The role of gravity in dexterous manipulation : a driving force rather than a perturbation

During their life, human beings learn how to interact with their surrounding environment and calibrate their motor system by matching movements and sensory inputs. On Earth, the omnipresence of gravity profoundly affects our everyday actions. For instance, an accelerated hand-held object generates an inertial force that adds to the object’s weight. These two components constitute the tangential load force. Remarkably, humans rarely drop objects, showing their ability to predict this load force by estimating the required normal grip force. However, when gravity is altered, dexterous manipulation becomes much more challenging. By modifying the gravitational forces in parabolic flights, we had the opportunity to structurally dissociate weight from inertia. We tested the adaptation of internal models that control grip force in various manipulations and in novel dynamic conditions. In rhythmic tasks, we demonstrated that the nervous system finely adjusts the grip controller in function of the physical context. In collisions, participants switched from a well known grip-load force coupling to a new strategy taking into account biomechanical properties of the impact to secure the grasp. Altogether, altered gravity environments allowed us to shed light on mechanisms used by the central nervous system to coordinate gaze, hand and grip motor actions during a variety of tasks that involve transport of an object and interactions with its environment through collisions.(FSA 3) -- UCL, 200