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Daniel R Coates, Dennis M Levi, Ramkumar Sabesan; The gradient of parafoveal crowding. Journal of Vision 2019;19(10):13b. doi: https://doi.org/10.1167/19.10.13b.
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The spatial extent of interference from nearby stimuli (“crowding”) has been well-mapped in the visual periphery, where its magnitude is strong. The exact dimensions of the crowding zone in the fovea and parafovea have been more elusive, however, since identification targets at threshold sizes in central vision are strongly affected by blur and eye movements. To combat these technical limitations we used adaptive optics (AO) to present optimally-corrected stimuli to the eye, permitting precise retinal targeting and the use of stimuli smaller and brighter than possible with conventional displays. In an AO scanning-laser ophthalmoscope (AOSLO), we measured crowding zones for Tumbling-E targets flanked by four Tumbling-Es in the fovea and at 1° eccentricity. The 1.5° raster resulted in a pixel size of ~0.18′. In separate experimental runs, QUEST controlled the size of letters flanked at nominal edge-to-edge spacings of 1, 2, or 4 times the letters size, as well as unflanked items. Letter sizes resulting in asymptotic (unflanked) performance of ~90% correct were used to compute crowding zones. The corresponding letters had a bar-width of ~0.7′ and ~1.27′ at the fovea and 1°, respectively. To determine the critical spacing for crowding, we fitted an exponential function to the performance versus flanker spacing curves. Using a strict criterion of >95% of the asymptotic performance level, we estimated critical spacings of 1.3′ and 5.1′ edge-to-edge, for the fovea and 1°, respectively. The corresponding E2 values (eccentricity where foveal thresholds double) were 1.3° for resolution and 0.34° for the critical spacing, in good agreement with previous work showing the anatomical versus neural loci of these tasks. While there remains some debate about the exact relationship between foveal and peripheral crowding, these results bridge these two regimes, revealing the smooth gradient when limiting factors can be effectively controlled.
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