Specific neocerebellar activation during out-of-phase bimanual movements.

We used fMRI to study cerebellar activation during index finger-thumb opposition of the right hand and index finger-thumb opposition of both hands in in-phase and out-of-phase modes. The right hand movement activates the contralateral anterior lobe of the cerebellum. During bimanual in-phase movements, this activity pattern becomes bilateral. More interestingly, bilateral out-of-phase movements recruit the cerebellar posterior lobe VIII, which likely corresponds to the second homunculus. As out-of-phase movements differ from the in-phase movements only by their temporal complexity and their attentional awareness, this study demonstrates the preferential involvement of the cerebellar second homunculus in the control of complex movements.