Unitary classification in a comparison task.

Sequential effects were used to diagnose whether elements in a two-object comparison task are represented as a perceptual unit or separately. The presence of sequential effects and absence of influences of individual elements on the subsequent trial in a successive comparison task favor the hypothesis that the elements in a pair are represented as a unit, and that a response is made to the perceptual unit. The patterns of response times on same and different trials differed in several ways; these suggested that the quality of the representations of same and different trials may differ.