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Hermet Yiltiz, Xiuyun Wu, Denis Pelli; Crowding area sets a lower bound on the neural noise that limits letter identification. Journal of Vision 2016;16(12):173. doi: 10.1167/16.12.173.
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We report a connection between effects of crowding and noise. In the periphery, it is impossible to identify a target in clutter, unless the clutter is at least a critical spacing away. The area enclosed by the critical spacing is the "combining field". Measuring thresholds in various levels of full-field white noise, we decompose threshold contrasts into efficiency and equivalent input noise (Pelli & Farell, 1999). Efficiency is the fraction of the contrast energy used by the human observer that would be needed by an optimal algorithm in the same amount of noise. Equivalent input noise is the amount of display noise needed to account for the human threshold, given the measured efficiency. We presented a 0.5, 2, or 8 deg letter at an eccentricity of 0-32 deg on a full-field background of white noise, at one of several noise contrasts. Measured threshold contrasts were decomposed into efficiency and neural noise. We find that efficiency is independent of eccentricity (0 to 32 deg) for all letter sizes within the acuity limit. For letters larger than the combining field, neural noise is proportional to letter area and independent of eccentricity. For letters smaller than the combining field, the neural noise corresponds to the combining field area, which is independent of letter size and grows with eccentricity. The foveal finding of equivalent noise proportional to area is consistent with scale-invariant letter recognition. The break of that proportionality in the periphery occurs when letter size equals combining field size, suggesting that there is a neural channel with that area, and perhaps there are similar channels with larger (but not smaller) areas.
Meeting abstract presented at VSS 2016
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