The gravitational forces as fundamental input for sensorimotor coordination

Any motor action results from a complex combination of motor commands that control the muscular forces and produce the desired movement. For this purpose, the motor commands must continuously compensate for the action of external forces acting on the body, of which the omnipresent gravitational forces. As a consequence, an internal knowledge of the movement dynamics, including the effect of gravitational forces, is essential in order to achieve successful movements. Evidence for such internal representation can be found in existing studies, but the role that gravity plays in the internal models of dynamics remains a challenging problem. The present thesis uses an experimental and modeling approaches in order to better understand the role played by gravity in sensorimotor coordination. This thesis shows that motor behavior in hypergravity (1.8 times terrestrial gravity) is consistent with the hypothesis the motor commands are optimized, taking into account the action of gravitational forces on the limb in the optimization process. In addition, forces applied by the subjects on a manipulated object were in direct support to the fact that the object weight was used by the brain for calibrating the internal models of dynamics. These results gave significant insight on the problem of sensorimotor coordination in weightlessness (0 g), given that the sensory information resulting from the action of gravity on the limbs is lost. The results confirm that internal representation of dynamics adapts to the 0 g condition, but it suffers from more uncertainty and, therefore, potentially affects the movement stability. A stabilizing mechanism that accounts for the experimental results is proposed as model for motor behavior under uncertain internal representation of dynamics.