Perceived contrast and contextual effects

The perceived contrast between two surfaces with different luminances is affected by all the perceptual attributes of the stimulus (Helson and Rohles, 1959 American Journal of Psychology 72 530 - 538). In the experiment, I used three pairs of chips, with different luminance contrasts. There were four series of stimuli. In series I and II adjacent or separate chips, respectively, were presented on a homogeneous background with three different luminances; in series III and IV, adjacent or separate chips, respectively, were presented on a bipartite background. Series III and IV were presented once with constant luminance of both parts of background for all the pairs of chips, and once with varying luminance of left part of the background. Participants rated the degree of contrast using integers from 0 (absence of constrast) to 100 (black vs white). Separate ANOVAs for each series showed that the perceived contrast was affected by background luminance only in stimuli of series II. Stimuli of series III produced a transparent figure seen on a bipartite background. An overall ANOVA for series I and III showed that transparency reduced the perceived contrast between the chips. For stimuli of series II and IV, the perceived contrast was reduced. This result agrees with Diamond's (1955, Journal of Experimental Psychology 50 144 - 152) findings that contrast effect decreases as distance increases.