ABSTRACT
This chapter describes some of the quantitative formulations that have been proposed to account for assimilation and contrast phenomena. The major tenet of Helson’s adaptation level theory is that an individual computes a weighted mean of three classes of stimuli to produce an adaptation level that serves as a reference, or base line, against which all new stimuli are compared. The three classes of stimuli used in the computation of the adaptation level or reference level include both components of the array that are physically present and coded traces of past stimuli within the observer. In quantitative terms, the adaptation level is computed by taking the geometric mean of all the stimuli in the field, including the focal stimuli, background stimuli, and residual stimuli relevant to the judgment at hand. Residual stimuli include the observer’s past experiences with similar perceptual situations and his physiological state, constitutional factors, and learned perceptual strategies or styles.
