Future research directions to identify risks and mitigation strategies for neurostructural, ocular, and behavioral changes induced by human spaceflight: A NASA-ESA expert group consensus report

Effect of artificial gravity on neurocognitive performance during head-down tilt bedrest

This study evaluated the acute and chronic effects of intermittent and continuous Artificial Gravity (AG) on cognitive performance during 60 days of Head-down tilt bedrest (HDTBR), a well-established ground-based spaceflight analogue method. Participants were randomly assigned to three groups: intermittent AG, continuous AG, and HDTBR control group without AG exposure. Task performance and electrophysiological measures of attention and working memory were investigated during Simple and Complex tasks in the Visual and the Auditory modality. Compared to baseline, faster reaction time and better accuracy was present during HDTBR regarding the Complex tasks, however, the practice effect was diminished in the three HDTBR groups compared to an ambulatory control group. Brain potentials showed a modality-specific decrease, as P3a was decreased only in the Auditory, while P3b decreased in the Visual modality. No evidence for acute or chronic AG-related cognitive impairments during HDTBR was found.

Weighing the impact of microgravity on vestibular and visual functions

Numerous technological challenges have been overcome to realize human space exploration. As mission durations gradually lengthen, the next obstacle is a set of physical limitations. Extended exposure to microgravity poses multiple threats to various bodily systems. Two of these systems are of particular concern for the success of future space missions. The vestibular system includes the otolith organs, which are stimulated in gravity but unloaded in microgravity. This impairs perception, posture, and coordination, all of which are relevant to mission success. Similarly, vision is impaired in many space travelers due to possible intracranial pressure changes or fluid shifts in the brain. As humankind prepares for extended missions to Mars and beyond, it is imperative to compensate for these perils in prolonged weightlessness. Possible countermeasures are considered such as exercise regimens, improved nutrition, and artificial gravity achieved with a centrifuge or spacecraft rotation.

Navigating the Unknown: A Comprehensive Review of Spaceflight-Associated Neuro-Ocular Syndrome

Spaceflight-associated neuro-ocular syndrome (SANS) is a complex and multifaceted condition that affects astronauts during and after their missions in space. This comprehensive review delves into the various aspects of SANS, providing a thorough understanding of its definition, historical context, clinical presentation, epidemiology, diagnostic techniques, preventive measures, and management strategies. Various ocular and neurological symptoms, including visual impairment, optic disc edema, choroidal folds, retinal changes, and increased intracranial pressure, characterize SANS. While microgravity is a primary driver of SANS, other factors like radiation exposure, genetic predisposition, and environmental conditions within spacecraft contribute to its development. The duration of space missions is a significant factor, with longer missions associated with a higher incidence of SANS. This review explores the diagnostic criteria and variability in SANS presentation, shedding light on early detection and management challenges. The epidemiology section provides insights into the occurrence frequency, affected astronauts' demographics, and differences between long-term and short-term missions. Diagnostic tools, including ophthalmological assessments and imaging techniques, are crucial in monitoring astronaut health during missions. Preventive measures are vital in mitigating the impact of SANS. Current strategies, ongoing research in prevention methods, lifestyle and behavioral factors, and the potential role of artificial gravity are discussed in detail. Additionally, the review delves into interventions, potential pharmacological treatments, rehabilitation, and long-term management considerations for astronauts with SANS. The conclusion underscores the importance of continued research in SANS, addressing ongoing challenges, and highlighting unanswered questions. With the expansion of human space exploration, understanding and managing SANS is imperative to ensure the health and well-being of astronauts during long-duration missions. This review is a valuable resource for researchers, healthcare professionals, and space agencies striving to enhance our knowledge and address the complexities of SANS.

Dry eye disease in astronauts: a narrative review

Long-duration spaceflight can have adverse effects on human health. One of the most common ocular conditions experienced by astronauts is dry eye disease (DED). Symptoms of DED include feelings of eye irritation, eye strain, foreign body sensation and blurred vision. Over 30% of International Space Station expedition crew members reported irritation and foreign body sensation. We reviewed the current literature on the prevalence and mechanisms of DED in astronauts and its potential implications for long-duration spaceflight, including the influence of environmental factors, such as microgravity and fluid shift on tear film physiology in space. DED has negative effects on astronaut performance, which is why there is a need for further research into the pathophysiology and countermeasures. As an in-flight countermeasure, neurostimulation seems to be among the most promising options.

Paving the way to better understand the effects of prolonged spaceflight on operational performance and its neural bases

Space exploration objectives will soon move from low Earth orbit to distant destinations like Moon and Mars. The present work provides an up-to-date roadmap that identifies critical research gaps related to human behavior and performance in altered gravity and space. The roadmap summarizes (1) key neurobehavioral challenges associated with spaceflight, (2) the need to consider sex as a biological variable, (3) the use of integrative omics technologies to elucidate mechanisms underlying changes in the brain and behavior, and (4) the importance of understanding the neural representation of gravity throughout the brain and its multisensory processing. We then highlight the need for a variety of target-specific countermeasures, and a personalized administration schedule as two critical strategies for mitigating potentially adverse effects of spaceflight on the central nervous system and performance. We conclude with a summary of key priorities for the roadmaps of current and future space programs and stress the importance of new collaborative strategies across agencies and researchers for fostering an integrative cross- and transdisciplinary approach from cells, molecules to neural circuits and cognitive performance. Finally, we highlight that space research in neurocognitive science goes beyond monitoring and mitigating risks in astronauts but could also have significant benefits for the population on Earth.

Methods for assessing change in brain plasticity at night and psychological resilience during daytime between repeated long-duration space missions

This study was designed to examine the feasibility of analyzing heart rate variability (HRV) data from repeat-flier astronauts at matching days on two separate missions to assess any effect of repeated missions on brain plasticity and psychological resilience, as conjectured by Demertzi. As an example, on the second mission of a healthy astronaut studied about 20 days after launch, sleep duration lengthened, sleep quality improved, and spectral power (ms²) co-varying with activity of the salience network (SN) increased at night. HF-component (0.15-0.50 Hz) increased by 61.55%, and HF-band (0.30-0.40 Hz) by 92.60%. Spectral power of HRV indices during daytime, which correlate negatively with psychological resilience, decreased, HF-component by 22.18% and HF-band by 37.26%. LF-component and LF-band, reflecting activity of the default mode network, did not change significantly. During the second mission, 24-h acrophases of HRV endpoints did not change but the 12-h acrophase of TF-HRV did (P < 0.0001), perhaps consolidating the circadian system to help adapt to space by taking advantage of brain plasticity at night and psychological resilience during daytime. While this N-of-1 study prevents drawing definitive conclusions, the methodology used herein to monitor markers of brain plasticity could pave the way for further studies that could add to the present results.

Homo sapiens—A Species Not Designed for Space Flight: Health Risks in Low Earth Orbit and Beyond, Including Potential Risks When Traveling beyond the Geomagnetic Field of Earth

Homo sapiens and their predecessors evolved in the context of the boundary conditions of Earth, including a 1 g gravity and a geomagnetic field (GMF). These variables, plus others, led to complex organisms that evolved under a defined set of conditions and define how humans will respond to space flight, a circumstance that could not have been anticipated by evolution. Over the past ~60 years, space flight and living in low Earth orbit (LEO) have revealed that astronauts are impacted to varying degrees by such new environments. In addition, it has been noted that astronauts are quite heterogeneous in their response patterns, indicating that such variation is either silent if one remained on Earth, or the heterogeneity unknowingly contributes to disease development during aging or in response to insults. With the planned mission to deep space, humans will now be exposed to further risks from radiation when traveling beyond the influence of the GMF, as well as other potential risks that are associated with the actual loss of the GMF on the astronauts, their microbiomes, and growing food sources. Experimental studies with model systems have revealed that hypogravity conditions can influence a variety biological and physiological systems, and thus the loss of the GMF may have unanticipated consequences to astronauts’ systems, such as those that are electrical in nature (i.e., the cardiovascular system and central neural systems). As astronauts have been shown to be heterogeneous in their responses to LEO, they may require personalized countermeasures, while others may not be good candidates for deep-space missions if effective countermeasures cannot be developed for long-duration missions. This review will discuss several of the physiological and neural systems that are affected and how the emerging variables may influence astronaut health and functioning.

Are head-down tilt bedrest studies capturing the true nature of spaceflight-induced cognitive changes? A review

Although a number of studies have examined cognitive functions in space, the reasons behind the observed changes described by space research and anecdotal reports have not yet been elucidated. A potential source of cognitive changes is the cephalad fluid shift in the body caused by the lack of hydrostatic pressure under microgravity. These alterations can be modeled under terrestrial conditions using ground-based studies, such as head-down tilt bedrest (HDBR). In this review, we compare the results of the space and HDBR cognitive research. Results for baseline and in-flight/in-HDBR comparisons, and for baseline and post-flight/post-HDBR comparisons are detailed regarding sensorimotor skills, time estimation, attention, psychomotor speed, memory, executive functions, reasoning, mathematical processing, and cognitive processing of emotional stimuli. Beyond behavioral performance, results regarding brain electrical activity during simulated and real microgravity environments are also discussed. Finally, we highlight the research gaps and suggest future directions.