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    • cerk An important fact for memory development

    2018-10-25

    An important fact for memory development is that some of the MTL circuitry has a protracted period of development, with the functions of the PRC developing early, CA1 volumes developing substantially over the first two years, albeit at different rates based on layer input origination, and CA3 and the DG volumes developing the latest both in human and primate development (Bachevalier, 2014; Lavenex and Banta Lavenex, 2013). These varied developmental trajectories led these authors to propose that maturation of these substructures should reflect the emergence of different memory processes in development. A challenge for this proposal is how to reconcile this protracted view of development with recent reports of early memory function in tasks known to elicit hippocampal processing in adults, such as relational binding of a face to a scene (Richmond and Nelson, 2009; Richmond et al., 2004; Chong et al., 2015), memory for spatial relations between objects in a display (Richmond et al., 2015), remembering temporal relations between events in a scene (Barr et al., 1996; Bauer et al., 2003), relational inference (Rovee-Collier and Giles, 2010), demonstrations of context effects (Richmond et al., 2004; Edgin et al., 2014), and better retention after sleep than after a similar period of wakefulness (Friedrich et al., 2015; Seehagen et al., 2015). Researchers have long noted early and late stages of memory development (Carver and Bauer, 2001; Jabés and Nelson, 2015; Mullally and Maguire, 2014; Nelson, 1995; Piaget, 1973; Schacter and Moscovitch, 1984) placing the emergence of the “late” stage at about 9 months in human children. However, this proposal is inconsistent with evidence on cerk development that exists in the literature that we also review (e.g., Bachevalier, 2014; Lavenex and Banta Lavenex, 2013). Our unique proposal is that 18–24 months of age reflects a major milestone in hippocampal development and its connections to cortex when circuitry among key hippocampal subfields and neocortical–hippocampal connections should be mature enough to support sleep neural replay. Before this time we propose that memory function is mostly supported by cortical structures characterized by an incremental learning profile with memories established through repeated exposure, inflexible representations and shallow retention profiles. In comparison, hippocampal memories are established rapidly in a couple exposures, objects and contexts are linked in memory but are also maintained separately, and retention profiles are robust, supported by neural replay during sleep. Consistent with proposals by Bachevalier (2014), Lavenex and Banta Lavenex (2013) and Olson and Newcombe (2014), it is only after basic circuitry is established among the subfields of the hippocampus that we should see more advanced hallmarks of memory function associated with relational binding, spatial relations, temporal order, and the binding of items in scenes. We focus here on episodic memory development supporting retrieval of memories of specific learning events that are functionally and anatomically separate from memories supported by procedural habit systems, such as memories formed using conjugate mobile reinforcement which are nondeclarative in nature, likely engaging the basal ganglia and cerebellum (see Bauer, 2007; Jabés and Nelson, 2015; Nelson, 1995; Schacter and Moscovitch, 1984 for similar arguments).
    Anatomical development of MTL Encompassing the amygdala and hippocampus, the MTL is surrounded by perirhinal and parahippocampal cortices, with entorhinal cortex connecting hippocampal and cortical structures (see Fig. 1). Critically, regions of the MTL and subfields of the hippocampus and their connectivity develop at different rates (Bachevalier, 2014; Jabés and Nelson, 2015; Lavenex and Banta Lavenex, 2013). Some patterns of local neural firing in the MTL develop early in rat models, with hippocampal CA1 place cells, which fire in response to an organism\'s position in the environment, emerging at postnatal day 16 (P16), and grid cells in entorhinal cortex developing at P20, substantially earlier than once thought (Wills et al., 2010). While glucose utilization and the number and density of synapses in most of the hippocampus are also adult-like by 6 months of age in humans (Seress and Ábrahám, 2008), the DG undergoes protracted development with rapid rates of neurogenesis at 8–16 months and achievement of adult like-morphology by 12–15 months (Bauer, 2007). Slow pruning of synapses to adult levels occurs after 4–5 years in DG (Bauer, 2007; Eckenhoff and Rakic, 1991). Myelination of hippocampus and its subfields also follows a protracted course (Arnold and Trojanowski, 1996), continuing to be modified into adolescence, with the DG showing the latest time frame to reach maturity (Ábrahám et al., 2010).