▪ REVIEW Plasticity in the Human Motor System

The organization of motor representation in the human brain is plastic in a number of situations. Plasticity may be caused by unmasking of pre-existing connections, activity dependent synaptic changes, changes in membrane excitability, or growth of new connections. In peripheral injury, such as amputation, regions of brain no longer have a peripheral target. Neighboring motor representation areas targeting muscles ipsilat-eral and proximal to the amputation stump enlarge into this territory. Some changes can occur quickly, whereas others require years; this probably reflects different processes. In early brain injury, including pa-tients with hemispherectomy, some recovery of function seems related to enhancement of ipsilateral path-ways. Stroke in adult life may cause some reorganization in the lesioned hemisphere, and the role of ipsilateral pathways is not yet well established. Patterns of use and motor learning can influence brain organization. Use of a body part increases its representation, whereas disuse reduces representation. In motor learning, such as learning a finger sequence, the representation of muscles involved with the learning increases in size. Plasticity can even be cross-modal, as demonstrated by the functional role of the visual cortex when the blind read Braille. NEUROSCIENTIST 5:324-332, 1999 KEY WORDS Brain plasticity, Brain injuries, Motor representation, Motor learning The brain has a complex wiring diagram. It is so com