Perceptual separability, decisional separability, and the identification-speeded classification relationship.

Three observers participated in several sessions of identification and speeded classification. Predictions from F. G. Ashby and W. T. Maddox's (1994a) response time (RT) theory of speeded classification were tested. A key theoretical concept is decisional separability (DS), that is, that the decision about the level of 1 component does not depend on the perceived value of the other component. The theory predicted that DS would hold in the filtering task but would be violated in the redundancy task, resulting in the redundancy gains. To summarize the results, (a) DS held in the filtering conditions, but not in the redundancy conditions; (b) redundancy gains occurred; (c) despite the redundancy gains, strong evidence was obtained that the stimulus components are perceptually separable; (d) 2 new models that each assumed RT decreases with the distance between the percept and the decision bound provided good accounts of the RT distributions and accuracy rates; and (e) the shift from identification to speeded classification influenced both perceptual and decisional processes.