Abstract
Summation refers to the small reduction of threshold contrast as a signal is extended in space or time, or when multiple signals are presented simultaneously. Facilitation refers to the large reduction in the threshold contrast increment when it is superimposed on a near-threshold pedestal contrast. Separate hypotheses relate each of these effects to distinct measures of the log-log steepness of the psychometric function. This note shows that for 2-alternative forced-choice (2afc) proportion correct, the Weibull function P( c)= 1-(1− γ)exp[-( c/α) β] and the d′ power law d′( c) = ( c/c 0.76) b are practically indistinguishable. Furthermore, their steepness parameters are proportional to each other, β = 1.247 b. This implies a simple relation between 2afc summation and facilitation which can be tested without measuring the psychometric function.
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