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Ziyun Zhu, Dongjun He, Fang Fang; Crowding suppresses cortical responses to the target in human early visual cortex. Journal of Vision 2015;15(12):101. doi: https://doi.org/10.1167/15.12.101.
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Crowding is the identification difficulty for a target in the presence of nearby flankers. Recent studies (Chen et al., 2014; Millin et al, in press) have shown that, using an indirect fMRI/ERP measure, the magnitude of crowding effect was closely associated with an early mutual cortical suppression between the target and flankers in V1. Here, we used the fMRI-based population receptive filed (pRF) technique to directly assess the effect of crowding on cortical responses to the target and flankers in human visual areas. The target was centered at 7.25° eccentricity in the upper-left visual quadrant with two adjacent flankers positioned radially. The target and flankers were a circular patch of a sine-wave grating (radius: 1.75°; contrast: 1.0; spatial frequency: 2 cycles/°) and were presented in a uniform gray background. The orientation of the flankers could be either perpendicular or parallel that of the target, resulting in a weak or strong crowding effect, which was confirmed by a separate psychophysical test. We identified voxels responding to the target and flankers based on their pRF parameters. We found that the responses of the target voxels in V1 and V2 were significantly weaker in the strong crowding condition than in the weak crowding condition, while the responses of the flanker voxels showed no difference in the two crowding conditions. We also found that the suppressive effect with the target voxels depended on subjects’ spatial attention to the stimuli. These results provide direct evidence that the suppressed cortical responses to the target in human early visual cortex and spatial attention may play a critical role in visual crowding.
Meeting abstract presented at VSS 2015
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