Brain displacement and nonlinear deformation following human spaceflight.

When humans return from space, their average brain position is higher within the cranial compartment. Here, we evaluated brain regional shift magnitudes and deformations and their behavioral correlates. We analyzed MRI data from 26 astronauts and 24 participants from a long-duration head-down tilt bed rest study. We aligned skull position across timepoints (before and after spaceflight or bed rest) as a reference and then used rigid body registration to quantify brain position change. The brain shifted backward, upward, and rotated back in the pitch direction from before to after spaceflight and bed rest in a fashion that correlated with exposure duration. Regional analyses revealed nonlinear deformations in all three directions that were not captured in previous reports of the average position shift. Moreover, we observed symmetrical left-right shifts that were not evident in the average position shift due to sign cancellation. In those who spent one year in space, the supplementary motor cortex showed the largest upward displacement [2.52 mm, 95% CI = (2.25, 2.79)]. Widespread recovery occurred in all three dimensions (particularly in the up-down direction) over 6 mo following spaceflight. Larger posterior insula translations were correlated with greater postflight balance declines. The brain shifts and deformations in astronauts were somewhat similar to head-down tilt bed rest, but there were notable differences. Thus, we demonstrate comprehensive brain position changes within the cranial compartment following spaceflight and an analog environment. These findings are critical for understanding the effects of spaceflight on the human brain and behavior.