The brain has evolved an internal model of gravity to cope with life in the Earth’s gravitational environment. How this internal model benefits the implementation of skilled movement has remained unsolved. One prevailing theory has assumed that this internal model is used to compensate for gravity’s mechanical effects on the body, such as to maintain invariant motor trajectories.
Alternatively, gravity force could be used purposely and efficiently for the planning and execution of voluntary movements, thereby resulting in direction-depending kinematics. We experimentally interrogated these two hypotheses by measuring arm kinematics while varying movement direction in normal and zero-G gravity conditions.
By comparing experimental results with model predictions, we show that the brain uses the internal model to implement control policies that take advantage of gravity to minimize movement effort. Moreover, our experiments in monkeys, in which we applied bilateral labyrinthectomy, showed that the vestibular provides crucial information to the motor system for the optimal integration of gravity force.