S's task was to decide as quickly as possible whether two simple visual stimuli were the same or different. Stimuli varied with respect to 3 attributes: size, color, and shape. Pairs of stimuli were presented either simultaneously or in sequence. “Different” RTs varied inversely with the number of attributes with respect to which the 2 stimuli of a pair differed, d. “Same” RTs were shorter than would be expected from an extrapolation of the results with d ≥ 1 (different stimuli) to include the case of d = 0 (same stimuli). Error rates were related to d in much the same way as was RT.
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