Abstract
A model of visual incremental threshold detection is presented, having the following features. 1. (1) From Crozier's Law we deduce that all thresholds, for conventional as well as for noisy images, are ultimately determined by noise. Other evidence, which is described, supports this conclusion. 2. (2) It is proposed that signal and noise are sampled by apertures at a hypothetical “decision site” in the visual cortex. These apertures, which may consist of cortical receptive fields, “match” the signal as projected on the cortex. 3. (3) For circular signals < 10 mrad, the diameter of the relevant sampling aperture is equal to that of the signal as modified by the point spread-function of the optical and neural components. The signal intensity and the noise power are therefore integrated over the modified signal diameter. The model accurately predicts the contrast threshold function over a wide range of conditions for signals <10 mrad. 4. (4) For circular signals > 10 mrad, one of three qualitative mechanisms is tentatively proposed: first, a limit to the size of receptive fields, second, a limit to the active area of receptive fields and third, the effect of lateral inhibition on the large-area signal-to-noise ratio.
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