Sleep-dependent Enhancements of Memories in Early Childhood

• In Event: 3-188 - Sleep, Memory and the Brain During Development

Integrative Statement

Sleep supports memory consolidation in young adults (Diekelmann & Born, 2010). Studies in animals suggest that memories are replayed in the hippocampus during sleep and simultaneous activation (cortical slow waves) across the cortex supports hippocampal-neocortical stabilization of the memory (O’Neill, Pleydell-Bouverie, Dupret, & Csicsvari, 2010; Born & Wilhelm, 2012). Sleep in children likewise supports memory consolidation (Kurdziel, Duclos, & Spencer, 2012). We have found evidence of sleep-dependent memory consolidation in children as young as 3 years and others have provided evidence even from infants (Gomez, Bootzin, & Nadel, 2006). Specifically, in preschool-aged children, memories encoded in the morning are reduced when probed in the afternoon when children stay awake (wake condition). However, if a nap takes place during this time (nap condition), memories are unchanged.

It is possible that the nap provides only passive protection from interference that occurs when awake and, thus, naps may provide only an indirect benefit on memories. Alternatively, sleep may play an active role, such as through neural replay. To better understand whether daytime naps in children play and active or passive role in memory strengthening, we considered the change in memory over the wake and sleep intervals. Given that encoding took place between 10-10:30 a.m. and recall took place between 3:30-4 p.m. and the nap filled only the latter portion of this interval (1-3 p.m.), we expected that memories would decay during the initial waking portion of the interval and that the naps would recover memories from this decay (Figure 1). We tested this by providing an additional assessment (recall 1) of encoding strength prior to the nap and again after the nap (recall 2). Indeed, memories decayed between encoding and recall 1 but returned to encoding level at recall 2 following a nap. When children remained awake between recall 1 and recall 2, memories did not change significantly. This provides strong evidence that naps actively enhance memories, by recovering them from decay.

To consider the underlying mechanism supporting nap-dependent memory consolidation, we assessed sleep physiology using polysomnography (a montage of EEG, EMG, and EOG recordings used to measure sleep). If naps played a passive role in memory strengthening, we would expect a correlation between nap length and performance protection (longer nap = longer interval of memory protection). Such was not the case. Rather, we found that performance changes over sleep were associated with sleep spindles in non-REM sleep. This result was replicated with two declarative memory tasks, a visuo-spatial learning task and a story learning task.

Together, these data suggest an active role of sleep spindles memory consolidation in young children. Memory replay is marked by hippocampal ripples which co-occur with sleep spindles. Sleep spindles lie in the troughs of slow waves in the cortex. Thus, associations between memory changes over sleep and sleep spindles may reflect the active replay of memories in the hippocampus and co-activation in the cortex, thus supporting the recovery of weak memories and long-term memory stabilization.

Author

• Rebecca M. C. Spencer, University of Massachusetts Amherst
  Presenting Author