PhD Abstract:

Accidental hypoxia (i.e., the lack of oxygen) can have detrimental effects on the brain, leading to neurological pathologies such as stroke, neonatal hypoxia, high-altitude-related disorders, or sleep apnea. The hippocampal subfields, essential for episodic memory, are particularly vulnerable to hypoxia. However, freedivers (FD), who are regularly exposed to voluntary hypoxic episodes during their training, do not seem to present major episodic memory impairments. What neural markers underlie this apparent resilience in FD? Using multimodal magnetic resonance imaging (MRI), we investigated the memory, neuroanatomical, and neurofunctional evolution of a group of trained FD over a freediving season (7 months). Results were compared to those of a group of athletes engaged in aerobic exercise training. The study was structured along three axes: (1) COGNITIVE AXIS: the HippoPS pattern separation task, assessing the involvement of hippocampal subfields in episodic memory processes, showed stable memory performance in both groups (Micaux et al., 2025). (2) NEUROANATOMICAL AXIS: the HSF tool (Hippocampal Segmentation Factory©), dedicated to automatic segmentation in anatomical MRI, was used to examine the integrity of hippocampal subfield volumes. This analysis revealed comparable volumes between the two groups, suggesting anatomical integrity is maintained despite prolonged hypoxia. (Micaux et al., 2025). (3) NEUROFUNCTIONAL AXIS: resting-state fMRI, acquired both in normoxia and during apnea, showed the acute effects of hypoxia on brain connectivity and revealed a specific functional reorganization of hippocampal networks, including a decrease in connectivity with sensorimotor regions and an increase in connectivity with cerebellar and visual regions, particularly at the end of the training season. Additionally, positive correlations between hippocampal functional connectivity and memory performance suggest a compensatory mechanism relying on adaptive functional plasticity (Micaux et al., In Prep).

In conclusion, these findings reveal that repeated exposure to voluntary hypoxia, in the context of controlled exercise training in recreational sport, does not induce either neuroanatomical alteration of hippocampal subfields or significant memory deficits. This resilience appears to be underpinned by a functional brain reorganization involving hippocampal subfields that support the maintenance of episodic memory functions. This work positions freediving as a natural model to study the mechanisms of brain resilience to hypoxia, opening therapeutic perspectives for neurological pathologies occurring in such context.

Keywords: Freediving, Hypoxia, Brain Plasticity, Hippocampus, Episodic Memory, Neuroimaging, Anatomical Segmentation, Functional Connectivity