| Summary: | Judgments of the brightness of a test patch are strongly influenced by contrast and assimilation. However, the experiments that document these effects typically do not use feedback. We wondered whether observers might have access to strategies that were cleansed of these effects if they were given trial-by-trial feedback. In this study, observers viewed a 3.33° diameter Test-disk surrounded by an annular ring of what appeared to be homogenous visual noise; their task was to judge whether the luminance of the Test-disk was higher or lower than that of the fixed gray background outside the annulus. Although the annulus looked like a ring of visual noise on each trail, it was actually composed of a random, weighted sum of 11 orthogonal basis images: 5 noise images (which were constrained to contribute 94.8% of the energy in the noisy annulus) and 6 concentric annuli (which collectively covered the same region as each of the noise basis images and which contributed the remaining 5.2% of the energy). Data were collected for four display durations: 13, 27, 53, and 107ms. In each case, logistic regression was used to determine the influence exerted on the participant's judgments by the 11 basis components. Performance was similar for the four display durations. The innermost annulus exerted a contrast effect: Test-disk contrast judgments were negatively correlated with inner-annulus contrast. However, for four of the five participants, judgments tended to be positively correlated with the contrasts of the outermost two annuli. Plausibly, this latter effect reflects assimilation by the Test-disk of annulus brightness induced by contrast of the outer annular strip with the background. Thus, the feedback supplied did not enable observers to escape contrast and assimilation effects.
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