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Jingyang Zhou, Noah Benson, Jonathan Winawer, Denis Pelli; Conservation of crowding distance in human V4. Journal of Vision 2018;18(10):856. doi: https://doi.org/10.1167/18.10.856.
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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: 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 a fixed number of 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 ±2.5°, ±5°, and ±10°, radially and tangentially, for letter targets on the horizontal meridian. Results confirmed the well-known dependence on eccentricity and orientation. Using anatomical and functional MRI, we also measured each observer's retinotopic maps, and quantified tangential and radial cortical magnification in areas V1-hV4. RESULTS & CONCLUSION: We find that all four areas conserve cortical crowding distance across eccentricity, but only hV4 conserves crowding distance across both eccentricity and orientation. After averaging measurements across observers (n=4), we find that the V4 crowding distance is 3.0±0.2 mm (mean±rms error across orientation and eccentricity). Across both dimensions, conservation fails in V1-V3, with rms error exceeding 0.7 mm. The conservation of crowding distance in hV4 suggests that it mediates the receptive field of crowding, i.e. the integration of features to recognize a simple object.
Meeting abstract presented at VSS 2018
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