Congenitally altered motor experience alters somatotopic organization of human primary motor cortex.

Human motor development is thought to result from a complex interaction between genes and experience. The well-known somatotopic organization of the primate primary motor cortex (M1) emerges postnatally. Although adaptive changes in response to learning and use occur throughout life, somatotopy is maintained as reorganization is restricted to modifications within major body part representations. We report of a unique opportunity to evaluate the influence of experience on the genetically determined somatotopic organization of motor cortex in humans. We examined the motor "foot" representation in subjects with congenitally compromised hand function and compensatory skillful foot use. Functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) of M1 revealed that the foot was represented in the classical medial foot area of M1 and was several centimetres away in nonadjacent cortex in the vicinity of the lateral "hand" area. Both areas had direct output to the spinal motor neurons innervating foot muscles and were behaviorally relevant because experimental disruption of either area by TMS altered reaction times. We demonstrate a unique, nonsomatotopically organized M1 in humans, which emerged as a function of grossly altered motor behavior from the earliest stages of development. Our results imply that during early motor development experience may play a more critical role in the shaping of genetically determined neural networks than previously assumed.