Effects of head-down tilt bed rest plus elevated CO2 on cognitive performance

Abstract

Microgravity and elevated CO₂ levels are two important environmental spaceflight stressors that can adversely affect astronaut cognitive performance and jeopardize mission success. This study investigated the effects of 6° head-down tilt bed rest (HDBR) with (n = 11 participants, 30-day HDBR) and without (n = 8 participants, 60-day HDBR) elevated ambient (3.73 mmHg) CO₂ concentrations on cognitive performance. Participants of both groups performed all 10 tests of NASA's Cognition battery and a brief alertness and mood survey repeatedly before, during, and after the HDBR period. Test scores were adjusted for practice and stimulus set effects. Concentrating on the first 30 days of HDBR, a modest but statistically significant slowing across a range of cognitive domains was found in both groups (controls: -0.37 SD; 95% CI -0.48, -0.27; adjusted P < 0.0001; CO₂: -0.25 SD; 95% CI -0.34, -0.16; adjusted P < 0.001), most prominently for sensorimotor speed. These changes were observed early during HDBR and did not further deteriorate or improve with increasing time in HDBR. The study found similar cognitive effects of HDBR irrespective of CO₂ levels, suggesting that elevated CO₂ neither ameliorated nor worsened the HDBR effects. In both groups, cognitive performance after 15 days of recovery was statistically indistinguishable from pre-HDBR performance. However, subjects undergoing 60 days of HDBR rated themselves as feeling more sleepy, tired, physically exhausted, stressed, and unhealthy during recovery compared to their 30-day counterparts.NEW AND NOTEWORTHY This study investigated the effects of prolonged head-down tilt bed rest with and without elevated (3.73 mmHg) levels of ambient CO₂ on cognitive performance across a range of cognitive domains and is one of the few studies investigating combined effects of environmental stressors prevalent in spaceflight. The study showed moderate declines in cognitive speed induced by head-down tilt bed rest and suggests that exposure to elevated levels of ambient CO₂ did not modify this effect.

Keywords: CO2; cognition; microgravity; performance; spaceflight.

Figure 1.

Cognitive speed, accuracy, and efficiency across cognitive domains relative to the 30-day head-down tilt bed rest (HDBR) period (gray background) for the CO₂ group (3.73 mmHg CO₂, n = 11, 6 men) and the control group (ambient CO₂, n = 8, 6 men). Estimates reflect unadjusted means ± standard error z-transformed based on baseline (pre-HDBR) performance within each of the 10 Cognition tests and then averaged across tests. To reflect the analytical approach (adjusting for baseline performance), means were shifted within groups to reflect a pre-HDBR baseline performance of 0. The dashed line between the last HDBR administration and the first recovery phase administration in the control group indicates that this group spent 30 additional days in HDBR relative to the CO₂ group. HDBR, head-down tilt bed rest.

Figure 2.

Change in cognitive performance in the head-down tilt bed rest (HDBR) period relative to pre-HDBR baseline. Estimates reflect z-scores based on the mean and standard deviation of pre-HDBR baseline performance. Error bars reflect unadjusted 95% confidence intervals. A: estimates for the control group (ambient CO₂, n = 8, 6 men) and the CO₂ group (3.73 mmHg CO₂, n = 11, 6 men); B: estimates for the difference between the CO₂ group and the control group; *adjusted P < 0.05; **adjusted P < 0.01; ***adjusted P < 0.001; ****adjusted P < 0.0001. ALL, scores averaged across cognitive domains; AM, abstract matching; BART, balloon analog risk test; DSST, digit symbol substitution test; ERT, emotion recognition test; F2B, fractal 2-back; HDBR, head-down tilt bed rest; LOT, line orientation test; MP, motor praxis; MRT, matrix reasoning test; PVT, psychomotor vigilance test; VOLT, visual object learning test.

Figure 3.

Change in survey responses during head-down tilt bed rest (HDBR) (A) and post-HDBR recovery (B) relative to pre-HDBR baseline for the CO₂ group (3.73 mmHg CO₂, n = 11, 6 men) and the control group (ambient CO₂, n = 8, 6 men). Estimates reflect points on an 11-point scale (variables are listed by anchors for high values). Error bars reflect unadjusted 95% confidence intervals. *adjusted P < 0.05; **adjusted P < 0.01; ***adjusted P < 0.001; ****adjusted P < 0.0001. HDBR, head-down tilt bed rest.

Figure 4.

Change in cognitive performance in post-head-down tilt bed rest (HDBR) recovery period relative to pre-HDBR baseline. Estimates reflect z-scores based on the mean and standard deviation of pre-HDBR baseline performance. Error bars reflect unadjusted 95% confidence intervals. A: estimates for the control group (ambient CO₂, n = 8, 6 men) and the CO₂ group (3.73 mmHg CO₂, n = 11, 6 men); B: estimates for the difference between the CO₂ group and the control group; **adjusted P < 0.01. ALL, scores averaged across cognitive domains; AM, abstract matching; BART, balloon analog risk test; DSST, digit symbol substitution test; ERT, emotion recognition test; F2B, fractal 2-back; HDBR, head-down tilt bed rest; LOT, line orientation test; MP, motor praxis; MRT, matrix reasoning test; PVT, psychomotor vigilance test; VOLT, visual object learning test.