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Peng Cai, Dongjun He, Fang Fang; The size of population receptive field in V2 and crowding effect. Journal of Vision 2014;14(10):769. doi: https://doi.org/10.1167/14.10.769.
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Crowding is the identification difficulty for a target in the presence of nearby flankers. Based on psychophysical findings, many theories have been proposed to explain crowding at multiple levels. However, the neural mechanism of visual crowding is largely unknown. Here, we used the fMRI-based population receptive field (pRF) technique to probe this issue. A target was centered at 6.25° eccentricity with two adjacent flankers positioned radially. The target and flankers were a circular patch of a sine-wave grating (radius: 1.25°; contrast: 1.0; spatial frequency: 2 cycles/°; orientation: -45° or 45°) and were presented in a uniform gray background. They rotated round the fixation point and were displaced 20° every 2 seconds. The orientation of the flankers could be either perpendicular or parallel to that of the target, which resulted in a weak or strong crowding effect, as confirmed by a separate psychophysical test. We acquired BOLD signals responding to the rotating stimuli, and then estimated the pRF of each voxel in early visual areas in the weak and strong crowding conditions. . We found that, for the voxels in V2 responding to the target, their mean pRF size was significantly smaller in the weak crowding condition than that in the strong crowding condition. Such a pRF size difference was closely associated with subjects attention. The difference completely vanished when subjects performed a demanding fixation task. We speculate that the pRF size reduction might serve to prevent interference from the flankers and consequently weaken the crowding effect. Attention might play a significant role in this process.
Meeting abstract presented at VSS 2014
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