Thanks to COVID-19, we are all aware of how important our sense of smell is in our daily lives and how we may have been taking it for granted for a long time. However, our olfactory system is also connected to many other aspects of our body and mind, such as our memory, sleep, behavior, and much more. In the review paper, “Scents and Reminiscence: Olfactory Influences on Memory Consolidation in the Sleeping Human Brain,” Laura K. Shanahan and Jay A. Gottfried discuss recent studies on odor and memory consolidation which use olfactory targeted memory reactivation (TMR), a method of manipulating memories through sensory cues–in this case, odors. In one of the reviewed studies, Rasch et al. found that odor presentation during encoding and again during slow-wave sleep has a positive impact on memory consolidation, resulting in better performance on declarative memory tasks. Many similarities to this experiment are seen in the results of Dr. Shanahan’s own research on olfactory flexibility. Previously, Dr. Shanahan conducted an experiment to demonstrate that delivering odors during sleep influences human brain activity as well as behavior. Through the use of EEG, fMRI, and an olfactometer to deliver the odor to participants, Dr. Shanahan found evidence to support Rasch et al.’s conclusions. She also discovered significant activity in the ventromedial PFC, known to play a role in memory consolidation. These findings were critical in proving that our sense of smell can influence our behavior, and understanding which neural regions might be involved in this process.
In her more recent work, Dr. Shanahan sought out to determine if our behavior influences odor perception (the opposite of the above phenomenon). She hypothesized that satiety modulates our decision-making about certain odors. In this experiment, Dr. Shanahan made mixtures of non-food odors and food odors, and asked hungry participants to complete a perceptual task to determine which odor (non-food or food) was more prominent in an odor mixture. After, participants received a large meal related to that odor, and then were instructed to participate in the perceptual task once more. The findings illustrated that participants are much less likely to detect odors as food dominant after they had an odor-matched meal than before, suggesting satiety does indeed affect our behavior in terms of making choices about odor perception.
Similar behavioral results can be seen in a different study conducted by Hoenen et al., discussed in the research paper, “The Impact of Stress on Odor Perception.” Like Dr. Shanahan, these researchers also explored the relationship between odor perception and behavior, but focused specifically on the effect of emotional and physiological stress responses. In an experiment using a version of a specific stress test (the Trier Social Stress Test), Hoenen et al. assessed whether these responses would impact male individuals’ performance in odor identification. Along with recording measures of the participants’ mood in terms of arousal, anger, and anxiety to determine behavioral effects, the researchers also measured cortisol levels through saliva in order to compare the physiological changes. From their results, Hoenen et al. first found increased cortisol levels to be associated with better odor identification (i.e. rating odors higher in intensity). This suggests that physiological stress responses such as the activation of the hypothalamic-pituitary-adrenal (HPA) axis, does indeed influence odor perception. Hoenen et al. note that this aligns with previous research showing a relationship between anxious personality traits and enhanced odor perception. Secondly, they identified that anger was associated with worse odor identification. These interesting divergent results suggest that emotional and physiological responses influence our olfactory system in different ways.
Several similarities can be seen in the work of Dr. Shanahan and Hoenen et al. Both Dr. Shanahan’s current research and Hoenen et al.’s study rely on the idea that how we perceive our surroundings is dependent on our current biological state, or motivated perception. This may seem like an obvious fact in some aspects of our lives; we may be more aware of the beauty of nature in a park when we’re already in a cheerful mood. Clearly, this idea does not just apply to the olfactory modality. However, olfactory perception has been shown to both influence and be influenced by many factors. In this case, these two researchers–Dr. Shanahan and Hoenen–have both demonstrated how odor perception can be modulated by an individual’s biological state. Dr. Shanahan’s work revealed that satiety impacts a person’s ability to detect food-related odors, while Hoenen et al. illustrate that physiological and emotional stress responses affects perception of odor intensity. Besides providing us with the interesting finding that our olfactory system is heavily intertwined with many aspects of our behavior (such as eating or our responses to stress), this research may also have clinical and therapeutic implications. With knowledge of the relationship between behavior and odor perception, we may be better able to detect disorders like anxiety, PTSD, or those related to aggression.
Sources:
Hoenen, M, Wolf, O.T., Pause, B.M. (2017). The Impact of Stress on Odor Perception. Perception, 46(3–4),366–376. https://doi.org/10.1177%2F0301006616688707
Shanahan, L.K., Gottfried, J.A. (2017). Scents and Reminiscence: Olfactory Influences on Memory Consolidation in the Sleeping Human Brain. Cognitive Neuroscience of Memory Consolidation,335–346. 10.1007/978-3-319-45066-7_20
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