Concurrent performance of rhythmically compatible or incompatible vocal and manual tasks: evidence for two sources of interference in verbal-manual timesharing.

Forty-two normal, right-handed adults performed rhythmic finger tapping with the left or right hand while concurrently reciting sentences that were either compatible or incompatible with the rhythm being tapped. Both kinds of sentences slowed tapping equally, and the interference was lateralized so that the right hand was affected more than the left. In contrast, the regularity of finger tapping was disrupted to a much greater degree by incompatible sentences than by compatible sentences, and the effect was comparable in the left and right hands. The results implicate two mechanisms of verbal-manual interference--capacity limitation and timing constraints--and thus reconcile certain inconsistencies among findings from previous concurrent-task studies.