Finally, sleep is critical for the consolidation of memories and for learning. This includes motor learning : the acquisition of skills that require physical manipulation and dexterity, as might be essential in teleoperation of remote manipulators on Mars or the moon. Impaired sleep, which, as we've noted, is common in space, could undermine the ability to learn such tasks. This is especially important because a mission that takes a long travel time might demand the performance of tasks learned during training, well before the flight began, allowing plenty of time to forget. In-flight training could provide a vital refresher for those tasks, and it would be best if this relearning were not impaired by the inability to consolidate learning during sleep.
However, even the small possibility of long-lasting neural damage due to brain shifting or increased fluid volume—along with a possibly dire impact on in-flight performance—calls for careful study and monitoring. Underlying Brain Function Issues There are other physiological changes that occur during spaceflight that might impact brain function, whose effects are likely to be subtle compared to those above, but could combine synergistically with stressors in ways that we do not yet understand. One example involves the gut microbiome: the millions of microorganisms that live in the intestinal tract. Among its varied functions, the microbiome appears to
influence cognition and emotion through the so- called gut-brain axis ; it turns out that maintaining a healthy, vibrant, diversified array of intestinal microbes is important for mental
FOR A FUTURE MISSION TO MARS, FOR
EXAMPLE, SUPPORT FROM EARTH-BASED EXPERTS WILL BE EXTREMELY LIMITED, AND AUGMENTATION OF SUPPLIES THROUGH CARGO DELIVERY ESSENTIALLY IMPOSSIBLE.
health. For reasons not fully understood, some aspects of this gut microbiome change during spaceflight—possibly the result of the stressors already discussed, along with an altered diet and use of medications for sleep, pain, and other purposes. It seems that bone health, as well, has an impact on brain health through the hormone osteocalcin . Due to unloading of gravity in weightlessness, the bones lose calcium. At least a part of this process seems associated with a reduction in the activity of osteoblasts, which are the cells that create new bone and also secrete osteocalcin. The vestibular system—the non-auditory part of the inner ear that regulates balance—also plays a crucial role in spaceflight. The brain depends on the presence of gravity as transduced by the vestibular system—a constant throughout evolution and development—to provide a reference frame for spatial orientation. In space, the lack of normal gravitational influence on this system can result in space motion sickness , disorientation, and a variety of related effects. The resultant difficulty in spatial perception and navigation could be a contributor to space fog. In fact, more and more evidence relates vestibular dysfunction to cognitive issues. Although the astronaut’s vestibular system remains intact, alterations in its function due to changes in gravity level could have subtle but cumulative cognitive effects.
Resilience The body has compensatory mechanisms for many adverse effects of spaceflight, and they work well. But at some point, they reach their limits. Even short of that limit, at what point does the confluence of these effects reduce and erode performance margins and resilience: the ability to recover from an unexpected perturbation or anomaly? This is a perilous problem that is difficult to address. Astronauts don’t get to be astronauts without a full complement of compensatory strategies that they can call upon (volitionally or subconsciously) to work around deficits that occur. They might simply increase their level of concentration, or give themselves extra time, or seek help from a crewmate, or defer some harder tasks until they are better rested. If the brain remains able to carry out this type of logistical re-organizational process, can we say there is actually a deficit? Yet, when faced with an additional stressor such as an emergency alarm, the sudden increase in mental demand might make a latent mental or cognitive deficit manifest—at exactly the wrong time. We do not yet have standardized tests for adaptability and the ability to reorganize in the face of a decrease in brain function. A similar effect is seen on Earth among some patients with balance disorders. They may present to the clinician performing normally on a specific test of balance, but fail when an additional demand is added. An example is walking
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