What and when: Parallel and convergent processing in motor control

Successful motor behavior requires making appropriate response (response se lection) at the right time (timing adjustment). Earlier psychological studi es have suggested that the response selection and timing adjustment process es are performed serially in separate stages. We tested this hypothesis usi ng functional magnetic resonance imaging. The subjects performed a choice r eaction time task in four conditions: two (on-line response selection requi red or not) by two (on-line timing adjustment required or not). We found th at the neural correlates for the two processes were indeed separate: the an terior medial premotor cortex (presupplementary motor area) was selectively active in response selection, whereas the cerebellar posterior lobe was se lectively active in timing adjustment. However, the functional separation w as only partial in that the lateral premotor cortex and the intraparietal s ulcus were active equally for response selection and timing adjustment. The lateral premotor cortex was most active when both processes were required, suggesting that it integrates the information on response selection and th e information on timing adjustment; alternatively, it might contribute to t he allocation of attentional resources during dual information processing. The intraparietal sulcus was equally active when either response selection or timing adjustment was required, suggesting that it modifies, rather than integrates, these processes. Furthermore, our results suggest that these a ctivations related to response selection and timing adjustment were distinc t from sensory or motor processes.