We used functional magnetic resonance imagery (fMRI) to study cortical activation during index finger-thumb opposition of both hands using in-phase and out-of-phase modes. In-phase movements activated the sensorimotor cortex. During out-of-phase movements, activations were also observed in the supplementary motor area (SMA), in the cingulate motor area (CMA) and, less frequently, in the anterior cingulate cortex (ACC). As we have previously shown and confirmed in the present study, the same out-of-phase bimanual movements specifically activate the cerebellar second homunculus, leading us to postulate that the cerebellar second homunculus and medial wall motor areas participate in a circuit specifically involved in timing complex movements.