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Juan Chen, Qing Yu, Yingchen He, Fang Fang; An early cortical suppression might contribute to crowding. Journal of Vision 2012;12(9):328. doi: https://doi.org/10.1167/12.9.328.
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Although crowding has been studied extensively with visual psychophysics for many years, its neural mechanism, especially when and where crowding occurs in the brain, remains elusive. Here, we tapped into this question using event-related potentials (ERPs). A target was centered at 8° eccentricity in the upper-left visual quadrant, either alone or with two flankers positioned radially. Both the target and flankers were a circular patch of a sinusoidal grating. We changed the center-to-center distance between the target and flankers to manipulate the magnitude of crowding effect. In the near flanker condition, the distance was 2.48°. In the far flanker condition, it was 5.07°. Only in the former condition, there was a significant crowding effect as manifested by orientation discrimination impairment with the target, consistent with Bouma’s law of crowding. We measured the earliest ERP component (C1) evoked by five stimulus configurations, including the target only, the target with the near flankers, the target with the far flankers, the near flankers only and the far flankers only. The C1 had a peak latency of about 80 ms and is believed to be generated in early visual cortical areas (e.g. V1 and V2). We found that, the sum of C1 amplitudes evoked by the target and the far flankers only was equal to the C1 amplitude by the target with the far flankers. However, the sum of C1 amplitudes evoked by the target and the near flankers only was significantly smaller than the C1 amplitude by the target with the near flankers, indicating a cortical suppression between the target and flankers. Notably, the suppression was not found when subjects’ attention was directed away from the stimuli. These results suggest that the early cortical suppression enabled by spatial attention might contribute to the crowding effect.
Meeting abstract presented at VSS 2012
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