Assimilation and contrast in perceptual judgments

An attempt is made to specify the differential conditions underlying two opposite context effects: assimilation and contrast. Assimilation occurs when the judgment of the current stimulus shifts in the direction of the preceding stimulus; contrast occurs when the judgment shifts in the opposite direction. Both effects are brought together and demonstrated in a single paradigm. It is hypothesized that the number of contextual stimuli is the differentiating factor. Two alternative hypotheses, the differential stimulation hypothesis and the temporal position hypothesis, are also tested. The differential stimulation hypothesis proposes that the intensity difference between the contextual and judged stimulus determines whether assimilation or contrast occur, and the temporal position hypothesis proposes that varying the temporal position of the contextual stimulus is the critical factor. While the results do not support either of these alternative hypotheses, it is shown that the number of contextual stimuli does determine whether assimilation or contrast occur.