Same-different judgments: a review of current controversies in perceptual comparisons.

A commonly used experimental paradigm for studying human information processing asks subjects to classify two stimuli with respect to their similarities and differences. The "same"-"different" judgment task, in particular, has useful applications in a variety of studies. The interpretation of "same"-"diiferent" reaction time data, however, is complicated by the existence of two seemingly anomalous effects, the fast-"same" effect and the criterion effect. This article examines explanations of these effects and brings evidence from the literature to bear on them. Current models of "same"-"different" judgments, two of which are widely accepted, can be rejected on the evidence of data reviewed here. Among the disconfirming data are several results that offer suggestions both for improving current models and for questioning some of their fundamental presuppositions. Sameness and difference are fundamental cognitive relations that enter, at least implicitly, into most forms of adaptive perceptual behavior. Accordingly, experimental tasks requiring subjects to discriminate sameness and difference among stimulus patterns have become standard investigative tools in a number of areas of psychological research, areas ranging from basic psychophysics and stimulus coding to concept learning and language processing. The time taken to make a "same"-"different" judgment has been a particularly useful measurement because reaction times (RTs) can often be used to isolate mental processes underlying phenomena of interest (Sternberg, 1969).' Aside from its methodological use as an adaptable and sensitive probe of extrinsic phenomena, the "same"-"different" judgment task has itself been the subject of considerable study over the past 15 years. The resulting body of empirical evidence has produced substantial agreement on the functional significance of information-processing operations that subserve perceptual comparisons. However, a debate, contentious at times and still unresolved, has surrounded "same"-"different" judgment