Sequential priming is not constrained by the shape of long-term learning curves

When multiple stimulus-to-response (S-R) mappings are randomly intermixed and repeated in a block of trials, immediate repetitions of an aspect of a stimulus and/or a response can facilitate stimulus detection, classification, and/or response selection - known as sequential priming. In addition to these short-term effects, response times (RTs) for almost any task diminish with extended practice; improvements can occur over many days, and RT learning curves typically assume exponential or power functions. We investigated whether short-term sequential priming and long-term practice modulate RT through a common mechanism, using a variant of the additive factors method. We tracked how various priming effects, presumably affecting different processing stages (e.g., stimulus selection, stimulus identification/classification, and S-R mapping), varied over training sessions as RT diminished. All the priming effects either were not reduced or reduced approximately linearly at rates much slower than those predicted by the shapes of the corresponding RT learning curves. The overall results suggest that short-term sequential priming and long-term practice modulate RT through relatively separate mechanisms, even though they appear to affect a common set of behaviorally defined processing stages.