Unveiling the role of the hippocampus in procedural motor learning

The discovery in the 1950s of the central role of the human hippocampus in the formation of episodic memories established a conceptual dichotomy in cognitive neuroscience between declarative learning, dependent on the hippocampus, and motor learning dependent on the cerebellum and basal ganglia. However, recent data from human and non-human animals are challenging this traditional view. For example, recent studies carried out in patients with reversible hippocampal lesions have reported a deficit in the speed of learning a motor sequence with their fingers (motor sequence learning, MSL). In this talk, I will present functional and structural neuroimaging evidence from our team supporting a role of the hippocampus in motor memory encoding and early consolidation. I will show functional evidence indicating that this structure is more active early during MSL training, especially during the quiet rest periods interspersed with practice, and that this increment in activity predicts gains in performance. Furthermore, I will show evidence from diffusion MRI suggesting that motor sequence learning induces fast structural plasticity in the hippocampus as soon as 30 min post-training. The striking overlap between functional and structural results suggests that the regions of the hippocampus that activate during encoding undergo structural plasticity, a finding compatible with the modern definition of an engram. Finally, I will discuss data from two additional studies from our team showing that training in other procedural motor learning tasks requiring no sequencing and no explicit processing at all produce similar rapid changes in hippocampal microstructure. Our work points to the involvement of the hippocampus in the reactivation of procedural memories and provides evidence for a possible common mechanism at the basis of the formation of declarative and non-declarative memories. None