Distinct growth regimes govern crowding in foveal and extrafoveal vision

Abstract

Visual crowding--where object recognition is impaired by nearby stimuli--is a well-documented phenomenon in the visual periphery, thought to reflect fundamental processes of grouping and segmentation. A hallmark of crowding is its spatial extent increasing linearly with eccentricity, as described by Boumas law, reflecting retinal convergence and cortical magnification. Although crowding is typically studied in the periphery--and Boumas law predicts no crowding at the center of gaze--it occurs even foveally. Within the foveola, the central 1 deg of the visual field, characterized by peak visual acuity and largely one-to-one connectivity between photoreceptors and ganglion cells, the Bouma law breaks and the extent of crowding is expected to remain constant with eccentricity. Yet, whether crowding varies at this fine spatial scale remains unknown. To investigate this, we combined high-resolution eye tracking with a gaze-contingent display system to precisely localize gaze and measure crowding thresholds at multiple foveolar eccentricities. Our results reveal that crowding does increase linearly within the foveola, but at a rate significantly slower--approximately 3.5 times-than in the extrafovea. By showing that even within the highest-acuity region of the visual field, spatial integration zones are not fixed but change with eccentricity these findings challenge the view of the central fovea as a spatially uniform processing zone and demonstrate that crowding at this scale follows a distinct regime. SignificanceWhen we view objects in clutter, recognition declines--a phenomenon known as crowding. This effect increases with distance from the center of gaze, following Boumas law, which predicts little or no crowding at the foveas center. As this law breaks at the center of gaze, it has been hypothesized that around this region, crowding remains constant with eccentricity. Using high-resolution eyetracking, we show that even within the central 0.4{degrees} of the visual field, crowding increases steadily with minute stimulus offsets from the point of fixation. This growth is much slower than in peripheral vision, revealing a distinct crowding regime in the foveola, likely reflecting cortical and retinal differences. Our findings offer new insights into the fine-scale organization of spatial vision.