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Hans Strasburger; Character recognition and Ricco's law. Journal of Vision 2005;5(8):219. doi: 10.1167/5.8.219.
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© ARVO (1962-2015); The Authors (2016-present)
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. 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 presented by Fischer & May (1970) for the cat.
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