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Dennis Levi, Gong-Liang Zhang, Cong Yu; Is there a “top-down” component of crowding in peripheral and amblyopic vision?. Journal of Vision 2011;11(11):426. doi: 10.1167/11.11.426.
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Crowding represents an important bottleneck for object recognition in peripheral and amblyopic vision; however there is intense debate about the nature and locus of crowding. The present study evaluated the role of top-down influences by comparing crowding when the target and flankers were drawn either from the same set of Sloan letters, or from different sets of Sloan letters, while maintaining the same target-flanker complexity. When target and flankers are drawn from the same set of letters, crowding in normal peripheral vision is characterized by substantial flanker substitutions, in that observers frequently report a flanker, especially the outer flanker, as the target. Surprisingly, the flanker substitution effect is not evident in the central vision of most strabismic amblyopes. For these observers the rate of reporting the flankers and reporting other unused letters is similar. Crowding, and the flanker substitution effect are both greatly reduced when the target and flankers have opposite polarities. Importantly, when target and flankers are drawn from different sets of letters, crowding is markedly reduced in peripheral vision, and in the central vision of the minority of amblyopes who showed flanker substitution. Our results suggest a top-down effect on crowding which inhibits flanker substitution. This top-down effect is eliminated when the target and flankers have opposite polarities. With opposite target-flanker polarities crowding becomes equally weak regardless of whether target and flankers are from the same letter set or not.
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