Remembering After We Sleep Could be a Smell Away

Just smelling my mom’s homemade lasagna evokes very particular memories from my childhood – the way the kitchen looked, silly conversations with my family over dinner, an outfit that I used to wear. Because smells can so effectively help us remember, they are a powerful tool for scientists studying memory. In a new study, researchers found that exposing people who were sleeping to the same odor they smelled when learning a memory task improved their memory.

“We know from studies in rats that neural firing patterns of specific memories that were learned before sleep replay during subsequent sleep,” says Susanne Diekelmann of the University of Tübingen in Germany. “Odors represent an optimal way to manipulate such reactivations externally during sleep in the human brain.”

Diekelmann and colleagues designed an experiment to test whether it matters if the odor present during the learning phase is the same as the odor induced during sleep. They found that not only did the exposure to the same odor boost memory but also that the memory-enhancing effects created increased brain oscillation activity associated with memory during sleep. The study thus provides some of the first clues scientists have had in understanding how certain memories become activated during sleep.

Diekelmann talked with CNS about the design and results of the study, involving a memory task and EEG recordings, just published in the Journal of Cognitive Neuroscience.

CNS: What do we know about how smell influences memory?

Diekelmann: We know that smells become exceptionally well associated with particular memories. Everyone knows from personal experiences that specific smells can readily elicit the memory of certain persons or past events, like the scent of a perfume that reminds us of a lover or the smell of grandma’s apple pie reminiscent of our childhood days. This is probably so because the brain areas that are involved in olfactory processing are strongly connected with regions for learning and memory, such as the hippocampus.

CNS: How then does smell influence memory during sleep?

Diekelmann: Odors are particularly well-suited to study memory during sleep because, first, odors become strongly associated with learning experiences and, second, odors can be easily applied during sleep without inducing arousal responses or awakenings.

In our study, participants learned the locations of card pairs showing animals and everyday objects. This task resembled the classical game ‘concentration’. While participants learned the card pair locations, they smelled a specific odor, so that the odor became associated with the new memories. During subsequent sleep, we presented participants with the same odor again or with a different odor or with an odorless oil.

When participants received the same odor during sleep as during learning they remembered more of the learned card pair locations in the next morning. We believe that the odor that is connected to the learning experience reactivates the associated memories during sleep, thereby strengthening the underlying memory traces so that eventually participants show better recall for these memories.

CNS: Can you describe what the odors you used smelled like to the participants?

Diekelmann: We used two distinct odors. One of the odors smelled of lemons; the other odor smelled a little like detergent. Importantly, the type of the odor did not affect how well the memories became reactivated during sleep. The only important thing was that the same odor was presented during learning and during subsequent sleep. Generally, the lemon smell and the detergent smell were equally effective to reactivate and improve memory.

CNS: How did you deliver the odors?

Diekelmann: Participants wore a nasal mask that was attached to an odor delivery machine via tubes. The odor delivery machine started and stopped odor delivery during learning and during sleep according to a computerized program. To minimize habituation to the odor during sleep, the odor was presented for 30 seconds followed by 30 seconds break. This 30 second on/30 second off procedure was repeated for about one hour during slow wave sleep, the deepest sleep stage.

CNS: How much better did people remember something when exposed to the same odor during sleep as during the memory task?

Diekelmann: Participants remembered about 15% more of the learned card pair locations when they received the same odor during sleep and learning compared to the different odor and the odorless vehicle. This improvement corresponded to about 1.5 more card pairs.

CNS: What was the significance of the EEG data you recorded from the participants?

Diekelmann: We already know that there are specific brain oscillations during sleep that are involved in memory processing, such as slow oscillations, delta activity, and fast spindles. These oscillations presumably coordinate the reactivation of memories in the hippocampus, as well as their redistribution to neocortical brain areas for long-term storage. In our new study, we found that the learning-associated odor presented during sleep increased activity in the delta and the fast spindle frequencies and resulted in steeper slow oscillations. The steeper slow oscillations during odor application predicted subsequent memory performance. These changes might indicate that the learning-associated odor influences memory reactivation, as well as the redistribution of memories from the hippocampus to long-term storage.

CNS: Why is it important to understand the link between odor and memory?

Diekelmann: What is actually important for us is to understand the role of sleep for memory. We use odors as a means to externally manipulate the processing of memories during sleep. We know that by this means we can enhance memory storage and memory strengthening. This knowledge might eventually provide new techniques to improve our memory capacities and to restore memory functions in particular disorders such as Schizophrenia and Alzheimer’s disease. Other applications are conceivable to improve therapeutic effectiveness, for example in the treatment of anxiety disorders.

CNS: What do you next want to explore in this connection?

Diekelmann: One important direction for future research will be to specify the exact neurophysiological mechanisms underlying the effect of odor on memory reactivation during sleep. … We also want to determine whether all memories benefit equally from odor reactivation or whether this procedure is most effective for specific types of memories and whether there are certain boundary conditions for the memory benefit to express.

-Lisa M.P. Munoz

The paper, “Reactivating Memories during Sleep by Odors: Odor Specificity and Associated Changes in Sleep Oscillations” by Julia S. Rihm, Susanne Diekelmann, Jan Born, Björn Rasch, was published in the Journal of Cognitive Neuroscience online on January 23, 2014.