Abstract
It’s well established that vision in the periphery and parafovea is characterized by asymmetries; humans are better at discriminating items along the horizontal meridian compared to the vertical meridian. Similarly, sensitivity in the lower visual field is better than in the upper visual field. Current evidence shows that the extent of these asymmetries decrease with eccentricity, suggesting that they may be absent in the central 1deg fovea. However, due to technical limitations this has never been examined. Thanks to high-precision eyetracking and a gaze contingent display control allowing for more accurate localization of gaze, we probed fine visual discrimination at different isoeccentric locations across the foveola and compared it with corresponding locations in the periphery. Participants (n=10) performed a two-alternative forced-choice discrimination task while maintaining fixation on a central marker. Performance was tested at 8 locations, approximately 20 arcmin from the preferred locus of fixation. The same task was replicated at 4.5 degrees eccentricity (n=7) and the stimuli size was adjusted to account for cortical magnification. Our results show that, similarly to what happens in the visual periphery, humans are more sensitive to stimuli presented along the horizontal than the vertical foveal meridian. While the magnitude of this asymmetry across the meridians is smaller in the fovea than extrafoveally, the magnitude of asymmetry along the vertical meridian is equal. Furthermore, foveal asymmetry on this meridian is flipped compared to what is found extrafoveally: objects in the upper foveal meridian are discerned more easily than those in the lower meridian. These findings show that even foveal vision is characterized by perceptual anisotropies and that their characterization is in part different from what is found in the rest of the visual field. Furthermore, while some asymmetries are larger extrafoveally, others are present to the same extent at both scales.