Abstract
Long duration head down tilt bed rest (HDBR) has been widely used as a spaceflight analog environment to understand the effects of microgravity on human physiology and performance. Reports have indicated that crewmembers onboard the International Space Station (ISS) experience symptoms of elevated CO2 such as headaches at lower levels of CO2 than levels at which symptoms begin to appear on Earth. This suggests there may be combinatorial effects of elevated CO2 and the other physiological effects of microgravity including headward fluid shifts and body unloading. The purpose of the current study was to investigate these effects by evaluating the impact of 30 days of 6° HDBR and 0.5% CO2 (HDBR + CO2) on mission relevant cognitive and sensorimotor performance. We found a facilitation of processing speed and a decrement in functional mobility for subjects undergoing HDBR + CO2 relative to our previous study of HDBR in ambient air. In addition, nearly half of the participants in this study developed signs of Spaceflight Associated Neuro-ocular Syndrome (SANS), a constellation of ocular structural and functional changes seen in approximately one third of long duration astronauts. This allowed us the unique opportunity to compare the two subgroups. We found that participants who exhibited signs of SANS became more visually dependent and shifted their speed-accuracy tradeoff, such that they were slower but more accurate than those that did not incur ocular changes. These small subgroup findings suggest that SANS may have an impact on mission relevant performance inflight via sensory reweighting.New And NoteworthyWe examined the effects of long duration head down tilt bed rest coupled with elevated CO2 as a spaceflight analog environment on human cognitive and sensorimotor performance. We found enhancements in processing speed and declines in functional mobility. A subset of participants exhibited signs of Spaceflight Associated Neuro-ocular Syndrome (SANS), which affects approximately one in three astronauts. These individuals increased their visual reliance throughout the intervention in comparison to participants who did not show signs of SANS.
Highlights
Successful spaceflight missions depend upon high levels of human performance
For the head down tilt bed rest (HDBR) + CO2 group, a significant main effect of time was observed for the time to complete the digit symbol substitution task
The 2D card rotation accuracy and the percentage of completed items increased with time, and the time to complete the task decreased as a function of time spent in the HDBR + CO2 environment (Supplementary Figure S1)
Summary
Successful spaceflight missions depend upon high levels of human performance. it is well established that astronauts experience altered sensory perception (Kornilova, 1997a; Clement et al, 2013) and declines in manual control, balance and locomotion both in- and post-flight (Lackner and DiZio, 1996; Kornilova, 1997b; Reschke et al, 1998; Bock et al, 2003, 2010). The environment aboard the ISS poses a potential risk to mission success and the long-term health of astronauts, as prolonged exposure to elevated ambient CO2 results in increased cerebral blood flow and mild performance impairments (Manzey and Lorenz, 1998; Satish et al, 2012; Fisk et al, 2013; Allen et al, 2015, 2018). Space station crewmembers have exhibited symptoms of elevated CO2 exposure such as headaches occurring at a more mild level of CO2 elevation than for headache symptoms in Earth-based studies of elevated CO2 (Law et al, 2014). There may be an interactive effect of elevated CO2 and the fluid shifts toward the head which we (Lee et al, 2019) and others (Roberts et al, 2017) have shown occur with spaceflight
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