Memory Specificity and Generalization through Neural Replay during Sleep


Research on how sleep contributes to memory has blossomed in recent years. These studies have generally focused on whether or not sleep impacts various types of memory independently. An open question is whether sleep interactively influences different memory types. My research focuses on two types of memory—specificity and generalization. Whereas we forget many memories encoded during our day, sleep can help counteract some of that forgetting and protect specific memories. However, we don’t know whether sleep improves generalization at the cost of specificity or vice versa. Based on prior work, I hypothesized that specificity and generalization exhibit a trade-off such that remembering detailed specifics blocks generalization. Additionally, I hypothesized that when generalization occurs, the specifics of memories are lost. In this dissertation, I describe two experiments designed to address this question. Chapter three is composed of two studies where I showed participants videos of outdoor scenes that fall into three categories. After learning, participants were tested on recognizing old or new videos (specificity) of the terrains as well as identifying to which of three highly-confusable categories the video belonged (generalization). In the first study of chapter three, participants had either a 12-hour delay that included sleep or was comprised of wake only. In the follow-up study we controlled circadian factors by having participants remain awake or nap for 90 minutes during the afternoon while we recorded brain activity. After the delay, they were once again tested on specificity and generalization. We did not find a significant difference between the sleep and wake groups in either of the experiments, but did find that forgetting of specific videos happened more rapidly than forgetting of categorization knowledge. Although these results did not provide evidence for the hypothesized specificity-generalization tradeoff, they did provide support for the prototype theory of memory as opposed to exemplar theory. In chapter four, I focused on a period of afternoon sleep after participants attempted to memorize a set of paintings created by six different artists. I used targeted memory reactivation (TMR) with sounds associated with each artist in an attempt to modulate memory processing during sleep. The goal was to test whether memory reactivation alters trade-offs between memory specificity and generalization. I hypothesized that TMR cues would improve memory for generalization over remembering specific paintings. While cuing did not alter participants’ performance on generalization, it did impact memory for specific, studied paintings. Participants were worse at identifying cued paintings compared to uncued paintings after the nap, possibly because memory reactivation during sleep facilitated storage of generic aspects of paintings at the expense of some details. These experiments provide evidence for several ways in which different neural mechanisms operative during sleep support memory for specifics and generalization. The results show that memory for specifics is forgotten at a faster rate than generalization, and that directed replay during sleep can impact that rate. These findings should guide future research, particularly with regard to developing experimental designs that can independently assess the two memory types. The experiments shed light on the varying forgetting rates across different memory types and provide a springboard for future research into mechanisms that govern memory consolidation during sleep.

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