Abstract
Crowding is the substantial interference of neighboring items on target identification. Crowding with letter stimuli has been studied primarily in the visual periphery, with conflicting results for foveal stimuli. While a cortical locus for peripheral crowding is well established (with a large spatial extent up to half of the target eccentricity), disentangling the contributing factors in the fovea is more challenging due to optical limitations. Here, we used adaptive optics (AO) to overcome ocular aberrations and employed high-resolution stimuli to precisely characterize foveal lateral interactions with high-contrast letters flanked by letters. Crowding was present, with a maximal edge-to-edge interference zone of 0.75-1.3 minutes at typical unflanked performance levels. In agreement with earlier foveal contour interaction studies, performance was non-monotonic, revealing a recovery effect with proximal flankers. Modeling revealed that the deleterious effects of flankers can be described by a single function across stimulus sizes when the degradation is expressed as a reduction in sensitivity (expressed in Z-score units). The recovery, however, did not follow this pattern, likely reflecting a separate mechanism. Additional analysis reconciles multiple results from the literature, including the observed scale invariance of center-to-center spacing, as well as the size independence of edge-to-edge spacing.
Highlights
Crowding is the substantial interference of neighboring items on target identification
Flom made a precise distinction between the term “contour interaction,” which he used to refer to low-level spatial interactions between nearby contours, and the more general “crowding effect,” which included eye movements and attentional factors[11]
The solid data points indicate average performance at each spacing, with error bars representing 95% confidence intervals from maximum likelihood estimation based on the raw data and binomial statistics
Summary
Crowding is the substantial interference of neighboring items on target identification. While a cortical locus for peripheral crowding is well established (with a large spatial extent up to half of the target eccentricity), disentangling the contributing factors in the fovea is more challenging due to optical limitations. Some of the earliest investigations of crowding were performed in the fovea of normal and amblyopic observers by Flom and colleagues[9] They found that the maximal extent of interference from bar flankers on identification of a Landolt C was correlated with the size of the minimum resolvable letter for each subject. Flom made a precise distinction between the term “contour interaction,” which he used to refer to low-level spatial interactions between nearby contours, and the more general “crowding effect,” which included eye movements and attentional factors[11]. Usage of the term “crowding” has been expanded to www.nature.com/scientificreports/
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