| Humans display a remarkable ability to adapt to new situations. With respect to sensory motor control, one way these adaptive capabilities have been quantified is by requiring subjects to attempt to point at a target while wearing prism goggles. During the exposure, the perturbation caused by the prism can be compensated for and the visual mapping of external space is realigned with the proprioceptive mapping of the felt position of the arm.; The purpose of this study was, first, to categorize different aspects of prism adaptation taking place in various visual conditions. Then the brain mechanisms underlying these different aspects of the adaptation were identified. Through an examination of the degree to which adaptation is disrupted in normal, healthy human subjects when transcranial magnetic stimulation (TMS) is applied over different brain areas, it was possible to make strong inferences about the function of the stimulated site in the given task.; Compared to a control condition without TMS, decreased amounts of the prism aftereffects as well as the proprioceptive realignment were observed when TMS was applied over the premotor cortex (PMC) at the beginning of reaching with the full visual feedback of the hand. Posterior parietal cortex (PPC) stimulation at the end of the movement caused decreased amounts of the aftereffects and the visual realignment only when the visual feedback of final performance was available. Kinematic analyses confirmed that the behavioral characteristics of reaching movements during the prism exposure correlated with the changes in the visual or proprioceptive representation. On the other hand, there was no change in these representations when the cerebellum was stimulated. However, decreased amount of on-line correction was observed.; Taken together, this evidence shows that the PMC contributes to the modification of the proprioceptive frame of reference during prism reaching movements, while the PPC is more involved with the shift of the visual frame of reference throughout the adaptation period. The cerebellum is not directly involved in these coordinate shifting mechanisms but is involved in the error correction function. |
