Neural Working Memory Changes During a Spaceflight Analog With Elevated Carbon Dioxide: A Pilot Study

Abstract

Spaceflight missions to the International Space Station (ISS) expose astronauts to microgravity, radiation, isolation, and elevated carbon dioxide (CO 2), among other factors. Head down tilt bed rest (HDBR) is an Earth-based analog for spaceflight used to study body unloading, fluid shifts, and other factors unrelated to gravitational changes. While in space, astronauts need to use mental rotation strategies to facilitate their adaptation to the ISS environment. Therefore, spatial working memory is essential for crewmember performance. Although the effects of HDBR on spatial working memory have recently been studied, the results are still inconclusive. Here, we expand upon past work and examine the effects of HDBR with elevated CO 2 (HDBR + CO 2) on brain activation patterns during spatial working memory performance. In addition, we compare brain activation between 30 days of HDBR + CO 2 and 70 days of HDBR to test the isolated effect of CO 2. Eleven subjects (6 males, 5 females; mean age = 34 ± 8 years) underwent six functional magnetic resonance imaging (fMRI) sessions pre-, during, and post-HDBR + CO 2. During the HDBR + CO 2 intervention, we observed decreasing activation in the right middle frontal gyrus and left regions of the cerebellum, followed by post-intervention recovery. We detected several correlations between brain and behavioral slopes of change with the HDBR + CO 2 intervention. For example, greater increases in activation in frontal, temporal and parietal regions were associated with larger spatial working memory improvements. Comparing the HDBR + CO 2 group to data from our previous 70-day HDBR study, we found greater decreases in activation in the right hippocampus and left inferior temporal gyrus for the HDBR + CO 2 group over the course of the intervention. Together, these findings increase our understanding of the neural mechanisms of HDBR, elevated levels of CO 2 and spaceflight-related changes in spatial working memory performance.

Visuomotor Adaptation Brain Changes During a Spaceflight Analog With Elevated Carbon Dioxide (CO2): A Pilot Study

“…Here, we did not observe any brainbehavior change correlations, complicating interpretations. In this same cohort, we found evidence for both neural compensation and dysfunction as a result of HDBR+CO 2 when participants performed a spatial working memory task (Salazar et al, 2020) or received vestibular stimulation (Hupfeld et al, 2019). However, the lack of associations found in the present study does not allow us to fully interpret whether identified longitudinal neural changes that occur during visuomotor adaptation across HDBR+CO 2 are compensatory or dysfunctional.…”

“…Both spaceflight and HDBR are associated with sensory reweighting, changes in cognitive/sensorimotor processes (Bock et al, 2010;Yuan et al, 2016Yuan et al, , 2018Hupfeld et al, 2019), and modifications of brain structure (e.g., Koppelmans et al, 2017;Roberts et al, 2017;Lee et al, 2019b;Hupfeld et al, 2020) and function (Pechenkova et al, 2019) in healthy individuals. Our group (Hupfeld et al, 2019;Lee et al, 2019a;Salazar et al, 2020;McGregor et al, 2021) and others (Zwart et al, 2019;Laurie et al, 2020;Basner et al, 2021) have reported findings from an analog NASA campaign combining HDBR with elevated CO 2 levels. Compared to HDBR alone, elevated CO 2 better mimics the ISS environment (∼0.4% CO 2 ), which has average atmospheric CO 2 levels approximately 10 times greater than those on Earth.…”

“…Compared to HDBR alone, elevated CO 2 better mimics the ISS environment (~0.4% CO 2 ), which has average atmospheric CO 2 levels approximately 10 times greater than those on Earth. We recently showed that 30 days of HDBR combined with elevated CO 2 (HDBR+CO 2 ) results in reduced brain activity during spatial working memory task performance (Salazar et al, 2020 ). In another study, we observed adaptive plasticity of the vestibular system during HDBR+CO 2 followed by recovery after the conclusion of the intervention (Hupfeld et al, 2019 ).…”

A review of alterations to the brain during spaceflight and the potential relevance to crew in long-duration space exploration

“…A recent study of HDBR in an elevated CO 2 atmosphere demonstrated altered activation of the brain during vestibular stimulation than during HDBR alone, indicating that increased CO 2 may alter vestibular processing and compensation 44 . HDBR with CO 2 has also been associated with greater decreases in activation of the left inferior temporal gyrus and right hippocampus compared to HDBR alone, regions that are involved in visual memory, cognition and spatial working memory 46 . Fascinatingly, a recent fMRI study demonstrated increased functional connectivity between visual, vestibular and motor brain regions during HDBR with CO 2 ; while decreases in functional connectivity were seen between somatosensory, cognitive, motor, vestibular and multimodal integration regions 45 .…”

“…Several recent studies have also explored HDBR with CO 2 as an enhanced model for sensorimotor re-weighting during spaceflight [44][45][46] . A recent study of HDBR in an elevated CO 2 atmosphere demonstrated altered activation of the brain during vestibular stimulation than during HDBR alone, indicating that increased CO 2 may alter vestibular processing and compensation 44 .…”