Asymmetries in foveal vision.

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-deg 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 deg 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 ∼4deg away from the center of gaze, resulting in overall different foveal and extrafoveal perceptual fields. The 1-degree foveola, the retinal region responsible for high-resolution vision, has traditionally been studied as a uniform unit. Our research challenges this notion by uncovering perceptual asymmetries in foveolar fine spatial vision. Using a high-resolution eyetracker, we demonstrate that humans discriminate objects above the center of gaze better than below, a pattern opposite to the asymmetries observed just a few degrees away from the foveola. These findings reveal not only that this region is not uniform but also that it is not simply a high-resolution extension of extrafoveal vision but it is characterized by unique features. The discovery of these perceptual asymmetries raises critical questions about the neural representation of foveal input and the underlying factors shaping these differences.