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Ashley Clark, Janis Intoy, Haizhu Yang, Martina Poletti; Fixational eye movements and crowding in the foveola.. Journal of Vision 2020;20(11):1514. doi: https://doi.org/10.1167/jov.20.11.1514.
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© ARVO (1962-2015); The Authors (2016-present)
Visual crowding, the inability to recognize objects in clutter, has been studied for decades. While most research has focused on its effects in the visual periphery, this phenomenon has been minimally studied in the foveola, the 1-degree retinal region where visual acuity is highest. Fixational eye movements, in particular ocular drift, continually shift retinal projections of objects across the foveola. While they may not influence peripheral crowding, they may play a role when crowded stimuli are presented foveally. To address these issues, we assessed the effects of crowding at the foveal scale while tracking gaze position with high precision.
Emmetropic subjects (n = 9) were instructed to identify a target, which was either presented alone (uncrowded condition) or surrounded by four flakers of the same size (crowded condition). Stimuli, presented foveally, consisted of Pelli-font numbers designed specifically for studying crowding in the fovea. The threshold strokewidth of the target was calculated in both conditions using an adaptive procedure. In the crowded condition, both target size and flanker spacing changed together.
Our findings show that crowding occurs at the foveal scale. With identical stimulus size, visual discrimination is impaired when a foveal stimulus is surrounded by flankers. On average, target size needs to increase 0.63 ± 0.39 arcmin (an increase of 40%) for performance to be comparable to the uncrowded condition. While ocular drift was unaffected by target size, our results indicate that individuals with larger strokewidth thresholds were characterized by larger drifts in both the uncrowded and crowded condition. Furthermore, the larger the difference in strokewidth thresholds, the larger the difference in drift span between crowded and uncrowded conditions. This evidence suggests that foveal crowding is not only determined by retinal and cortical factors, but fixational eye movements may play a crucial role in modulating its strength.
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