Force related activations in rhythmic sequence production.

Brain imaging studies have implicated the basal ganglia in the scaling of movement velocity. Basal ganglia activation has also been reported for movement timing. We investigated the neural correlates of scaling of force and time in the production of rhythmic motor sequences using functional magnetic resonance imaging (fMRI) of the human brain. Participants (N = 13) were imaged while squeezing a rigid force transducer in a near isometric manner between thumb and index finger, to reproduce four different rhythmic sequences. The responses were separated by either equal (600 ms) or alternating (400, 800 ms) intervals, and produced with either equal (12 N) or alternating (8, 16 N) forces pulses. Intervals and force levels were balanced across each condition. The primary motor cortex (M1), supplementary motor area (SMA), basal ganglia, thalamus, and cerebellum were activated during the production of sequences marked by equal interval and force. There was no reliable main effect of alternating interval. In contrast, greater activation of these regions was associated with the extra demands of responding with alternating force pulses. We interpret the data as identifying a significant role of the BG in the control of force. In addition, the results indicate the importance of monitoring force when studying brain activation associated with motor timing.