Asymmetry of verbal-manual interference: dissociation between rate and variability in left-handers.

In two experiments, 48 right-handed and 48 left-handed adults, respectively, performed speeded and consistent finger tapping with and without concurrent oral reading. Interference was measured in terms of change in tap-to-tap rate and variability. Experiment 1 confirmed a previous report that concurrent reading decreases the rate of speeded finger tapping and increases the rate of consistent tapping in right-handers, and that the right hand is affected more than the left. Experiment 2 showed that, for left-handers, concurrent reading decreases the rate of left-hand tapping more than right-hand tapping but increases the variability of the right hand more than the left. The double dissociation in left-handers between hand and dependent variable suggests that the speed and variability reflect different mechanisms of intertask interference. More generally, the findings illustrate the multidimensionality of motor performance and the risk of making inferences about neural organization on the basis of a single dependent measure.