What and When: Parallel and Convergent Processing in Motor Control

Successful motor behavior requires making appropriate re-sponse (response selection) at the right time (timing adjust-ment). Earlier psychological studies have suggested that the response selection and timing adjustment processes are per-formed serially in separate stages. We tested this hypothesis using functional magnetic resonance imaging. The subjects performed a choice reaction time task in four conditions: two (on-line response selection required or not) by two (on-line timing adjustment required or not). We found that the neural correlates for the two processes were indeed separate: the anterior medial premotor cortex (presupplementary motor area) was selectively active in response selection, whereas the cer-ebellar posterior lobe was selectively active in timing adjust-ment. However, the functional separation was only partial in that the lateral premotor cortex and the intraparietal sulcu