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AnonTechie wrote with news of research into the effects cosmic rays have on the brain. From the article published in Science Advances:

As NASA prepares for the first manned spaceflight to Mars, questions have surfaced concerning the potential for increased risks associated with exposure to the spectrum of highly energetic nuclei that comprise galactic cosmic rays. Animal models have revealed an unexpected sensitivity of mature neurons in the brain to charged particles found in space. Astronaut autonomy during long-term space travel is particularly critical as is the need to properly manage planned and unanticipated events, activities that could be compromised by accumulating particle traversals through the brain.

Using mice subjected to space-relevant fluences of charged particles, we show significant cortical- and hippocampal-based performance decrements 6 weeks after acute exposure. Animals manifesting cognitive decrements exhibited marked and persistent radiation-induced reductions in dendritic complexity and spine density along medial prefrontal cortical neurons known to mediate neurotransmission specifically interrogated by our behavioral tasks.

This was stated a little more readably at ScienceDaily:

What happens to an astronaut's brain during a mission to Mars? Nothing good. It's besieged by destructive particles that can forever impair cognition, according to a radiation oncology study. Exposure to highly energetic charged particles -- much like those found in the galactic cosmic rays that bombard astronauts during extended spaceflights -- cause significant damage to the central nervous system, resulting in cognitive impairments.

[Related]: Space Radiation On the Long Trip To Mars Could Make Astronauts Dumber