Functional and Structural Emergence of Memory Networks in Infant Brains

Abstract

The hippocampus is integral to memory formation, yet our understanding of its development in an infant's brain remains quite limited. Although infants can perform well in some memory tasks, they cannot actively create episodic memories in the hippocampus until the age of 3-4 years old, a phenomenon known as “infantile amnesia” (Miles, 1893). Understanding this phenomenon requires studying the growth in both structure and functional connectivity of the hippocampus to the rest of the cortex. To answer the question of how the hippocampus develops structurally and functionally, we divided the hippocampus of infants, aged 3 to 24 months, into eight equidistant bins as the hippocampus is known to serve differential functions along the anterior-posterior axis. We first used a ratio of T1-weighted (T1w) and T2-weighted (T2w) images to detect myelin content and found increased myelination with development, with an increased development rate in the posterior hippocampus. We then used each hippocampal bin in the right hemisphere to serve as a seed region whose average time course from resting-state functional magnetic resonance imaging (rsfMRI) was functionally correlated with all cortical voxels across ages. We correlated each infant’s functional connectivity map with those of 24-month old infants to see functional connectivity maturation. Not only did the connectivity pattern of the hippocampus to cortex develop, becoming more adult-like with age, but we also observed a higher rate of connectivity development in the posterior hippocampus, mirroring the myelin development data. Ultimately, we saw a higher rate of development in the posterior hippocampus after correlating the functional and structural development rates. Therefore, this study suggests that infantile amnesia likely results from underdeveloped functional connectivity of the hippocampus to the cortex, and that its development is likely constrained by myelination.