Evidence of hippocampal learning in human infants

The hippocampus is essential for human memory. The protracted maturation
of memory capacities from infancy through early childhood is thus often
attributed to hippocampal immaturity. The hippocampus of human infants has
been characterized in terms of anatomy, but its function has never been
tested directly because of technical challenges. Here we use recently
developed methods for task-based fMRI in awake human infants to test
whether the infant hippocampus supports statistical learning. Hippocampal
activity increased with exposure to visual sequences of objects when the
temporal order contained regularities to be learned, compared to when the
order was random. Despite the hippocampus doubling in volume across
infancy, learning-related functional activity bore no relationship to age.
This suggests recruitment of the hippocampus for statistical learning at
the youngest ages in our sample, around three months. Within the
hippocampus, evidence of statistical learning was clearer in anterior than
posterior divisions. This is consistent with the theory that statistical
learning occurs in the monosynaptic pathway, which is more strongly
represented in the anterior hippocampus. The monosynaptic pathway develops
earlier than the trisynaptic pathway, linked to episodic memory,
suggesting that the infant hippocampus participates in statistical
learning before it can form durable memories. Beyond the hippocampus, the
medial prefrontal cortex showed evidence of statistical learning,
consistent with the adult role of this region in memory integration and
generalization. These results suggest that the hippocampus supports the
vital ability of infants to extract the structure of their environment
through experience.