On the minimization of task switch costs following long-term training.

Flexible, context-dependent linkages between stimulus and response are fundamental to adaptive behavior. In the present article, we evaluate the limits of this flexibility by exploring the asymptotic efficiency of people's ability to switch between two different sensorimotor mappings. Two stimulus-response (S-R) mappings were learned, either both on the same hand (unimanual condition) or one mapping per hand (intermanual condition). The S-R mappings presented bivalent stimuli and employed the same response keys. A novel training regimen successfully reduced task-switch costs to approximately 20 msec, suggesting that residual switch costs cannot be eliminated. These costs cannot be entirely attributed to the cognitive control process of task-set reconfiguration, because they are observed over cued switch intervals of several seconds. Two additional issues in motor learning were addressed: the single or dual loci of manual motor control and the coordinate system of task representation. First, the results favored the notion of independent controllers for each hand instead of a single dominant controller, since intermanual performance was superior to unimanual performance. Second, a transfer task tested internal (egocentric) and external (allocentric) coordinate systems. Transfer was more effective using the external coordinate system, suggesting that the S-R mappings reflected the association between the bivalent stimuli and external goals (i.e., the individual keys) rather than the concrete pattern of muscle contractions (i.e., the finger pressing the key). Finally, retention tests revealed that these learned S-R associations were remarkably durable, since no decrements in fluent task-switching performance were observed after 10 months without practice.