After studying the brain scans of 13 Russian cosmonauts who participated in space missions to the ISS from 2014 to 2020, scientists discovered that prolonged exposure to microgravity had an impact on the connectivity of the brain structures responsible for adapting to unfamiliar conditions. The study revealed that these connections may not always return to their original state after the flight. The paper has been published in Communications Biology.
Prolonged stays in space cause physical changes to the human body, such as deterioration of bones and muscles. As a result, both health professionals and scientists closely monitor the health of astronauts. However, little is known about the effect of extreme conditions in space on the brain.
The human brain is characterised by neuroplasticity, or adaptability to environmental changes, that helps people learn new things and recover from injuries. One of the mechanisms of neuroplasticity involves changes in the strength of connections between different regions of the brain, as well as the loss of certain connections and the acquisition of new ones.
The main environmental change that affects cosmonauts on the International Space Station (ISS) is weightlessness, or microgravity. Over time, they gradually become accustomed to the absence of gravity. However, it is currently unclear which brain structures are responsible for this adaptation, and whether cosmonauts' brains recover their prior states upon returning to Earth.
An international team of researchers analysed the changes that occurred in the brains of 13 Russian cosmonauts who participated in long-duration space missions to the ISS between 2014 and 2020. The scientists specifically focused on changes in brain connectivity that occurred after prolonged exposure to microgravity. During the study, the participants underwent brain scans using functional magnetic resonance imaging (fMRI) at three different points in time: before the flight, immediately after the flight, and again eight months later during a follow-up examination. During the study period, two cosmonauts participated in two separate space missions and underwent complete scanning sessions during each mission.
Using the fMRI method, it is possible to visualise brain activity both when the brain is at rest and when it is engaged in specific tasks. When a specific area of the brain is activated, it results in an increase in metabolic activity and blood flow, and these changes can be detected by fMRI.
The researchers utilised the fMRI data to measure changes in the strength of connections between various brain regions both before and after the space flight. The brain activity of the cosmonauts at rest was compared to scans taken from 14 subjects in the control group. The researchers identified changes in the cosmonauts' brain connections which may be responsible for sensory adaptation.
In particular, they found a decrease in connectivity in the posterior cingulate gyrus which persisted at eight months after the flight. As the cingulate gyrus, in conjunction with the precuneus, constitutes a crucial cluster within the brain's default mode network, changes in its connectivity may indicate the broader cerebral effects of adaptation to unfamiliar sensory sensations experienced during a space flight.
Another cerebral structure, the thalamus, also demonstrated decreased connectivity, especially with the prefrontal cortex. The connections between the prefrontal cortex and the thalamus play a crucial role in adaptive decision-making and the function of working memory.
The right angular gyrus exhibited increased participation in whole-brain connectivity, both immediately postflight and at eight months after the space mission. In conditions of microgravity, the angular gyrus assumes a critical role in comparing sensory input to expected action outcomes and subsequently generating smooth motor patterns, as habitual movements are executed differently in space than on Earth. It is widely recognised that cosmonauts adapt to the spaceflight environment over time, and this adaptation may manifest in increased connectivity within the angular gyrus.
The only structure whose connectivity returned to the pre-flight status at eight months was the insular cortex. The insular cortex is functionally connected to the salience network in the human brain, which is responsible for identifying significant stimuli in the environment and selecting an appropriate response.
The researchers also noted reduced connectivity between the anterior insula and middle cingulate cortex, which may indicate a suppression of autonomic responses in the absence of normal gravity. During the initial stages of a flight, many cosmonauts experience space motion sickness, which is akin to other types of motion sickness, but they typically recover after a certain period. The rewiring of connectivity between the anterior insula and middle cingulate cortex may be indicative of these changes.
Taken together, these findings offer evidence of persistent functional changes in vestibular, motor, and multisensory brain regions caused by prolonged microgravity, which may be a manifestation of the brain's adaptation to a new environment.
These functional changes are not particularly dangerous and are similar to other types of adaptations to challenging conditions. People who pursue extreme occupations or hobbies on Earth face similar risks to their brain function. Therefore, the main way we plan to address these challenges experienced by cosmonauts is to develop a series of exercises to better prepare them for longer space flights (including potential interplanetary missions) and to facilitate their post-flight recovery.