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 (2 females, 9 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 load force 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 grip forces 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. Significance statement In measurements of grip force performed while astronauts manipulated objects in weightlessness, we found evidence of an overcompensation of the absence of weight. We postulate an Anti-Bayesian phenomenon wherein the CNS modulates GF responses in 0G to an erroneous prediction that physical objects will float upwards (negative weight). By studying the influence of object mass and movement speed on grip force control we were also able to propose a new model to describe grip-load force coordination. This model considers kinetic energy as a determining factor and emphasizes the importance of integrating the consequences of accidental slips into the control strategy.
Opsomer et al. (Mon,) studied this question.