The possibility that subjective contours are an artifact of brightness contrast was explored. In one experiment, inducing luminance was found to have different effects on the clarity of subjective contours and the magnitude of brightness contrast. The results of a second experiment indicated that differences of luminance in a stimulus display are necessary for subjective contours to be sustained, whereas chromatic differences are not. It is concluded that subjective contour and brightness contrast are distinct perceptual phenomena but share a dependency on the processing of edge information transmitted through the achromatic channels of the visual system.

Magnitude estimates of target brightness and clarity were obtained in a metacontrast paradigm as a function of stimulus onset asynchrony (SOA), target and mask duration, and fixation location. Under conditions of backward masking, effects on brightness and clarity were in opposite directions at short SOAs (i.e., judgments reflecting less clarity were accompanied by judgments that indicated increases in target brightness). However, at long SOAs and under conditions of forward masking, estimates of target brightness and clarity moved in the same direction. The results are discussed in the context of the "sustained-transient" theories of visual masking, and it is concluded that, in their present forms, these theories cannot completely handle the data we have reported.

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