Blackshot: An unexpected dimension of human sensitivity to contrast

Purpose. We studied the perceptual segregation of texture pairs consisting of a grid of uniform, square texture elements, differing only in the distribution of intensities across those elements. If two textures differ in mean intensity ("lightness") or in the variance of intensity ("contrast"), then they are easily segregated. Chubb, Econopouly & Landy (JOSA A, 11, 2350, 1994) demonstrated the existence of a 3rd mechanism B (in addition to mechanisms L and C, coding lightness and contrast respectively) that was solely responsible for the segregation of textures equated for mean and contrast. Here, we determine the sensitivity of B to textures differing in mean or variance, thus fully specifying its (highly nonlinear) contrast response function. Method. Two textures have gray-level histograms H1 and H2 chosen so as to equate both mean and variance so that B alone discriminates them. The magnitude of the difference D=H1-H2 was varied to find a segregation threshold t*D. Then, D was perturbed (e.g., D'=t*D+P) so that the two textures differed in mean or variance, and segregation performance was assessed. Results. Differences in texture mean and in texture variance traded off linearly with changes in D. This implies that even when the textures differ slightly in mean or variance, it is still B alone that discriminates between them. The slopes of the lines relating changes in mean and variance to changes in the amplitude of D reflect the sensitivity of B to texture mean and variance. Combining these findings with our previous results reveals that B is (i) highly sensitive to texture elements of the lowest contrast (near -1), and (ii) has a response that saturates by a contrast of -3/4 (dark gray). That is, this "blackshot mechanism" discriminates textures by comparing the number of the blackest pixels in each