Reaction time signatures of discriminative processes: differential effects of stimulus similarity and incentive.

In three experiments with pigeons, the similarity of unreinforced test stimuli to a reinforced stimulus and the frequency of reinforcement associated with a stimulus were varied. The stimulus on each trial was a small spot that appeared in different hues or, in Experiment 3, different forms. Differential response frequency and reaction time (RT) patterns emerged: Changes in similarity affected the percentage of stimuli responded to but left the shape of RT distributions about the same, whereas changes in reinforcement shifted RT distributions but had little effect on the percentage of responses. When the similarity and reinforcement variables were applied to the same stimuli (Experiment 2), their effects were largely independent. A generalization procedure (Experiment 3) replicated the similarity effects of the initial discrimination procedure. The RT distributions were modeled by a diffusion process, and implications for a memory-instance model were suggested.