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Jun-Yun Zhang, Gong-Liang Zhang, Cong Yu; Revealing the mechanisms underlying inner-outer asymmetry and visual crowding. Journal of Vision 2018;18(10):853. doi: 10.1167/18.10.853.
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© ARVO (1962-2015); The Authors (2016-present)
The outer flanker induces stronger crowding than does the inner flanker (Bouma, 1973). This classical yet unsolved mystery is considered a hallmark property of visual crowding. Here we tested a hypothesis that the roles of different crowding mechanisms may be responsible for the asymmetric crowding effects by inner and outer flankers. Observers reported a Sloan-letter target (7.5° eccentricity) that was flanked by either an inner or outer Sloan letter along the horizontal meridian. Crowding was quantified by threshold changes or critical spacing. Results: (1) When the target and flanker were randomly drawn from the same 5 Sloan letters, crowding was strongly asymmetric (baseline-normalized thresholds with an outer/inner flanker = 1.75/1.18); (2) When the target and flanker were each drawn from a separate 5-letter group and the observers were shown with a printed target list, to avoid stimulus confusion in visual memory, the corresponding thresholds changed to 1.57/1.14; (3) When the flankers were 10x10 scrambled, as well as replaced by same-sized black squares, to remove stimulus features, the corresponding thresholds further reduced to 1.19/1.14. (4) The Bouma factors with an outer flanker were 0.37, 0.26, and 0.13 in conditions 1-3, respectively. (5) When a flanker was placed above/below the target (tangential arrangement), the thresholds were unchanged (1.08-1.12) across conditions 1-3, and similar to unchanged thresholds with an inner flanker. Our results suggest: First, outer-flanker crowding consists of target-flanker confusion in visual short-term memory (grouping effect in 1-2), feature confusion (feature scrambling effect in 2-3), and lateral masking (3), while inner-flanker crowding includes lateral masking only. This difference explains inner-outer crowding asymmetry. Second, the Bouma factor can be divided into three components, each reflecting the effect of one crowding mechanism. Third, top/bottom-flanker crowding, like inner-flanker crowding, is caused by lateral masking and significantly weaker than outer-flanker crowding, which explains radial-tangential crowding asymmetry.
Meeting abstract presented at VSS 2018
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