While Roscosmos is discussing future manned flights to Mars, NASA plans to open the International Space Station for commercial tourism, and SpaceX is testing its Starship Mars prototype, scientists are seriously concerned about the impact of prolonged stay in space on the human body. While the effects of weightlessness on bones, muscles and the vestibular system are well known, how the human brain copes with microgravity has yet to be fully examined. IQ.HSE has compiled the latest research on this topic.
During flights, cosmonauts are continuously exposed to weightlessness, which requires adaptation and causes changes within the body. Life on colonised planets and satellites — humanity's likely future — will demand special conditions to become safe for our body. Recent studies using neuroimaging show that space travel does not leave the brain unaffected either.
An international team which included scientists from the HSE,Federal Center of Treatment and Rehabilitation,Moscow State University, Russian Academy of Sciences, and Gagarin Cosmonaut Training Centre used functional magnetic resonance imaging (fMRI) to measure functional brain connectivity in a group of cosmonauts in a groundbreaking research project.It turned out that adaptation to microgravity and related changes in motor activity can cause the modifications of functional connectivity between the brain areas.
The researchers performed brain fMRIs on eleven cosmonauts before and after space missions lasting on average six months and then compared their fMRI data to those of healthy volunteers who had stayed on Earth. The researchers were looking for changes in connectivity between brain areas underlying sensorimotor functions such as movement and perception of body position. These brain areas were activated using gait-imitating plantar stimulation.
On Earth, the perception of space and body position is regulated by the vestibular systemconsisting of vestibular sacs and the semicircular canals of the inner ear. In weightlessness, however, the vestibular function is impaired because the system needs gravity to work properly. Therefore, cosmonauts often experience dizziness and disorientation while their bodies struggle to adapt to the new environment.
The researchers discovered changes in cosmonaut brain connections responsible for perception and movement. To compensate for the lack of information from the organs of balance, which cannot provide reliable information in micrograviry, the brain develops an auxiliary system of somatosensory control, with increased reliance on visual and tactile feedback instead of vestibular input. Indeed, the fMRI data showed increased connectivity between the islet lobes — responsible for integrating sensations coming from different systems — and other parts of the brain.
Under Earth’s gravity, vestibular nuclei are responsible for processing signals coming from the vestibular system. But in space, according to the researchers, the brain may downweight the activity of these structures to avoid conflicting information about the environment.
This study is not the first to examine the effects of weightlessness on the brain using neuroimaging. Some earlier studies also focused on the health risks faced by cosmonauts.
Researchers at the Medical University of South Carolina found an upward shift of the brain hemispheres within the skull and increased density of brain tissue at the vertex, both caused by weightlessness.
The researchers suggest that such a shift may cause compression of the cerebral veins and elevated intracranial pressure which, in turn, can lead to optic disc oedema and visual impairment.
Cerebrospinal fluid (CSF) surrounds the brain and spinal cord, protecting the nervous system from injury and providing it with nutrients. Weightlessness causes a disruption of normal CSF circulation, alongside a change in the volume of ventricles, i.e. cavities containing CSF in the brain.
In a study simulating the effects of microgravity, volunteers were asked to observe head-down tilt bed rest for 11-14 days. Changes in the subjects' ventricular volume ranged from a 10% increase to a 20% decrease, in line with what has been observed in cosmonauts after space flight.
Fluid build-up inside the skull may be another cause of visual impairment. Two-thirds of all cosmonauts suffer from blurred vision, which can persist upon their return to Earth.
A Russian-Belgian team of scientists found that cosmonauts tend to experience a decrease in the volume of grey and white matter in those areas of the brain which control movement and process sensory information. The maximum change observed was a 3.3% tissue decrease in the right middle temporal gyrus.
However, seven months after the space flight, brain tissue volume was nearly fully restored. Possible causes of the decrease and its effect on cognitive abilities are not yet known.
Once back on Earth, cosmonauts' bodies begin to recover. But it is unclear how long-term exposure to microgravity can affect the brain. While today cosmonauts spend several months in space, future colonisation of other planets may require years of space travel.
According to researchers, creating artificial gravity in space stations to mimic the conditions on Earth may protect cosmonauts from many health risks.