The question of whether first-order (luminance-defined) and second-order (contrast-defined) stimuli can be combined in order to improve perceptual accuracy was examined in the context of two suprathreshold discrimination experiments, one spatial and the other temporal. The stimuli were either gratings of one type of image alone or else the sum of two gratings of the same orientation, spatial frequency, temporal frequency, and phase, but of different types. For both spatial frequency discrimination (static gratings) and speed discrimination (1-c/deg drifting gratings), performance was markedly better for a combined grating stimulus than predicted on the basis of independent processing of the two types of stimulus. But this was true only for stimuli of low contrast. Facilitation of discrimination performance occurred only in the contrast range where discrimination performance is contrast dependent. At higher contrasts, where performance has reached an asymptote for each type of pattern alone, there was no facilitation. The results suggest that first- and second-order stimuli, although believed by most researchers to be detected separately, can subsequently be combined in order to improve perceptual accuracy in conditions of low visibility.
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