WHAT ARE NORMAL MOVEMENTS IN ATYPICAL POPULATIONS

Redundancy of the motor control system is an important feature that gi ves the central control structures options for solving everyday motor problems. The choice of particular control patterns is based on priori ties (coordinative rules) that are presently unknown. Motor patterns o bserved in unimpaired young adults reflect these priorities. We hypoth esize that under certain atypical conditions, which may include disord ers in perception of the environment and in decision making, structura l or biochemical changes within the central nervous system (CNS), and/ or structural changes of the effectors, the central nervous system may reconsider its priorities. A new set of priorities will reflect the c urrent state of the system and may lead to different patterns of volun tary movement. Under such conditions, changed motor patterns should be considered not pathological but rather adaptive to a primary disorder and may even be viewed as optimal for a given state of the system of movement production. Therapeutic approaches should not be directed tow ard restoring the motor patterns to as close to ''normal'' as possible but rather toward resolving the original underlying problem. We illus trate this approach using, as examples, movements in amputees, in pati ents with Parkinson's disease, in patients with dystonia, and in perso ns with Down syndrome.