br A view of late hippocampal development predictions
A view of late hippocampal development: predictions One prediction is that children relying on cortical networks should require many more exposures for learning and retain less. An excellent example comes from word learning. Bergelson and Swingley (2012) report that by 6–9 months of age children know the meanings of many common words, knowledge that must certainly emerge through massive exposure. If learning were mediated by rapid-learning trisynaptic circuitry we would expect 6 month olds to have much larger vocabularies and they do not. Consistent with the proposal that cortical learning requires many exposures compared to learning supported by trisynaptic circuitry (McClelland et al., 1995), 13 month olds in an experimental setting required 9 exposures to a single word-referent pairing for retention (Woodward et al., 1994). In PD 0325901 a well-documented increase in the rate of word learning occurs between 18 and 24 months in children (Goldfield and Reznick, 1990). This increase coincides with emerging trisynaptic connectivity and also reflects individual differences noted for the timing of the word spurt in different children. The hippocampus is highly vulnerable to insult (Lowenstein et al., 1992), often reflecting individual differences in learning, consistent with the wide range of individual variation noted for word learning in typical and atypical groups, including infants born prematurely (Foster-Cohen et al., 2007). A recent study reported a positive correlation between expressive vocabulary and hippocampal volume in typically-developing preschool age children (Lee et al., 2015). A second prediction is that sleep should contribute to greater accuracy in children with more mature trisynaptic circuitry. Cortical learning (before 18–24 months) should be incremental or should reflect less forgetting than in a wakefulness condition, consistent with our findings of greater generalization after sleep in infancy and greater fidelity of memory in toddlers (Gómez et al., 2006; Hupbach et al., 2009; Werchan and Gómez, 2014). Sleep findings in infants and young children are still scarce, but in distinguishing our discontinuous view of memory development from continuous theories (e.g., Rovee-Collier and Cuevas, 2009; Rovee-Collier and Giles, 2010), it will be important to test younger and older children in experiments with the same tasks before and after sleep. With more exposures for younger children, we predict encoding to the same performance levels across development. However sleep after learning should produce qualitatively more precise and more robust memorial outcomes in children 18–24 months and older aided by sleep neural replay. These patterns should be greatest for tasks tapping trisynaptic function such as those supporting retention of temporal order, allocentric spatial relations, pattern separation, and binding of information from different cortical pathways (e.g., objects of the ventral stream with contexts from the dorsal stream). A third prediction is that children with compromised hippocampal development and function such as premature children and children with Down syndrome (Pennington et al., 2003) should exhibit fairly robust cortical learning, but limitations in learning signatures of trisynaptic function involving more precise, single trial acquisition, episodic detail, and greater retention after sleep versus wake. Such children should need more exposures to reach criterion levels of immediate memory performance and they should retain less after a delay. These memory deficits are present in patients with developmental amnesia due to hippocampal damage (i.e., patient “Jon” who was born at 26 weeks gestational age; Gardiner et al., 2008). Further, mouse models of Down syndrome show a pattern of worse retention after a 24h delay for equally encoded information compared to wild-type mice (Smith et al., 2014). In our own work with children with Down syndrome of an age to have some maturity of hippocampal function (albeit compromised) we find larger vocabularies for children with better sleep and smaller vocabularies in children with obstructive sleep apnea and fragmented sleep (Breslin et al., 2014; Edgin et al., in press). A strong prediction that we are presently testing is that degree of developmental hippocampal compromise in these populations should predict the precision of memories and their retention functions over sleep and wake delays. Another prediction is that the extended trajectory of hippocampal development should relate to a later onset of memory difficulties in these populations. It has been noted that the hippocampal patient\'s difficulties are not always recognized until the school age years (Gadian et al., 2000), and other recent studies have suggested greater memory impairments measured in later vs. early development in Down syndrome (Roberts and Richmond, 2014).