Neuroimaging the sleeping brain: Insight on memory functioning in infants and toddlers

Abstract

Episodic memory, or the ability to remember past events with specific detail, is central to the human experience and is related to learning and adaptive functioning in a variety of domains. In typically developing children, episodic memory emerges during infancy and improves during early childhood and beyond. Developmental processes within the hippocampus are hypothesized to be primarily responsible for both the early emergence and persistence of episodic memory in late infancy and early childhood. However, these hypotheses are based on non-human models. In-vivo investigations in early human development of hippocampal processes have been significantly limited by methodological challenges in acquiring neuroimaging data, particularly task-related functional neuroimaging data, from infants and toddlers. Recent studies in adults have shown neural activity in the brain regions supporting episodic memory during slow-wave sleep using functional magnetic resonance imaging (fMRI), and fMRI has been increasingly utilized in infancy and early childhood to address other research questions. We review initial evidence and present preliminary data showing the promise of this approach for examining hippocampal contribution to how infants and toddlers remember individual events, and their association with information about the context in which the event occurred. Overall, our review, integrated with the presentation of some preliminary data provides insight on leveraging sleep to gain new perspectives on early memory functioning.

Keywords: Episodic memory; Functional magnetic resonance imaging; Infancy; Memory development; Sleep; Toddlers.

Figure 1.

Functional MRI analysis for recent memory. Group analyses were constrained to the left (LH) and right hippocampus (RH) using age-appropriate hippocampal templates and a cluster thresholding process (z = 2.30, cluster corrected, P < 0.025). (A) These analyses isolated active bilateral clusters for the memory (target + reversed) > novel contrast (left, max: x = −26, y = −33, z = 1; 1,839 voxels; right, max: x = 23, y = −27, z = 1; 1,107 voxels). (B and C) Mean parameter estimates (PE) were extracted for the T > N contrast. The relation between PE (y axis) for T > N in the RH and the number of correct items in our two behavioral measures (x axis), room choice (B) [r = 0.43, t(19) = 2.10, P = 0.049, d = 0.77] and the composite memory measure (C) (ρ = 0.60, P = 0.004) was significant. Adapted from “Memory-Related Hippocampal Activation in the Sleeping Toddler.” By J. Prabhakar, E. G. Johnson, C.W. Nordahl, and S. Ghetti, 2018, Proceedings of the National Academy of Sciences, 115(25), p. 6502. Copyright 2018 National Academy of Sciences. Adapted with permission.

Figure 2.

Preliminary functional clusters in the hippocampus for remote memory. Contrast comparison of previously heard song to a novel song resulted in two clusters in the right hippocampus [Z > 1.97, no cluster correction (Cluster 1, max: x = 18, y = −17, z = −9; 19 voxels) (Cluster 2, max: x = 18, y = −11, z = −13; 10 voxels)]. (A) Sagittal view, x = 18; (B) Coronal view, y = −11.