REM Sleep Facilitates Novelty Recognition of Visual Stimuli

Abstract

Research on memory encoding has long debated the intricate relationship between novelty and memory retention. The classical novelty/encoding hypothesis [1] suggests that the hippocampal activation elicited by novel stimuli is due to a memory encoding process instantiated by novelty detection. Therefore, memory performance for novel items should consequently be better than for familiar ones. Yet, this hypothesis encounters challenges, as familiar stimuli are sometimes better remembered than novel ones, and neural responses to novel stimuli diminish over time.

This discrepancy highlights a key aspect of the methodology employed in much of the research on novelty and learning—the distinction of complete and relative novelty. Complete novelty is attributed to a stimulus that the observer has never encountered before, while a stimulus with relative novelty may only be novel due to the unfamiliar combination of well‐known features. However, categorizing novelty in this binary manner may oversimplify the phenomenon, as several results show that repetition of novel stimuli results in the gradual decrease of the brain responses associated with novelty detection. 

In alignment with recent cognitive neuroscience findings [2], we propose conceptualizing novelty as a continuum which is theoretically crucial in that it allows us to study the dose–response effects of novelty on memory. Specifically, if the novel stimuli differ in the degree of theoretical novelty in a quantifiable manner, we can infer what degree of theoretical novelty elicits behavioural or neurophysiological phenomena suggestive of novelty detection. 

The present research employs a graded novelty encoding task [3] to manipulate novelty continuously. Using R Studio, stimuli are generated as colored geometric shapes arranged in a 3x3 matrix, with variations in three key components (layout, shapes, and colors). These different configurations are further converted into numerical values for data analysis.

The study includes a two-hour nap session with polysomnography recording, positioned between an incidental phase and a recognition task. This setup aims to investigate how spindle index and eye movement during the REM phasic stage correlate with the memory consolidation of familiar stimuli acquired during the study phase.

Our hypothesis posits that items categorized as more novel will exhibit better recognition in memory tests, with the memory-enhancing effects of novelty manifesting in a graded manner. Furthermore, we anticipate that REM sleep will enhance memory performance, particularly by reducing the recognition threshold between novel and familiar items, thereby facilitating better discrimination between original figures and distractors in subsequent recognition tests, even when the differences are subtle.

References

[1] Tulving, E., & Kroll, N. E. A. (1995). Novelty assessment in the brain and long-term memory encoding. Psychonomic Bulletin & Review, 2(3), 387–390. https://doi.org/10.3758/bf03210977

[2] Van Kesteren, M. T. R., Ruiter, D. J., Fernández, G., & Henson, R. N. (2012). How schema and novelty augment memory formation. Trends in Neurosciences, 35(4), 211–219. https://doi.org/10.1016/j.tins.2012.02.001

[3] Reichardt, R., Polner, B., & Simor, P. (2022). The graded novelty encoding task: Novelty gradually improves recognition of visual stimuli under incidental learning conditions. Behavior Research Methods, 55(4), 1587–1600. https://doi.org/10.3758/s13428-022-01891-8