HAND TRAJECTORIES OF VORTICAL ARM MOVEMENTS IN ONE-G AND ZERO-G ENVIRONMENTS - EVIDENCE FOR A CENTRAL REPRESENTATION OF GRAVITATIONAL FORCE

The purpose of the present experiment was to study the way in which th e central nervous system (CNS). represents gravitational force during vertical arm pointing movements. Movements in upward (against gravity) and downward (with gravity) directions, with two different mass loads (hand empty and with a handheld 0.5-kg weight) were executed by eight subjects in a normal gravitational environment. Movements by two cosm onauts, in the two directions, were also tested in a state of weightle ssness. Analyses focused upon finger trajectories in the saggital plan e. Subjects in a normal gravitational environment showed curved paths for both directions and weight conditions. In addition, downward movem ents showed significantly smaller curvatures than upward movements. Mo vement times were approximately the same for all the experimental cond itions. Curvature differences between upward and downward movements pe rsisted during space flight and immediately postflight. Movement times from both cosmonauts increased slightly during flight, but returned t o normal immediately on reentry in a one-G environment. Results from t he present study provide evidence that gravity is centrally represente d in an anticipatory fashion as a driving force during vertical arm mo vement planning.