Astronaut Cognitive Issues: Houston, Are the Neurons Ready to Take Off?

You’re an astronaut looking up at the stars sparkling in the void of darkness. Your dreams, passion, hard work—it’s all come to this, yet it’s lonely being in this void without your family and friends. It’s scary to be left alone with the fear of not making it back to Earth. It’s difficult to be in this harsh environment when your body is built to survive on Earth. 

When researching the effects of space on the human body, we often focus on physical aspects like muscle loss and also on designing better spacecraft and spacesuits. As a result, the mental health of astronauts such as cognitive issues—problems with a person’s learning ability to think, learn, remember, and make judgments—are typically overlooked. Fortunately, the Artemis missions’ aim to revisit the Moon to create human habitation highlights the importance of maintaining cognitive sharpness.

Two factors that researchers have noted when studying the causes of astronauts’ cognitive issues after missions are space radiation and microgravity. Radiation is when energy is transported to another entity and emitted as rays, electromagnetic waves, and/or particles. Space radiation consists of only the nucleus—neutrons and protons—atoms. It forms after atoms approach the speed of light and the internal structures of the atom are stripped away except the nucleus. Imagine an astronaut in space and tiny particles that have enough energy to bypass their spacesuits hit them quickly. The energy transferred to their body may lead to degradation, resulting in degenerative diseases. Additionally, microgravity, or “very small” gravity, is the perceived weightlessness astronauts experience when floating in space due to low gravitational force.

University of California researchers investigated the effects of chronic low-dose space radiation on astronauts in an article published in the National Library of Medicine. Researchers exposed mice and rats to low dose-rate neutron irradiation in order to mimic the galactic cosmic radiation (GCR) that affects the functionality of astronauts’ central nervous systems. GCR comes from charged particles—protons, electrons, and atomic nuclei—outside our solar system that were accelerated by the energy released in supernovas—explosions from stars in their last stage of life. GCR is highly penetrating, hard to shield, and its risks to astronauts include neuronal damage and cognitive deficits.

Neutron irradiation exposes materials to a controlled flow of neutrons because, as mentioned, space radiation consists mostly of neutrons. The neutron irradiation process and GCR have similar energy transfer processes. Both involve high Z—high atomic number elements (elements with many protons and neutrons)—and energy particles.

To conduct the experiment, researchers used two groups of 40 male mice, each around eight months old and with the same genetic background. Through a six-month period, the researchers delivered a dose of one milligram per day, which consisted of 80 percent neutrons since space radiation mostly consists of neutrons and 20 percent photons due to their indirect interaction with human tissues in space through ionization. Ionization is when photons collide with atoms in human tissues and transfer enough energy for the atoms to lose electrons to possibly disrupt cellular structures. Additionally, a control group was placed in a separate facility with similar conditions but without the radiation. 

Following the six months, eight control and seven experimental mice were anesthetized and decapitated to have their brains removed. 29 cells from the control mice and 28 cells from the experimental mice were used to test for properties of how neurons process signals and generate electrical impulses. Since neurons use electrical and chemical signals to relay information, scientists hoped to gather information on how space radiation affects the brain's ability to communicate with itself.

Furthermore, 24 cells from the control group and 25 cells from the experimental group were used to identify potential differences in how the neurons form connections with each other. Slices of the brain containing the hippocampus, an area in the brain associated with learning and memory, were incubated in a 35°C cutting solution for an hour to recover. Microelectrodes were then inserted into the neurons in the hippocampus to measure electrical activity.

The neurons in the hippocampus from the experimental group exposed to the radiation had a more hyperpolarized resting membrane potential, decreased excitability, and a decrease in the frequency of spontaneous excitatory postsynaptic currents. Usually, a neuron has a slightly more negative charge inside compared to outside its cell membrane. An increase in hyperpolarization means the negative charge increases even more on the inside, which makes it more difficult to produce electrical signals as electrical flow slows down at a higher rate when the electrical signal passes through the neurons. A decreased excitability means a decrease in a neuron’s response to stimulation. Finally, neurons use postsynaptic currents for processing and delivering information. When it’s harder to stimulate the currents without external stimuli, there is a decrease in the frequency of spontaneous excitatory potentials, meaning neurons are communicating less. Essentially, space radiation makes it more difficult for neurons to produce a response to a stimulus, which makes it more difficult for the brain to communicate with itself. 

National Library of Medicine Department of Radiology and Radiological Science researchers explored another factor in astronaut cognitive issues—prolonged microgravity as a result of long-duration space flights. 

This experiment was rather straightforward: it analyzed the MRI scans pre and post-flight of seven astronauts with long-duration flights on the Interational Space Station (ISS) and 12 astronauts with short-duration flights on the ISS. Short-duration lights lasted an average of 14.7 days while long-duration ISS missions lasted an average of 162.7 days. This revealed that for long-duration flights, there was an increase in the total ventricular volume—a system of cavities that hold the fluid of the central nervous system in the center of the brain—associated with deteriorative brain disease, and there was also crowding of brain tissue at the vertex—the top and highest part of the brain—which negatively impacted neural connectivity and brain communication. 

Additionally, post-flight, the astronauts took the WinsCAT, a collection of five subtests used to measure the cognitive ability of astronauts. The most notable results from the test come from the CDS and SPT subsets, which measure processing speed/efficiency/learning and attention span, respectively. Results included a significant reduction in accuracy on the CDS processing speed and learning subtest but faster reaction times for the sustained attention subtest. This decrease in accuracy and increase in reaction time may reflect a trade-off between speed and accuracy in cognitive processing. 

A limitation of this study is the small sample size and lack of uniform protocols. Ultimately, these research articles are just a few steps toward improving astronauts’ mental health when they are fighting to advance us in the merciless environment of space. Future research will help us develop interventions and countermeasures so when we go beyond the Moon and Mars for habitation, astronauts do not have to compromise their cognitive abilities.