An analysis of the brains of 11 male cosmonauts who spent an average of about 6 months in space found no evidence that this extensive time in microgravity led to neurodegeneration. Most changes the researchers observed shortly after the cosmonauts returned to Earth were reversed when their brains were examined again within 7 months, although recovery in the superior portion of the brain was more pronounced than in the inferior portion.

The findings provide insights into how the brain reorganizes itself during long spaceflights on a macro- and micro-scale, including through shifts in cerebrospinal fluid.

While neuroimaging techniques, such as magnetic resonance imaging (MRI), have recently enabled scientists to uncover changes in the brain’s structure and function after spaceflight, there are still little data on long-term effects, and while some previous studies measured macroscopic tissue changes, they could not assess underlying changes in the tissue microstructure.

To study space travel’s impacts on the brain in greater detail, Steven Jillings and colleagues determined the fraction of gray matter, white matter, and cerebrospinal fluid per voxel (a point in 3-dimensional space) in a series of diffusion MRI (dMRI) images using a technique that relies on each type of tissue’s unique dMRI signal decay. Jillings et al. used this approach to evaluate the brains of cosmonauts from Roscosmos, Russia’s space program, before and about 9 days after long-duration space missions on the International Space Station averaging 171 days.

The researchers performed additional scans on 8 of the cosmonauts about 7 months after their return. While Jillings et al. did detect increases in the quantity of gray matter tissue per voxel in the superior part of the brain and decreases in gray matter in the ventricles and the Sylvian fissure (which divides the frontal and parietal lobes from the temporal lobe), these were changes in the distribution of tissue caused by shifts in cerebrospinal fluid, not reductions in the net quantity of gray matter.

The shifts in cerebrospinal fluid the researchers observed support previous observations that microgravity causes the brain to shift upward inside the skull, and, additionally, suggest the cerebellum shifts upward, too.

The data also suggests that larger decreases in the sharpness of cosmonauts’ vision after spaceflight – a symptom caused by a condition called spaceflight-associated neuro-ocular syndrome (SANS) – are associated with larger expansions in the brain’s ventricles. However, since this result contradicts the finding of a previous study, Jillings et al. conclude further research involving a larger dataset is needed to determine the link between SANS and brain-related changes.