Long-Term Memory Engrams From Development to Adulthood

Memories formed in adulthood can last a lifetime, whereas those formed early in life are rapidly forgotten through a process known as infantile amnesia. In recent years, tremendous progress has been made in understanding the memory engram-the physical trace of a memory in the brain-and how it transforms as memories evolve from recent to remote. This review focuses on engram cells and examines their roles in memory encoding, consolidation, retrieval, and forgetting from development to adulthood. We concentrate on four key brain regions: the hippocampus, the retrosplenial cortex, the medial prefrontal cortex, and the anterior thalamic nuclei. We first focus on the adult brain, highlighting recent studies that reveal the distinct contributions of engram cells in each brain region, with particular emphasis on synaptic plasticity and memory consolidation. We then explore how coordinated activity across these regions supports long-term memory. In the second section, we review emerging knowledge of engram cells in the developing brain, examining how developmental differences in their functions contribute to infant memory generalization and infantile amnesia. Compared to adults, much less is known about how, or to what extent, early-life memories undergo consolidation. In the final section, we synthesize current knowledge of memory consolidation and retrieval in the adult brain with what is known about the development of the four brain regions we discuss. We then propose key directions for future research. In sum, this review brings together recent findings that deepen our understanding of the dynamic changes in memory engrams that underlie consolidation and long-term storage and explores how developmental differences may shape the maturation of memory processes.