Head-Down Tilt Body Positioning Impairs Brain Function and Cognitive Performance

There is limited but increasing evidence from both human and animal studies that exposure to microgravity may affect brain function and cognitive performance. Anecdotal reports and observations provide evidence of occasional disturbance in cognitive abilities during spaceflight [1,2] and effects on cortical activity have been reported, particularly in the lower frequency range [3,4,5]. We hypothesized that changes in cortical activation during micro gravity might also be related to baroreceptor control concomitant with fluid shifts to the upper extremities. Thus, in order to assess the impact of immediate head down tilt positioning, we simulated fluid shifts observed during parabolic flight using a tilt-table experiment. 24 (12 men and 12 women) healthy, young subjects were exposed to a supine position for 10 min (baseline), then tilted 9 degree head down (HDT) for 30 seconds after which they were immediately tilted to 45 degree head-up tilt (HUT). EEG was recorded at Cz during all phases (PowerLab 8/30, ADInstruments, CO, USA) and assessed with respect to individually determined alpha- and theta frequency bands. During each phase subjects were asked to perform a visual cognitive test. As indicated in Fig. 1 Theta-Power increased during HUT and decreased during HDT with a significant higher Theta-Power during HUT compared to HDT (P < 0.05), while Alpha-Power revealed a diametrically adverse effect. In addition, mean reaction time significantly decreased during HUT compared to HDT (P < 0.05). Low Alpha-Power and increased Theta-Power during cognitive performance has been associated with increased cortical activation. Thus, the reduced reaction times observed during HUT are in line with the changes in spectral power from HDT to HUT. These findings are very well in line with previous short-term HDT studies on cortical excitation [6,7,8]. While it cannot be excluded that these findings are related to the vestibular system and /or changes in cerebral perfusion, we suggest that the effects might also be explained by baroafferent feedback control as various studies have shown that intense baroreceptor stimulation can provoke a reduction in cortical excitability [9]. In conclusion, we found that HDT can impair cognitive performance and is associated with respective changes in neurophysiological correlates.