Investigating the Relationship Between Sleeping Brain Activity and Neural Differentiation of Competing Memories

Abstract

To accurately remember information from our past, we must not forget it and we must not confuse it with related information. In other words, we want our memories to be stable in our brain’s long-term storage and to be organized in such a way that we can access them without interference from competing memories. Memory models suggest that memory reactivation, which can occur during either wakefulness or sleep, is the key to stabilizing long-term memories and differentiating them from competing ones. In regards to sleep specifically, there is evidence that non-rapid eye movement (NREM) sleep spindle events and rapid eye movement (REM) sleep are linked to memory reactivation. Here, we used a pre-existing fMRI dataset to measure how much two memory representations move apart from one another, a measure called neural differentiation (study design by Dr. K Norman and Dr. E McDevitt; data collection by Dr. E McDevitt). We investigated if spindles and REM sleep, which occurred during an intervening period of sleep between neural measurements, were correlated with neural differentiation using EEG data and source localization techniques. We contrasted periods of spindle activity with equivalent periods of non-spindle activity to spatially localize brain areas with spindle-related activity during NREM sleep and found that neural differentiation was not significantly correlated with spindle activity. Next, we contrasted periods of phasic REM sleep (the segments of REM sleep containing rapid eye movements) with periods of tonic REM sleep to spatially localize REM-related brain activity. Neural differentiation was most strongly, though not significantly, correlated with REM activity localized to the superior frontal gyrus, though our REM pipeline needs to be re-evaluated. We also investigated if the relationship between NREM spindle activity and neural differentiation required a subsequent period of REM sleep. We examined the correlation of spindles and neural differentiation in two experimental nap groups, a NREM sleep only group and a NREM+REM sleep group. A number of small areas had a significantly higher correlation in the NREM+REM nap group compared to the NREM only group, but further analyses is needed to evaluate if these results are significantly different from chance. Finally, we discussed these results in the context of how understanding basic mechanisms of memory transformation during sleep can inform hypotheses about how sleep might be related to memory impairments in disease states (e.g., Alzheimer’s disease).