How linking odors to learning during sleep can help memories form
Scientists have understood for several years how connecting learning to specific odor cues can enhance memorization if the same smells are then presented during certain sleep phases. An impressive new trial from researchers at the University of Freiburg has now verified the method in a cohort of middle school students, and also found adding the odor cues to an exam scenario may further enhance successful recall.
A landmark 2007 study discovered learning can be improved if it is accompanied by odor cues that are replayed during a key sleep phase. The idea is based on the theory our brains shift short-term memories into long-term storage during the slow-wave phase of sleep. This process is often referred to as memory consolidation, and scientists have long explored ways to improve general learning by enhancing the process.
The 2007 study suggested if a certain smell was particularly tied to a learning moment, the memory consolidation of that moment could be better integrated if the same odor was presented specifically during the slow-wave sleep period that night. A subsequent study by the same research team fascinatingly discovered this odor cue seemed to work regardless of how pleasant or unpleasant a particular smell was.
Methods to improve memory consolidation during sleep have been dubbed targeted memory reactivation (TMR). Pragmatically, however, smell-based TMR in real-world conditions has been significantly understudied, with many researchers suggesting odor cues need to be specifically triggered during slow-wave sleep, and only slow-wave sleep, to be completely effective. Of course, this makes implementing the process in real-world scenarios practically impossible without complex EEG monitoring and timed odor releasing devices.
This new research set out to resolve several unanswered questions, including whether odor cue application in real-world scenarios works if applied for the whole night, and whether a third odor cue presented during subsequent memory retrieval improves results?
The trial recruited 54 German sixth grade students. The students were undergoing English vocabulary learning, and were directed to use rose-scented incense while studying for a test at home. The researchers broke the cohort down into several different groups, some using incense overnight to test memory consolidation enhancement, others using incense days later accompanying an exam, and a final group using the incense at all three points in the process – during learning, consolidation and retrieval.
"The students showed a significant increase in learning success by about 30 percent if the incense sticks were used during both the learning and sleeping phases," says Franziska Neumann, one of the authors on the new study.
This effect size is similar to prior studies investigating odor cues administered specifically during slow wave sleep. This suggests the method can be effectively deployed in real-world scenarios without trying to cue the odor trigger during a specific phase of sleep.
"We showed that the supportive effect of fragrances works very reliably in everyday life and can be used in a targeted way," explains lead on the study, Jürgen Kornmeier.
The use of odor cues during memory retrieval tasks delivered positive, albeit not statistically significant, results. The researchers note the particular sample size of this cohort was small, so, although there currently does seem to be extra benefit in odor cues presented at the time of memory retrieval, further study is necessary to really clarify whether combined cueing is effective.
Ultimately, this study suggests the simple incorporation of an odor cue to learning and sleep has the potential to slightly improve general memory performance. And most importantly, even though the slow-wave sleep phase is critical to memory consolidation, the timing of the fragrance during sleep is not significantly relevant.
"Our study shows that we can make learning during sleep easier,” concludes Kornmeier. “And who would have thought that our nose could help considerably in this.”
The new study was published in the journal Scientific Reports.