Models of the visual perceptual matching process are evaluated in two experiments in which the relative speeds of “same” and “different” responses were manipulated. In the first experiment, subjects were tested under two bias conditions: they were instructed to respond “same” only when sure or to respond “different” only when sure. Such bias is found to have a very large effect on the speed and accuracy of “same” and “different” responses, changing “same” responses from being faster than “different” responses (the usual result) in the sure “different” condition to being slower in the sure “same” condition. In the second experiment, the relative speed of “same” responses was slowed significantly by the addition of difficult “different” judgments. These results are used to argue against models that contain an identity matcher and against models that have separate components to account for errors and reaction time. We also point out that, although it is well known that the relative speeds of positive and negative responses are subject to bias manipulations, this fact has been ignored by many researchers in developing models in which the reaction time difference between positive and negative responses is used as a measure of a stage of processing.

In two choice reaction time (RT) experiments, subjects verbally predicted the stimulus alternative that was either most or least likely to be presented on each trial. An expectancy interpretation of prediction effects requires that stimuli matching least likely predictions yield the longest RTs. In Experiment 1, with four stimulus alternatives, RTs to stimuli matching the verbalized stimulus, both most likely and least likely predictions, were shorter than RTs to stimuli that matched neither the most nor the least likely predictions. However, matching most likely predictions produced faster responses with two alternatives. In Experiment 2, multiple predictions of both most and least likely stimuli were compared with joint predictions of first and second most likely alternatives. RTs to stimuli matching least likely predictions were longer than RTs to most likely matches, implicating prediction type as a partial determinant of RT. Matching least likely stimuli were responded to more quickly than unpredicted alternatives, demonstrating the importance of generating a prediction per se over the implied level of expectancy. Results from both experiments were contrasted with nonparametric predictions from an ordered memory scanning model.

A related FORTRAN program allowing the user to specify nontabled values of P and M is available from the first author.2. Equation 2 is obtained from Equation 1 following