Cerebrum Spring 2021

flights. (A Mars mission would be about three years, albeit with a long period of Martian gravity which might halt the progression of such weightlessness-induced problems.) In fact, a number of recent imaging studies have examined the brains of astronauts before and after spaceflight. In some cases, these studies show a slight upward shift of the brain in the skull, and an increase in the size of the ventricles (fluid- filled spaces inside the brain where cerebrospinal fluid is made and stored). Not all of these changes are reversed after returning to Earth. There are also changes in neural gray matter, which increases or decreases depending on the functions carried out by the brain area involved, and alterations in connectivity between regions, indicating neural plasticity and reorganization. Researchers have also observed white matter changes in several areas of the brain, including the cerebellum, which is involved in motor control and vestibular processing. So far, it seems that no permanent or dramatic damage to the neural tissue of the brain has resulted from spaceflight. In fact, many of the changes seen in imaging studies may be appropriately compensatory and adaptive for weightlessness—they are the body’s natural adaptive response to an unusual environment and serve the person well as long as he or she is in space.

inappropriate, or are the self-assessments incorrect? Or, as is more likely, is it the fact that motivated, high-performing individuals can rise to the occasion and perform well on virtually any well-defined task, as long as other distractors can be ignored? In the real world (and especially in space), people rarely have this luxury, which suggests why cognitive testing fails to capture the effects of spaceflight stressors in a realistic way. What Happens to the Brain? One such effect relates to changes in visual function . Astronauts returning from early missions of several months on ISS sometimes reported changes in visual acuity. Given all the demands of spaceflight, and the fact that these astronauts aged several months during their missions at a time in life when normal aging often produces decrements in visual function, it was hard to know what to make of these reports. Eventually, it became clear that there were changes in the structure of the eye—mostly temporary but some apparently long-lasting—that were very troubling. Alterations in visual function in high-performing individuals in a dangerous and demanding environment is something that gets a lot of attention, and a great deal of research has been devoted to characterizing and addressing this concern. What is the cause? It has been recognized for decades that, in weightlessness, there is a shift of fluids (i.e., blood, cerebrospinal fluid, lymph) from the lower to the upper body. This results in puffy faces (easily seen in astronaut photographs), sinus congestion, and blunted sensations of taste and smell. There are also changes in fluid drainage from the head, resulting in a buildup of fluid and, presumably, an (as yet unproven) increase in intracranial pressure. Some of the fluid makes its way down the sheath—the outer covering—of the optic nerve that leads to the eye. Among other things, this flow of fluid slightly distorts the shape of the eyeball, which changes its optical properties and hence visual acuity. There are also indications of damage to the retina , the layer of nerve cells in the eye that senses light and sends signals to the brain and that can detach from the epithelium. In advanced degeneration, the macula may bleed and leak fluid. Yellow deposits appear and vision becomes blurry. So far, these ocular effects have not produced dramatic deficits in astronauts’ ability to perform their duties, but they are worrisome. They began to generate serious concern when they became more consistent in ISS crews who spent several months in space (as opposed to crews of Space Shuttle missions who spent a maximum of 17 days there). Particularly disturbing is the possibility that these eye changes are but the canary in the coal mine: an indication of broader and more substantial neural damage that might result from longer

An astronaut's mental health challenge: seeing the same people day after day, month after month.


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