Nominal and physical decision criteria in same-different judgments.

We propose a model in which the physical and nominal dimensions of letter pairs are compared independently of whether subjects use physical (shape task) or nominal (name task) identity as the decision criterion. We attempt to explain the fast-same effect, the preponderance of false-different errors, and the nominal-physical disparity as results of congruent and incongruent outputs of physical and nominal comparison devices that function in both tasks. Subjects performed both tasks with and without response deadlines. The stimuli were presented foveally or unilaterally to one or the other hemisphere. With foveal presentations, the nominal-physical disparity disappeared when congruent and incongruent cells were compared, the fast-same effect occurred only in the shape task, and there was a preponderance of false-different errors only in the name task. Response times and error patterns from centrally presented trials conformed to the predictions of the model. Performance patterns from the lateralized trials conformed only partially. The implications of the data are discussed in the context of several theoretical models of same/different judgments.