A blueprint for movement: functional and anatomical representations in the human motor system

Despite a clear somatotopic organization of the motor cortex, a movement can be learned with one extremity and performed with another. This suggests that there exists a limb-independent coding for movements. To dissociate brain regions coding for movement parameters from those relevant to the chosen effector, subjects wrote their signature with their dominant index finger and ipsilateral big toe, and we determined those areas activated by both conditions using functional magnetic resonance imaging. The results show that movement parameters for this highly trained movement are stored in secondary sensorimotor cortices of the extremity with which it is usually performed, i.e., the dominant hand, including dorsal and ventral lateral premotor cortices. These areas can be accessed by the foot and are therefore functionally independent from the primary representation of the effector. Thus, somatotopy in secondary structures in the human motor system seems to be defined functionally, and not on the basis of anatomical representations.