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Augustin Burchell, Noah C Benson, Jing Y Zhou, Jonathan A Winawer, Denis G Pelli; Using fMRI to link crowding to hV4. Journal of Vision 2019;19(10):14a. doi: https://doi.org/10.1167/19.10.14a.
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© ARVO (1962-2015); The Authors (2016-present)
Crowding, the unwanted perceptual merging of adjacent stimuli, is well studied and easily measured, but its physiological basis is contentious. We explore its link to physiology by combining fMRI retinotopy of cortical area hV4 and psychophysical measurements of crowding in the same observers. Crowding distance (i.e. critical spacing) was measured radially and tangentially at eight equally spaced sites at 5° eccentricity, and ±2.5° and ±10° on the horizontal midline. fMRI mapped the retinotopy of area hV4 in each hemisphere of the 5 observers. From the map we read out cortical magnification, radially and tangentially, at the 12 sites tested psychophysically. We also estimated the area of hV4 in mm2. Combining fMRI with psychophysics, last year we reported conservation of a roughly 1.8 mm crowding distance on the surface of hV4 (the product of cortical magnification in mm/deg and crowding distance in deg) across eccentricity and orientation, in data averaged across observers (Zhou et al. 2018 VSS). Crowding distances were less well preserved in the V1–V3 maps. Conservation of the hV4 crowding distance across individual observers would mean a fixed product of visual crowding distance and cortical magnification, which implies a negative correlation between log crowding distance and log magnification. Separate linear mixed-effects models of log crowding area and log cortical magnification each account for about 98% of the variance. Log areal hV4 cortical magnification shows a trend toward a negative correlation with log areal crowding across 10 hemispheres (r=−0.53, p=0.11); log hV4 surface area and log areal crowding show a similar negative correlation (r=−0.55, p=0.1). The trend toward larger crowding distances in observers with less surface area in hV4 is consistent with the possibility that crowding distances, though highly variable when measured in the visual field, are approximately conserved on the surface of the hV4 map.
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