First-order and second-order signals combine to improve perceptual accuracy

The question. of whether first-order (luminance-defined) and second-order ( contrast-defined) stimuli can be combined in order to improve perceptual ac curacy was examined in the context of two suprathreshold discrimination exp eriments, one spatial and the other temporal. The stimuli were either grati ngs 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 diffe rent 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 inde pendent processing of the two types of stimulus. But this was true only for stimuli of low contrast. Facilitation of discrimination performance occurr ed 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 sugg est that first- and second-order stimuli, although believed by most researc hers to be detected separately, can subsequently be combined in order to im prove perceptual accuracy in conditions of low visibility. (C) 2001 Optical Society of America.