Effects of responding hand and concurrent verbal processing on time-keeping and motor-implementation processes.

Two experiments examined the effects of concurrent verbal processing on the ability to tap at a consistent rate with the left and right index fingers. Variability in the intertap intervals was divided into two components that reflect different processes involved in such paced tapping. One component reflects the consistency of an internal time-keeping mechanism and the other component reflects the consistency of motor implementation. The imposition of a concurrent verbal processing task increased variability attributable to the internal time-keeping mechanism, but had no effect on variability attributable to motor implementation. Furthermore, the interference caused by concurrent verbal processing was equivalent for the two hands. Variability attributable to motor implementation was consistently smaller for the right than for the left hand and was not increased by imposing a concurrent verbal processing task. This pattern of results indicates that there is an important dissociation between time-keeping and motor-implementation mechanisms. Furthermore, the results suggest that a single time-keeping mechanism serves both index fingers and that the left hemisphere of neurologically intact right-handers plays an important role in the time-keeping circuit regardless of which hand is tapping.