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Siu-Fung Lau, Sing-Hang Cheung; Effects of Kanizsa's illusory contours on crowding strength. Journal of Vision 2010;10(7):1345. doi: 10.1167/10.7.1345.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Crowding is the detrimental effect of surrounding objects on the identification of a target object. The cortical locus of crowding remains a controversy. Processing of Kanizsa′s illusory contours has been shown to start at V3. Here we attempt to localize crowding relative to V3 by asking if illusory contours influence crowding. Methods: Five normally sighted young adults participated. Target stimulus was a Kanizsa′s square inducer (0.5° diameter) presented at 4.5° eccentricity in the lower right visual field for 200ms. Subjects identified the target orientation in a 4AFC task. In the 4-flanker condition, 3 inducers were positioned to form a Kanizsa′s square with targets of 1 orientation. Center-to-center distance between the target and the lower right inducer was 1°. The fourth flanker was another inducer placed at 1° radially from the target on the fixation side. In the 2-flanker condition, the upper right and lower left inducers were removed. Discrimination index (d′) for each target orientation was calculated from 60 trials for both conditions. Results: Illusory contours (IC) could be perceived only in 1 of the 4 target orientations in the 4-flanker condition. The 4-flanker condition had lower d′, indicating strong crowding, than the 2-flanker condition for all target orientations. Average differences in d′ between the 4-flanker and 2-flanker conditions for the IC-present and IC-absent trials were 0.50+0.16 and 0.72+0.16 respectively. IC-present trials resulted in significantly smaller d′ difference than IC-absent trials (t(4) = 2.60, p = .03, one-tailed), indicating higher resistance to crowding from 2 additional flankers. Conclusion: Effect of additional flankers was reduced by the formation of illusory contours. The results suggest that illusory contour processing occurs before crowding, and thus, the cortical locus for crowding would be after V3. Preliminary results from a follow-up experiment with classification image supported the utilization of illusory contours in our task.
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