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.

A model of information transmission in the visual system which describes the effect of attention on apparent brightness is examined. This model states in part that the luminance of the portion of the visual field which captures the attention is overweighted in arriving at an overall average luminance level across the visual field. As this average must be computed with respect to both luminance and relative area, it is hypothesized that increasing the relative area of the portion of the visual field that captures the attention will result in a greater effect on the apparent brightness of all parts of the visual field. Two predictions, which involve the effect of relative area on apparent brightness, are experimentally tested and confirmed. Festinger, Coren, and Rivers (1970) have recently proposed a model of information transmission in the visual system which describes the effect of attention on apparent brightness. It is based in part on studies which imply that the best information on the retina is information about change in stimulation (Riggs, Ratliff, Cornsweet, & Cornsweet, 1953;Hartline, 1969). The model assumes that in order to assess the brightness levels of different portions of the visual field, the visual system computes an overall average luminance level (Buhler, 1922; Katz, 1935) and information about luminance differences is symmetrically superimposed on the average. The visual system then assumes uniformity of stimulation between the contours (Krauskopf, 1963;Yarbus, 1967; Gerrits, 1967) such that the brightness level at the contours becomes the brightness level between the contours. Included in the determination of the overall average luminance level is the overweighted luminance of the portion of the visual field which captures the attention (Helson, 1943).It follows from this model that the apparent brightness of various portions of the visual field should be affected by the luminance of the part which captures the attention. When considering a gray and white display, the gray should be seen .as darker if it captures the attention (contrast) and lighter if the white part captures the attention (assimilation). In a gray and black display, the gray should be seen as lighter if it captures the attention (contrast) and darker if the black part captures the attention (assimilation). By manipulating which portion of a gray and white or gray and black display would be likely to capture and hold the *Based on a dissertation presented for the degree of Doctor of Philosophy at the Graduate Faculty of Political and Social Science of the New School for Social Research. The research reported in this paper was supported by NIH Grant 16327, of which the principal investigator is Leon Festinger. The author wishes to thank Leon Festinger and Hiroshi Ono for their comments on the manuscript. tNow at Department of Psychology, York University, 4700 Keele Street, Downsview, Ontario, M31 IP3, Canada. attention, Festinger, Coren, and Rivers (1970) have shown that brightness assimilation seems to occur only if the p...

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