Effect of Risks, Consequences, and Gravitational Priors on Sensorimotor Coordination: Insights from Weightlessness.

When manipulating objects, our brain continuously adjusts grip force (GF) to the variations in load force (LF) generated by our own movements. GF-LF coordination provides a window into the predictive capabilities of the brain. To better understand how gravity influences these predictions, we analyzed grip dynamics and movement kinematics in astronauts (two females, nine males) manipulating objects on the ground (in 1G) and during spaceflight in a stable weightless environment (in 0G). We found that the imprint of gravity remains visible in the way we manipulate objects even after months of living in weightlessness. Empirical evidence showed that humans overcompensate for the absence of weight when manipulating objects in 0G, suggesting an anti-Bayesian anticipation of object buoyancy or negative weight. Shortly after returning to Earth, progressive kinematic adjustments were observed during the first movements with the object, as well as signs of incorrect LF predictions. The gradual and incomplete adjustments when passing from one gravitational context to the other underlines the predictive nature of the neural processes underlying these behaviors. In addition, a detailed examination of GF in weightlessness revealed a heretofore unrecognized link between the parameters of the GF/LF coupling, best described by a quadratic dependence of GF on both LF and the kinetic energy of the object. We conclude that not only is the risk of slip a determining factor in the control strategy, the impact of potential accidental slips is important as well.