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

Abstract

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.

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

“…While some aspects of these space-associated changes in the brain can be captured by analogs such as head-down tilt bedrest studies [ 89 ], it is not clear that all of the changes will be detected on Earth, due to the complexity of the changes in space. Interestingly, use of artificial gravity can alleviate some of the neural changes occurring as a result of the bedrest analog condition [ 90 ].…”

“…Clearly, alterations to the brain occur in LEO [ 89 , 99 , 173 , 174 , 175 ], potentially in part due to the microgravity and the associated fluid redistribution that occurs. Therefore, if the GMF-related systems are active, and the brain has already been impacted by microgravity-related changes, then it is likely that the loss of the GMF could exacerbate the changes.…”

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

“…microgravity, cosmic radiation, elevated CO2 levels, altered light exposure, increased ambient noise) and challenges related to the spaceflight environment (e.g. high workload, isolation, and confinement) (for a review see 1 ). Astronauts experience physiological deconditioning during spaceflights 2 , including loss of muscle mass and strength, bone density, sensory-motor deconditioning, altered central blood volume regulation, and headward fluid shift 3 .…”

“…Growing neuroimaging evidence shows that brain structure is affected by spaceflight including the upward shift of the brain, increased total ventricular volume 4 along with decreased gray matter and white matter volume 5 , 6 . Despite the clear impact of spaceflight on the central nervous system, results regarding spaceflight-related cognitive symptoms are mixed: while some studies report cognitive deficits, such as disturbed visuospatial performance 7 – 9 , others failed to demonstrate significant changes in cognitive performance (for reviews see 1 , 10 , 11 ). The reason behind inconsistent results includes low sample sizes and relatedly low statistical power, high variation in cognitive test batteries and targeted cognitive domains, and lack of more controllable settings, such as ground-based analogs 1 , 12 ).…”

“…Head-down tilt bedrest (HDTBR) presents a well-established ground-based space analog model for microgravity-induced physiological alterations including cardiovascular changes 3 , 13 , cephalic fluid shift 14 , upward brain shift, structural brain changes 15 , 16 . Even though it is suggested that HDTBR has a similar impact on cognitive functions as spaceflight, results regarding HDTBR on cognitive performance are also far from being conclusive (for reviews see 1 , 17 ). HDTBR also presents a great opportunity to test the effectiveness of countermeasures against microgravity-induced physiological changes under terrestrial conditions, such as short-arm centrifugation.…”