Episodic memories for autobiographical events that happen in unique spatiotemporal contexts are central to defining who we are. Yet, prior to about two years of age, children are unable to form or store episodic memories for recall later in life, a phenomenon known as infantile amnesia. The three to five years that follow this period are characterized by fewer episodic memories than would be predicted based on a simple forgetting function alone, a phenomenon referred to as childhood amnesia. To date, the neurobiological bases for these phenomena have remained highly hypothetical. In contrast, research conducted over the last 30 years has established that multiple memory systems are subserved by different brain circuits in adult individuals. In particular, declarative memory, which includes episodic memory and semantic memory (i.e., factual knowledge about the world), is dependent on the integrity and function of the hippocampal formation in adult humans, whereas non-declarative memories are not.
Allocentric spatial memory, the memory for locations coded in a relational manner to the surrounding environment, is a fundamental component of episodic memory, the "where" component of the defining "what, where and when" of episodic memories. Like semantic and episodic memory, allocentric spatial memory is also dependent on the integrity and function of the hippocampus in adult individuals, whereas egocentric spatial memory, the memory for locations coded in relation to the body such as "on my left", "on my right" or "in front of me", is not. However, despite our ever-increasing comprehension of the role that the hippocampal formation serves in learning and memory in adult individuals, our understanding of how different memory systems develop and why different types of hippocampus-dependent memory emerge when they do in early childhood is much less complete. In our laboratory, we study the development of allocentric spatial memory, a fundamental component of episodic memory, in children from 18 months to five years of age in order to shed light on infantile and childhood amnesia and the development of hippocampus-dependent memory processes in children.
In a series of three seminal studies, we have shown that (1) allocentric spatial memory emerges in children around 24 months of age, but continues to mature over the next 3 years ; and (2) improvements in the abilities of 2-to-5-year-old children to spatially resolve closely-apposed locations, and to temporally resolve individual events, underlie their improvements in allocentric spatial memory.
Our behavioral studies, in combination with results on the neuroanatomical development of the monkey hippocampal formation, support the hypothesis that the differential maturation of distinct hippocampal circuits underlies the differential emergence of specific "hippocampus-dependent" memory processes, culminating in the emergence of episodic memory concomitant with the maturation of all hippocampal circuits.
