Hand differences in the rate and variability of rapid tapping were evaluated for the intertap interval and its constituents-the key depression and key release phases of each tap. To accentuate potential hand differences, only subjects with a clear manual superiority in one hand were included. Relative manual proficiency on Fitts' reciprocal tapping task was used to exclude individuals with less-defined hand superiority or dominance, and to categorize subjects as having a dominant left (n=13) or right (n=11) hand. Analysis of variance indicated the dominant hand to have a shorter average intertap interval and thus a faster tapping rate. This hand difference in rate was found to be significant for the key-depression phase, but not the key-release.phase, of the tap. In each handedness group the dominant hand exhibited less variability in the intertap interval and both constituents. Potential associations of these findings with hemispheric asymmetries in sequential ability are discussed.

On the assumption of contralateral control, lateral asymmetries in motor performance are often associated with the functional specialization of the cerebral hemispheres. However, anatomical as well as previous behavioral studies, indicate this assumption may not be valid for all joints of the limbs. To test this assumption, right-handed male subjects performed a rapid tapping task using movements of the index finger, the wrist, or the shoulder. The hand used and order of conditions were counterbalanced across subjects. The rate and variability data were submitted to a 2 x 3 (hand x joint) Analysis of Variance with repeated measures on both factors. For both rate and variability significant (all p less than .01) main effects were evident for hand and joint with no significant interactions. It is argued that the variability of tapping reflects the processing mode of the contralateral hemisphere and thus leads to the conclusion that crossed pathways are the principal mechanism for controlling rapid movements of all limb joints. It is tentatively concluded that where either crossed or ipsilateral pathways are available, the type of movement control required will dictate which pathway is used.