Feedback-driven adaptation of gravity-related sensorimotor control to an upside-down posture

The ability to move is a vital and essential feature of human existence. We are experts at producing a variety of movements and have refined their control through evolution. As gravity is a major feature of our every-day environment, we have learned to take advantage of it by optimising its effects to minimise the cost of our actions. This can be illustrated by systematic differences in the temporal organisation of our movements according to their direction. Studying motor control in the face of various gravitational level modifications (hypergravity, hypogravity or weightlessness), the scientific literature has shown that movement kinematics are rapidly adapted to new gravitational conditions. Hitherto, most researchers varied gravitational intensity to probe its neural integration into sensorimotor control. Here, we investigated the effects of a reversal of gravitational direction in the egocentric reference frame. Our results reveal a major effect of body-orientation reversal on motor control. This effect then progressively disappears, such that arm kinematics reached values that were close to the known baseline optimal ones. These results reveal that the effects of a simple reversal of body-orientation cannot be fully anticipated to produce arm movements. Furthermore, comparing the evolution of varied parameters that were collected at different time points during each movement, our results reveal that adaptation first occured during the late movement phases (i.e., around the time to peak velocity and time to peak deceleration), where online feedback contributions are typically larger, while not ruling out trial-to-trial updates of predictive control.