Asymmetries in Foveal Vision

Abstract

Visual perception is characterized by known asymmetries in the visual field; human’s visual sensitivity is higher along the horizontal than the vertical meridian, and along the lower than the upper vertical meridian. These asymmetries decrease with decreasing eccentricity from the periphery to the center of gaze, suggesting that they may be absent in the 1 ∘ foveola, the retinal region used to explore scenes at high-resolution. Using high-precision eyetracking and gaze contingent display, allowing for accurate control over the stimulated foveolar location despite the continuous eye motion at fixation, we investigated fine visual discrimination at different isoeccentric locations across the foveola and parafovea in 12 human observers (both sexes). Although the tested foveolar locations were only 0.3 ∘ away from the center of gaze, we show that, similar to more eccentric locations, humans are more sensitive to stimuli presented along the horizontal than the vertical meridian. Whereas the magnitude of this asymmetry is reduced in the foveola, the magnitude of the vertical meridian asymmetry is comparable but, interestingly, the asymmetry is reversed: stimuli presented slightly above the center of gaze are more easily discerned than when presented at the same eccentricity below the center of gaze. Therefore, far from being uniform, as often assumed, foveolar vision is characterized by perceptual asymmetries. Further, these asymmetries differ not only in magnitude but also in direction compared to those present just ∼ 4 ∘ away from the center of gaze, resulting in overall different foveal and extrafoveal perceptual fields.