Perceived contrast in the fovea and periphery.

Subjects estimated the perceived contrast of 2 degrees-diameter sine-wave grating patches for spatial frequencies of 2, 4, 8, and 16 cycles/deg, at eccentricities from 0 degrees to 40 degrees and contrasts up to 0.8. The data were well fitted in all cases by power functions of contrast minus threshold, with exponents of the order of 0.5 implying similar mechanisms in both fovea and periphery. The data also demonstrate that, at high physical contrast, the visual system is generally driven toward an operating state in which two stimuli of equal physical contrast have equal perceived contrast even if the thresholds are quite different. As a consequence, peripheral perceived contrasts produced by high physical contrasts show almost no change with eccentricity, whereas thresholds increase by at least an order of magnitude. This implies that mechanisms mediating threshold detection and suprathreshold perception may be different.