Stimulus proba]bility and choice reaction time

Two experiments, in which three stimuli were paired with two responses in a 2∶1, 1∶1 arrangement, showed choice RT to be a decreasing function of the relative frequency of stimulus occurrence when response frequencies are equated. The size of effect varied significantly with stimulus type (Experiment 1), being larger for circular light patches of differing diameter than for sets of colors, letters, and spatially distributed light sources, but was independent of differences in the similarity of stimulus alternatives within a set (Experiment 2). The results are discussed in terms of various strategies of stimulus identification.