Limiting Factors in Threshold and Suprathreshold Vision

Abstract

THE threshold contrast of object detail decreases with increasing size; the full curve of Fig. 1 shows the threshold contrast of circular object details as a function of their angular diameter¹. To explain the form of this function some^2–4 have proposed that noise is less effective in “obscuring” the larger details; others^5,6 have suggested that the contrast loss in the optical and neural processes is less for the larger details. This fundamental uncertainty has not yet been resolved⁷. According to the first view, noise is sampled over the equivalent area of the object detail. Because a larger number (N) of random events would be integrated in a larger detail area, the relative standard deviation (N^−½) would be less, and would in fact be inversely proportional to the detail diameter (Fig. 1, dotted line). According to the second view, the “obscuring” effect of the noise is constant for different diameters of object detail, and the form of the contrast threshold function results solely from variation in the contrast loss. The loss arises from imperfections in the optical and the neural processes. The contrast loss may be quantitatively described in terms of a contrast transfer function. For the present purpose, this is defined as the ratio of the subjective brightness contrast to the objective luminance contrast of the object detail, expressed as a function of detail diameter. Both contrasts refer to the centre of the detail in relation to its background.