Hippocampal and large-scale functional connectivity reorganization following freediving training, and relationships with episodic memory

The hippocampus is particularly vulnerable to hypoxia, yet freedivers, who repeatedly undergo voluntary breath-hold hypoxia, do not appear to show the memory impairments commonly reported after involuntary hypoxic exposure. We investigated whether freediving training is associated with functional connectivity reorganization at the whole-brain and hippocampal levels, and whether hippocampal connectivity relates to preserved episodic memory performance. Seventeen male recreational freedivers underwent fMRI before and after a 7-month standardized training period, and 20 matched aerobic-trained controls were assessed at baseline. Functional connectivity analyses examined (i) whole-brain ROI-to-ROI connectivity, (ii) seed-to-voxel connectivity from the bilateral hippocampus and selected cortical seeds, and (iii) associations between hippocampal connectivity and episodic memory. Compared with controls, freedivers showed distinct connectivity patterns during apnea and rest. After training, whole-brain analyses revealed reorganization involving salience, frontoparietal, sensorimotor, visual, and cerebellar systems. Hippocampal analyses showed reduced connectivity with sensorimotor and default mode regions, alongside increased connectivity with cerebellar and visual areas, with stronger effects after training. In freedivers, specific post-training hippocampal connectivity was associated with episodic memory performance. These findings suggest that repeated voluntary hypoxia during freediving training is associated with selective functional reorganization of hippocampal and large-scale brain networks. This pattern may reflect adaptive neuroplasticity linked to preserved episodic memory under intermittent hypoxic exposure.