Hypothesis testing, attention, and 'Same'-'Different' judgments.

Logic and common sense say that judging two stimuli as "same" is the converse of judging them as "different". Empirically, however, 'Same'-'Different' judgment data are anomalous in two major ways. The fast-'Same' effect violates the expectation that 'Same' reaction time (RT) should be predictable by extrapolating from 'Different' RT. The criterion effect violates the expectation that RTs measured when sameness is defined by a conjunction of matching attributes should predict RTs measured when sameness is defined by a disjunction of matching attributes. The two criteria are symmetrical, yet empirically they differ greatly, disjunctive judgments being by far the slower of the two. This study sought the sources of these two effects. With the aid of a cue, a selective-comparison method deconfounded the contributions of stimulus encoding and comparisons to the two effects. The results were paradoxical. Each additional irrelevant (uncued) letter in a random string incremented RT for conjunctive judgments as much as an additional relevant letter did. Yet irrelevant letters were not compared and relevant letters had to be compared. These results appeared again in a second experiment that used words as stimuli. Contrary to intuition, a distinct comparison mechanism-the heart of relative judgment models-is not necessary in judgments of sameness and difference. It is shown here that encoding can carry out the comparison function without the operation of a separate comparison mechanism. Attention mediates the process by selecting from the set of stimulus alternatives, thereby partitioning the set into the 'Same' and 'Different' subsets. The fast-'Same' and criterion effects result from a structural limitation on what attention can select at any one time. With attention mediating the task, 'Same'-'Different' judgments become, in effect, the outcome of a testing of a hypothesis, bridging the distinction between absolute stimulus identification and relative judgments.