual asymmetries are present at this scale and, using high-resolution anatomical data from Reinger et al (2021), how they relate with asymmetries in cone density across the foveola. Participants (n = 12) were required to discriminate the orientation (± 45deg) of a 2’x7’bar presented 20’ away from the center of gaze. Stimuli were briefly flashed for 50ms at four possible locations along the cardinal directions and their contrast was set to yield ~70% correct responses. Our results demonstrate that vision is asymmetric even in the foveola; fine discrimination is better for stimuli in the upper visual field and along the horizontal meridian compared to the vertical meridian. A re-examination of data presented in Reinger et al (2021) showed that these differences qualitatively match with asymmetries in cone densities when eccentricity is calculated with respect to the preferred retinal locus (PRL i.e., the foveal location where stimuli are centered on during sustained fixation). Interestingly, behavioral and anatomical data do not match when eccentricity is calculated with respect to the region of highest density in the foveola, which is generally offset from the PRL. These findings suggest that cone density and the specific location chosen as the preferred retinal locus contribute to the observed perceptual asymmetries in the central fovea likely leading to an overall higher acuity in the upper visual field.