The cerebellar second homunculus remains silent during passive bimanual movements.

In a previous study, we showed that the second homunculus in lobule VIII of the cerebellum is activated during bilateral out-of-phase index finger-thumb opposition, implying a role in motor coordination. However, several recent studies indicate that the cerebellum could be more actively involved in sensory information processing during movement. Therefore, as lobule VIII activation could involve either a motor or a proprioceptive component, these two components must be distinguished and their relative contribution must be determined. Using functional imaging, we studied cerebellar activation of the same region during passively induced index finger-thumb opposition of both hands in in-phase and out-of-phase modes, thereby excluding the voluntary movement component. No significant activation was detected in lobule VIII. Intense activation of lobule VIII, obtained during active, out-of-phase bimanual movements, therefore does not involve a significant sensory component related to direct proprioceptive feedback. This result is strongly in favour of the specific recruitment of lobule VIII during out-of-phase movements related more to complex motor timing than to sensory function.