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H. Strasburger; Character Recognition and Ricco's Law . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4583.
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Purpose: The contrast threshold for the detection of patches of light depends upon stimulus size as described by Ricco’s classical law of areal summation; the critical diameter within which Ricco’s law holds increases with retinal eccentricity. Here we present an analogon of Ricco’s law for the recognition of characters at low contrast, and describe its variation with retinal eccentricity. Methods: Michelson contrast thresholds for the recognition of singly presented digits were determined in a 10–afc, maximum–likelihood adaptive procedure (ML–Pest/R_Contrast), as a function of character size (0.2° – 5°), at 13 retinal eccentricities on the horizontal meridian up to 36°. Thresholds were converted to Weber contrast CW = ΔL/L to be comparable to the classical formulations of Ricco’s law. Log–log contrast–size functions were analysed with respect to maximum slope and slope of –2. Results: Stimulus size has a more pronounced effect on character recognition than it has on stimulus detection, such that the maximum slope of the (log–log) areal–summation function is much steeper than Ricco’s (–2) slope. It ranges from –3 in the fovea to –7.5 at 30° eccentricity. At larger stimulus sizes there is a range at which Weber contrast threshold CW is proportional to stimulus area S2 (i.e. slope is –2); we denote this as the Ricco size range. The latter increases with retinal eccentricity at the same rate as receptive field size. Furthermore, the effect size CW × S2 is a constant multiple of Spillmann’s perceptive field size. The law will be formally related to that of Fischer & May (1970) for the cat. Conclusions: Areal summation at the ganglion cell level does not predict areal dependency for character recognition. However, the dependency of the area–dependency function on retinal eccentricity is closely related to receptive and perceptive field size. It is well described by a compact set of equations.
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