Effects of Parabolic Flight on Spatial Cognition and Hippocampal Plasticity

The main objective of this experiment is to assess the impact of parabolic flight on (1) spatial memory and orientation, (2) the changes in brain structure and function, and particularly the hippocampal formation, and (3) biochemical markers of stress and neuroplasticity. A preliminary study will be performed with the aims (1) to test during weightlessness hardware used to administer visuo-spatial testing and (2) to determine the optimal body posture for testing under microgravity conditions (i.e. body posture for which change in visuo-spatial testing induced by weightlessness is maximal). The main criteria are: * Structural and functional changes of brain evaluated by fMRI; * Cognitive performance, and specifically spatial cognition using a series computer-based tasks. * Key neurotrophins determined in venous blood samples The criteria used in the preliminary experiment is visuo-spatial performance evaluated with a computer-based test assessing spatial cognition. The investigators hypothesize that hippocampal structure and function, and particularly the dentate gyrus, will be altered after parabolic flight compared to ground based controls. Specifically, the 3 tested hypotheses are: * Parabolic flight induces changes in hippocampal structure; * Weightlessness induces changes in cognitive performance, and specifically spatial cognition; * There is a correlation between changes in hippocampal structure and changes in visuo-spatial memory performance. * There is a correlation between changes in hippocampal structure and changes in key neurotrophins. The hypothesis tested during the preliminary experiment is that body posture affects the changes in spatial cognition performance.