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 6⁰ head-down tilt bed rest (HDBR) with (N=11 participants, 30 days HDBR) and without (N=8 participants, 60 days 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.