Upright posture: a singular condition stabilizing sensorimotor coordination.

It has long been hypothesized that the nervous system uses the direction of gravity to align the various sensory systems when interacting with the external world. In line with this hypothesis, systematic drift in hand-path orientation was recently observed during targeted arm motions performed with eyes closed in weightlessness or, on Earth, for longitudinal movements in a supine posture. No such drift was observed in upright posture on Earth. But the precise conditions under which participants exhibit, and the factors that influence the magnitude of the drift, are not yet known. The objective of our study was to investigate if the upright posture, by virtue of being at a biomechanical singularity induced by the force of gravity, represents a unique condition in which drift in hand-path orientation is prevented. Human participants (male and female) performed sequences of repeated point-to-point arm move- ments between two visual targets aligned with the longitudinal body axis, first with eyes open, then with eyes closed. Participants performed these movements in various body orientations: seated upright, and tilted backward at 45, 90, and 135°. We observed drift in hand-path orientation in the eyes-closed condition when the body was tilted, but not when it was upright. The direction and rate of drift were indistinguishable between the three tilted orientations tested (45, 90, and 135°). These findings support the hypothesis that the upright posture is a unique configuration which facilitates sensorimotor transformations and prevents drift in path orientation when the eyes are closed.