Sensorimotor function may be fundamentally limited in hypogravity

During spaceflight, humans are exposed to unfamiliar gravitational fields and to rapid transitions in the magnitude of these gravitational fields. Many aspects of sensorimotor neural function are altered by these transitions, and adaptation after transitions has been characterized. However, it is important to know whether human physiology has inherent limitations in hypogravity (i.e., gravity between 0 and 1.0 G) that cannot be overcome by adaptation. To address this critical gap, we studied manual control performance using a laboratory-based centrifuge that was configured and used to mimic hypogravity. Ten healthy human subjects performed a manual control task using a joystick to control the tilt of the motorized chair upon which they were sitting. Manual control performance worsened immediately after transition from 1.0 G_c to 0.5 G_c (69%), partly adapted over 18 min in 0.5 G_c, and remained significantly worse despite adaptation (42%) (1.0 G_c = 9.81 m/s² of centripetal acceleration). We propose that in hypogravity, any particular body tilt will result in diminished shear force on sensory graviceptors relative to 1 G, reducing signal relative to intrinsic neural noise. This necessitates larger tilt angles before manual control inputs can be determined, thus worsening performance. These results add to prior studies providing evidence supporting the hypothesis that closed-loop sensorimotor performance may be fundamentally limited by signal-to-noise ratio, including in hypogravity. This may contribute to risk during lunar piloting and ambulation. We also studied underlying mechanisms using a computational model of closed-loop control and found that adaptation was associated with increasing control gain (K_P).NEW & NOTEWORTHY Prior studies suggest that humans can adapt to hypogravity (i.e., 0-1.0 Earth G). We examined human manual control performance during the transition from an Earth-gravity to a hypogravity condition. We found that performance worsened significantly after the transition. With practice in the hypogravity condition, performance partially improved with practice over repeated trials, but remained impaired in the hypogravity condition relative to the Earth-gravity condition.