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Jingyang Zhou; Conservation of crowding distance in human V4. Journal of Vision 2017;17(15):19. doi: 10.1167/17.15.19.
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© ARVO (1962-2015); The Authors (2016-present)
Crowding — the inability to recognize objects in clutter — severely limits object recognition and reading. In crowding, a simple target (e.g. a letter) that is recognizable alone cannot be recognized when surrounded by clutter that is less than the psychophysical crowding distance away (deg). Prior work shows that crowding distance scales linearly with target eccentricity and varies with the direction of crowding. The crowding distance is approximately double for flankers placed radially rather than tangentially. Multiplying the psychophysical crowding distance by the cortical magnification factor yields the cortical crowding distance (mm of cortex). In V1, radial cortical crowding distance is about 6 mm and conserved across eccentricity, but not across orientation (Pelli, 2008). Since crowding distance in V1 is conserved radially across eccentricity, we imagined that there might be some downstream area, more involved in crowding, where the crowding distance is isotropic, conserved across both eccentricity and orientation.
METHOD: We measured psychophysical crowding distances on 4 observers at eccentricities of 0°, ±4°, and ±8°, radially and tangentially, for letter targets on the horizontal meridian. Results confirmed the well-known dependence on eccentricity and orientation. Using fMRI, we also measured each observer's retinotopic maps, and quantified tangential and radial cortical magnification in areas V1, V2, V3, and hV4.
RESULTS & CONCLUSION: All four areas showed conservation of cortical crowding distance across eccentricity, but only hV4 conserved crowding distance across orientation. The very high symmetry of hV4 with respect to crowding distance suggests that hV4 mediates the receptive field of crowding, i.e. the integration of features to recognize a simple object.
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