The ability for a certain smell or sound to seemingly transport you back in time to a specific time, place, and even emotional state, is undoubtedly a testament to the existence of a remarkable relationship between sensory information and memory formation. As discussed by researchers Laura Shanahan and Jay Gottfried, there is substantial evidence in favor of the existence of a strong connection between sensory input, such as smell and sound, and the consolidation of memories. Furthermore, the stabilization of these memory networks can be accomplished not only in conscious individuals but during certain stages of sleep. The sleep stage that has been associated with optimal memory consolidation is slow-wave sleep, and through their review of several studies with sleep and memory consolidation in focus, Shanahan and Gottfried create a compelling argument concerning the role of stimuli exposure during this critical period, and its resulting increase in memory recall. This re-exposure to stimuli is known as targeted memory reactivation (TMR), and recent studies have emphasized its application during periods of slow-wave sleep (SWS), thus implying a significant relationship between SWS sleep and the reinforcement of memories. The evidence surrounding the application of TMR in adolescents is convincing, however, establishes avenues to explore its pertinence in other age groups, particularly in populations with impaired memory function.
There is an astounding amount of research that has sought to uncover the mechanisms underlying memory decline in elderly individuals. With a focus on addressing whether or not there is an application of TMR in aiding memory recall in this population, researchers M.J Cordi, T. Schreiner, and B. Rasch conducted a study testing auditory TMR in 26 healthy individuals between the ages of 62 and 83. In their study “Reactivating Vocabularies in the Elderly”, Cordi et al. utilized vocabulary pairing tests in which the German-speaking participants learned 60 Dutch word pairs before resting. They then exposed the participants to the words at a hearing-threshold tested volume once they were observed to be in non-REM sleep in N2 or N3 stages. After 90 minutes of reactivation and a minimum of 3 hours of sleep, the participants were awakened and asked to recall words learned in the initial pre-sleep session. The researchers employed EMG, ECG, EOG, EEG electrodes, and event-related potential analysis to detect sleep state, wakefulness, and relative spindle, theta, and slow-wave activity.
Contrary to the results concluded in TMR studies with adolescent participants, there was no widespread evidence of significantly improved recall as a result of the auditory TMR implemented in this study. However, there were improvements to be noted on a more individual-based level. They were able to divide their participant group into 2 groups- improvers and non-improvers. The improver group showed a trend for higher sleep efficiency, faster falling-asleep times, and more time spent in NREM sleep and slow-wave sleep, however, these results did not reach significance. Overall, the oscillatory patterns between improver-individuals and non-improver-individuals were different, with improver time-frequency results that resembled those observed in younger participants, but the sum of the findings of this research did not indicate a benefit of TMR in memory fortification in elderly populations. While the null results of this study can initially seem discouraging, they illuminate possible mechanisms that may underlie memory loss in older-aged populations. Cordi et al. explain that based on findings of previous studies, they are able to conclude that the role of sleep in memory consolidation is rather dynamic, as it changes with time on an inter-individual level. Their findings also pose a possible avenue of research with a possible decline in the effectiveness of the memory reactivation mechanism as a focus. Furthermore, Cordi et al. state that there is a known reduction in SWS as aging progresses, thus providing another area of interest for future studies.
In all, the application of TMR in improving memory recall remains an encouraging technique in adolescent populations, as explained by Shanahan and Gottfried. Unfortunately, the use of TMR in attempts to increase memory recall in aged individuals does not hold the same promise. As research continues to progress with hopes of identifying fundamental causes of memory loss in aged individuals, the two studies mentioned in this paper highlight possible routes of further investigation within themselves. It proves to be a consistently pertinent avenue of experimentation, and one that continues to build upon itself even with the aid of null results.
References:
Cordi, M.J., et al. “Reactivating Vocabularies in the Elderly.” BioRxiv, Cold Spring Harbor Laboratory, 1 Jan. 2017, www.biorxiv.org/content/10.1101/216283v1.abstract.
Shanahan, Laura K., and Jay A. Gottfried. “Scents and Reminiscence: Olfactory Influences on Memory Consolidation in the Sleeping Human Brain.” Cognitive Neuroscience of Memory Consolidation, 2017, pp. 335–346., doi:10.1007/978-3-319-45066-7_20.
Fascinating! Great work Ms. Dean.
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