Central control of grasp: manipulation of objects with complex and simple dynamics.

We performed whole-brain fMRI to explore the neural mechanisms that contribute to the ability to manipulate an object with complex dynamics. Subjects grasped a weighted flexible ruler and balanced it in an unstable equilibrium position as an archetype of grasping an object with complex dynamics. This was contrasted with squeezing a soft foam ball as an archetype of grasping an object with simple dynamics. We hypothesized that changes in activity in primary motor cortex (MI) would be similar under the two conditions, since muscle activation was matched, which was confirmed. We hypothesized further that the cerebellum would be selectively activated when manipulating the flexible ruler because the ability to make the adjustments necessary to balance the ruler would require an internal dynamics model, represented in the cerebellum. As predicted, the ipsilateral cerebellum was strongly activated when balancing the weighted ruler whereas only moderate activation was found when squeezing the foam ball. We also found evidence for selective activation of areas, previously implicated in tactile object recognition, when holding the flexible ruler. We speculate that these areas, which include secondary somatosensory cortex (SII), Brodmann area 40 and insula, integrate tactile and proprioceptive information in the context of controlling the orientation of the flexible ruler and provide appropriate feedback to MI. We speculate that the failure to find activation of these areas when squeezing the ball was due to the fact that tactile stimulation was entirely self-produced, resulting in the attenuation of cortical sensory activity (Blakemore, S.-J., Wolpert, D.M., Frith, C.D., 1998. Central cancellation of self-produced tickle sensation. Nat. Neurosci. 1, 635-640, Blakemore, S.-J., Frith, C.D., Wolpert, D.M., 2001. The cerebellum is involved in predicting the sensory consequences of action. NeuroReport 12, 1879-1884).