The human hippocampus is involved in implicit motor learning
Recent evidence suggests that the human hippocampus, traditionally associated with declarative memory, plays a role in motor sequence learning (MSL). However, the classic MSL paradigm depends initially on declarative learning. Therefore, it is critical to discern whether the participation of the hippocampus relates to its canonical role or to processing a general aspect of learning that transcends the declarative/non-declarative distinction. To address this issue, here we turn to visuomotor adaptation -a type of motor learning involving skill recalibration-which unlike MSL can be easily manipulated to eliminate the explicit component. Here, we examined the broader involvement of the hippocampus in procedural motor learning by using diffusion MRI to indirectly assess structural plasticity associated with memory consolidation in visuomotor adaptation (VMA) and an implicit-only version (IVMA). We found that both VMA and IVMA engaged the left posterior hippocampus in a learning-specific manner. Remarkably, while VMA induced only transient hippocampal alterations, IVMA elicited structural changes that persisted overnight, underscoring the reliance on implicit learning for enduring neuroplasticity. As expected, training on both visuomotor tasks impacted the microstructure of the cerebellum, the motor and the posterior parietal cortex. Notably, the temporal dynamics of changes in these regions closely paralleled those of the left hippocampus, suggesting that motor and limbic regions operate in a coordinated manner as part of the same neural network. Collectively, our findings support an active role of the hippocampus in procedural motor memory and argue for a unified function in memory encoding regardless of the declarative or non-declarative nature of the task.