Why are odors important for memory formation

In the study by Matsunaga et al. Correspondingly, the authors surmised that the positive emotions elicited by the odor-evoked memories were responsible for these positive physiological responses.

Masaoka, Sugiyama, Katayama, Kashiwagi and Homma [ 35 ] further showed that when men and women ranging in age from 29 to 50 years were presented with a self-selected odor that evoked an autobiographical memory odor and two control fragrances, a pleasant odor rose and a neutral odor chamomile that did not elicit personally meaningful recollections, the autobiographical memory odor promoted deeper, slower and more relaxed breathing compared to odors that did not evoke a memory.

Slow, deep breathing is associated with relaxation and may stimulate whole-brain synchronization as it is seen in slow-wave sleep and meditation [ 49 ], both of which have neurological benefits. Masaoka et al. Importantly, the evocativeness of the autobiographical memories triggered by odors and depth of breath were found to be greatest among individuals who scored high in trait anxiety, implying that the stress reducing benefits of odor-evoked memories may be strongest for those with the most need for it.

Chronic stress is correlated with a host of negative psychological and physiological consequences ranging from depression and memory-loss to stroke and cancer for a few reviews see [ 50 , 51 , 52 ]. A positive and calm mood is an antidote to stress, though when one is in a state of stress, pleasant moods are very hard to experience and maintain.

Nevertheless, many people intuitively know that odors have the potential to elicit comfort. Following from the intuitive behavior of comfort smelling, the deliberate activation of comforting odor-evoked-memories by smelling an article of clothing, a fragrance, or any specific odor that for a given individual evokes soothing feelings of relaxation, may reduce stress. Using odor-evoked memory to reduce stress can minimize the negative consequences that often accompany self-medicating methods such as excessive drug, alcohol and food intake, and may even be helpful in reducing cravings for various substances of abuse.

Sayette and Parrot [ 38 ] found that smelling odors decreased the urge for cigarettes among chronic smokers, and odors that elicited autobiographical memories trended towards being most effective.

Several studies have now also reported that, at least in laboratory settings, odors can disrupt cravings for highly desirable calorie dense foods [ 56 , 57 ]. These studies on food craving did not assess whether the odors involved evoked personal memories. Nevertheless, it is likely that the mechanisms through which odors reduced food cravings were memory based.

For example, aromas connected to rich and filling foods could have elicited feelings of satiation, other odors may have triggered reminders of dieting intentions, and memories triggered by odors may have distracted individuals from their immediate cravings and thus disrupted capitulating to these urges [ 55 ].

These possibilities now need to be addressed in further research. Neuroimaging studies investigating brain-immune interactions have revealed that prefrontal regions, such as the ventromedial prefrontal cortex vmPFC and the orbitofrontal cortex OFC , regulate peripheral immune activities such as the proportion of natural killer cells among peripheral circulating lymphocytes [ 58 , 59 ].

Since the OFC is the secondary olfactory cortex it follows that odors may be able to exert a direct influence on immune responses. Recent research that has examined physiological parameters associated with odor-evoked memory has found that odor-evoked memories may decrease systemic inflammation. IL-2 is normally produced by T-lymphocytes during an immune response. A decrease in peripheral IL-2 therefore implies an inhibition of inflammation.

As such, Matsunaga et al. To further investigate the relationship between odor-evoked memory and brain-immune interactions, Matsunaga and colleagues [ 37 ] conducted an experiment where healthy men and women underwent positron emission tomography PET while they were exposed to a perfume that they had selected as eliciting an autobiographical memory and a pleasant unfamiliar perfume that did not have any personal significance for them control fragrance and plasma concentration of several inflammatory cytokines were measured.

Moreover, recent studies in psychoneuroimmunology have shown that emotional experiences can modulate the secretion of proinflammatory cytokines from immune cells, and that psychological stressors, such as anxiety, can promote secretion of proinflammatory cytokines [ 61 ]. It has also been demonstrated that happy feelings can suppress the secretion of proinflammatory cytokines and that they have favorable effects on health and wellbeing [ 62 ].

Excessive inflammation has been implicated as the precursor to nearly every deleterious physical condition [ 63 ]. If odor-evoked-memories have the ability to reduce inflammation, even if the effects are minimal, it would be beneficial.

A caveat to over-reaching on this conclusion, however, is that blood tests in the Matsunaga experiments [ 36 , 37 ] were taken within two minutes of odor exposure so it is not known how long the positive effects from odor-evoked memory on immune function would last.

Cautiously, one might posit that as long as a positive emotional state were maintained from an odor-evoked memory a reduction in inflammatory cytokines may persist. Conversely, based on the extant research on stress and inflammation and studies demonstrating the link between olfaction and brain-immune interactions, odor-evoked memories that elicit substantial distress, such as which occur in PTSD, may have the potential to increase inflammation, though this has not yet been empirically assessed.

Olfactory adaptation and habituation are perceptual factors that are important to consider when odors are frequently experienced. When an odor is continuously or repeatedly smelled it loses its ability to be perceived. After constant exposure to an odor for more than 20 min sensitivity is greatly reduced and when one is exposed to the same odor on a daily basis olfactory sensitivity to that odor dramatically diminishes [ 64 , 65 ].

Therefore, limited exposure to odors that elicit autobiographical memories and use of multiple odors with this capacity is advised when using odors that evoke memories for therapeutic purposes. There is also some evidence that odors that stimulate the trigeminal nerve e.

Therefore, specific odorant qualities may also be important to consider in treatment applications. Another important issue is that olfactory cognition is unlike cognition mediated through our other senses in that it is extremely resistant to retroactive interference [ 67 , 68 ]. That is, the first association formed to an odor typically remains tied to that odor despite multiple future experiences of the same odor in different contexts.

This is an advantage in that the same odor can be used to repeatedly recapture a specific past event—so long as issues of odor habituation have been mitigated—but also means that negative odor-evoked memories, such as in PTSD, are extremely hard to extinguish. In order for an odor to be therapeutically effective it must be both associated to, and perceived as, possessing specific positive emotional qualities. That is, the scent of lavender may induce positive feelings of relaxation [ 69 , 70 ], but only if lavender scent is known to be and associated to pleasant calming emotions for a given individual.

We have recently shown that olfactory sensitivity fluctuates with circadian phase and is most acute in the late afternoon and worst in the early morning [ 72 ]. This suggests that the time of day when odors are presented in therapeutic applications would also be important to consider. The occurrence of personally meaningful odor-evoked memories is relatively rare.

Therefore, even if a scent is generally denoted as positive, the odor-evoked memory that a given individual experiences to it may not be. For instance, the scent of a popular cologne though evocative of romance for many, can trigger PTSD flashbacks for a rape victim [ 34 ]. Thus, in addition to idiosyncratic experience, the predictive emotional effects of a certain odor evoking a memory are limited to the culture s in which the acquired associations fit the expected responses [ 47 ].

It is also now known that each of us, unless we are an identical twin, possess a unique complement of functional olfactory receptors [ 74 , 75 , 76 ]. This means that there are subtle differences in how we all perceive odors. The olfactory receptors that we possess reflect which genes are expressed as functional receptors and how many copies of a specific receptor a person may have.

For example, people who despise the aroma of cilantro are missing the gene for detecting the herbal floral quality of this aroma and therefore only detect the soapy note [ 77 ].

Additionally, the more copies of a specific olfactory receptor one has the more sensitive one will be to certain odorants. Moreover, the stronger an odor is perceived to be the more likely it is to be unpleasant, as regardless of the sensory system, high intensity stimuli are perceived as more unpleasant than the same stimuli at moderate intensity [ 78 ].

These issues imply that past experience and genetic variability need to be evaluated before adopting an odor-evoked memory regimen for a given individual.

Women are generally more sensitive to odors than men are [ 79 ], and this enhanced sensitivity may influence the degree to which odor-evoked memories elicit physiological and emotional responses. Women have been shown to be more emotionally reactive to odors [ 80 ], and more susceptible to emotional conditioning with odors [ 81 , 82 , 83 ]. Lehrner, Eckersberger, Walla, Potsch and Deecke [ 84 ] also found that women, but not men, experienced less anxiety and a calmer and more positive mood when they were exposed to an orange aroma while awaiting an anxiety provoking event.

However, in an extended replication of Lehrner et al. It should be noted that many studies examining odor-evoked memory have involved a preponderance of female participants e. In descriptive studies that directly compared odor-evoked memories experienced by men and women it was observed that women described more emotional memories than men did [ 85 , 86 ].

However, other odor-evoked memory investigations involving experimental paradigms have not reported gender differences [ 31 , 87 ]. Gender also interacts with age with respect to how odors elicit memories. However, with participants between the ages of 65—70 years, the effectiveness of odors to elicit autobiographical memories was equivalent between the genders—and, notably, the number of autobiographical memories generated to odors increased with age.

The qualitative nature of odor-evoked memories as a function of age or gender was not examined in this study. In any event, generalizing the psychological and physiological results from odor-evoked memory research equivalently between men and women should to some extent be tempered, and the effects may also be mediated by personality factors.

Personality can modulate the degree to which odors elicit emotional states. Devriese and colleagues [ 89 ] reported that neurotic individuals a moody temperament, and a tendency towards anxiety and negative affect were more likely to generalize acquired somatic symptoms, such as hyperventilation, in response to odors.

There is also some evidence that emotionally labile individuals have greater absolute sensitivity to some odors [ 87 ]. Chen and Dalton [ 80 ] further found that women high in trait anxiety perceived hedonically polarized odors odors rated as distinctly pleasant or unpleasant more intensely than a neutral odor, and that men who were high in neuroticism or anxiety detected hedonically polarized odors faster than a neutral odor.

In the specific realm of odor-evoked memory, high trait anxiety subjects were shown to feel more brought back in time, experienced higher arousal, and took slower, deeper breaths during the experience of odor-evoked memories than participants low in trait anxiety [ 35 ].

The general personality predisposition towards nostalgic reverie has also been found to correlate with experiencing more odor-evoked nostalgia [ 32 ]. Mood improvement is desirable at numerous levels and this review has shown that the positive emotional states elicited by odor-evoked memories can ameliorate psychological and physiological responding and lower stress [ 35 , 36 , 43 ]. Moreover, through psychoneuroimmune interactions the emotionally positive qualities of odor-evoked memories appear to be able to reduce inflammation [ 36 , 37 ].

Inflammation is the basis of the majority of disease states and premature death [ 63 ]. Thus harnessing the power of positive odor-evoked memories has appreciable implications for benefiting health. Any odor that for a given individual evokes a happy autobiographical memory has the potential to increase positive emotions, decrease negative moods, disrupt cravings, lower stress and decrease inflammatory immune responses, and thereby have a generally beneficial effect on psychological and physiological wellbeing.

Odor-evoked memories may also be able to stimulate specific emotions, such as self-confidence, motivation and vigor, and thus energize behavior as a function of the specific emotions that a given odor-evoked memory evokes. For example, an odor that triggers the memory of winning an important race could inspire all of these states and trigger positive physiological consequences.

Moreover, the special resistance to retroactive interference that is fundamental to olfactory cognition can make a specific odor linked to a meaningful past personal event extremely reliable as a therapeutic agent. The potential therapeutic value of odor-evoked memory is not a testament to aromatherapy—the general proposition that various plant-based aromas have the ability to influence mood and wellness—which has mostly unsuccessfully waded into the therapeutic domain due to a lack of scientific rigor and confusion regarding the mechanisms involved [ 90 ].

Rather, when odors are capable of eliciting emotional and physical changes it is due to the emotions, memories and associations that have been linked to an odor through past personal experiences, which are then elicited when the odor is encountered, and the psychological and physiological responses connected to the odor are recapitulated [ 45 , 47 ].

It is in this way that odor-evoked memories have been empirically verified to alter emotional, mental and physical states and thus how they may be used in therapeutic applications.

A methodologically sound understanding of the mechanisms that elicit beneficial responses from odor-evoked memories makes the implementation of odor-evoked memories a plausible adjunctive treatment for numerous conditions.

For example, in addition to manipulations involving explicit odor-evoked memories, olfactory associations can also be used to implicitly alter behavior in a healthy manner, as Gaillet and colleagues have shown with food choices [ 91 ]. Future research should now use this empirical foundation to explore the precise parameters involved. Additionally, further research is needed to systematically investigate how odor-evoked autobiographical memories may be used therapeutically in conditions ranging from depression to addiction to systemic inflammation, both in the immediate case and in the long term.

This work will also need to take into account issues of odor exposure, such as adaptation, habituation, trigeminal activation, and time of day, as well as a number of individual characteristics such as idiosyncratic experience, gender, age, genetics and personality. The uniquely emotional nature of odor-evoked memories brings our past back to us more viscerally and transportatively than any other type of memory experience. This exceptional feature of olfaction is witnessed most poignantly in the tragic case where a person suddenly loses their sense of smell in an accident and along with it their connection to themselves and others, their emotional wellbeing, and their overall quality of life [ 48 , 92 , 93 ].

From numerous perspectives it is evident that the autobiographical memories and emotional associations that are triggered by odors are essential to our psychological and physiological health.

National Center for Biotechnology Information , U. Journal List Brain Sci v. Brain Sci. Published online Jul Rachel S. Elizabeth Race, Academic Editor. Author information Article notes Copyright and License information Disclaimer. Received May 22; Accepted Jul This article has been cited by other articles in PMC. Abstract This article discusses the special features of odor-evoked memory and the current state-of-the-art in odor-evoked memory research to show how these unique experiences may be able to influence and benefit psychological and physiological health.

Keywords: odor, emotion, psychology, autobiographical-memory, immune-response, stress physiology, aromatherapy, gender, personality. Introduction Odor memory is a central feature of olfactory cognition, and can be divided into two distinct cognitive-perceptual processes [ 1 , 2 , 3 ].

Table 1 Methods and results in odor-evoked memory research with implications for psychological and physiological health. Post-scanning, odors and labels were re-assessed and participants provided subjective evaluations of their memories. Odors elicited more emotional and positive memories, and more activity in parahippocampal, amygdala and tempopolar regions than labels. Miles and Bernsten [ 13 ] 12 familiar stimuli presented in verbal, visual and olfactory formats were compared for various qualitative dimensions of memory.

Higher levels of odor-evoked nostalgia elicited higher levels of positive affect, self-esteem, self and social connectedness, optimism, and life meaning. Toffolo et al. One week later memories for the documentary were compared.

The olfactory condition elicited more arousing, detailed and unpleasant memories of the documentary than the auditory condition. Odor-only cues elicited the most positive, emotional, and evocative memories. Odors that evoked AMs promoted deeper, slower and more relaxed breathing compared to the CFs.

More evocative memories and deeper breathing was observed among individuals who scored highest in trait anxiety. Evoked emotions and associations were evaluated using rating scales. AM fragrances elicited more positive emotion, increased feelings of comfort and happiness, and decreased anxiety compared to the CF. Emotions and memories evoked were evaluated. PET scans of neurological activity during odor exposure were assessed. No IL changes were observed. Sniffing odors reduced cravings for cigarettes compared to the control condition.

There was a trend for odors that evoked a memory to be the most effective at diminishing urges. Open in a separate window. Odor-Evoked Memory and Psychological Responses Positive moods and emotions are known to be beneficial for psychological health [ 30 ] and odor-evoked memories have been shown to be more positive than memories elicited by other cues. The latest study tried to boost memory processes using smell, which is a powerful cue for memory and, unlike sound, is unlikely to wake people from their sleep.

In the study, the researchers exposed 32 people to the smell of a rose while asking them to remember the location of words on a computer screen, either on the left or right side. As they slept, the researchers again exposed them to the rose smell, but this time only to one nostril. The reason for this was because memories associated with locations on the left are storied in the right side of the brain, and vice versa.

Delivering the memory evoking scent to only one nostril, therefore, allowed the researchers to re-activate memories stored in the same side of the brain as the chosen nostril. If the researchers activated the left side of the brain, the participants were better at remembering words shown on the right side of the computer screen, and vice versa.

Recordings made while the participants were sleeping showed different patterns of electrical activity between the two sides of the brain. The side of the brain that received the scent showed activity the scientists associated with memory consolidation.

The discovery helps scientists to understand exactly how sleep aids in the process of memory consolidation. By triggering memory building in just one side of the brain, the study identified the specific brain activity associated with memory formation, helping to solve this mystery. The findings that we can manipulate memory consolidation with smell might also have clinical applications.

The researchers say the technique could help restore memory to people who have experienced a brain injury, such as a stroke, or help those with post-traumatic stress disorder PTSD , who show more activity on the right side of the brain when remembering a trauma.

The diet can have a significant impact on the brain's function. To answer these questions, Ramirez and members of his lab created fear memories in mice by giving them a series of harmless but startling electric shocks inside a special container.

During the shocks, half of the mice were exposed to the scent of almond extract, while the other half were not exposed to any scent. The next day, the researchers returned the mice to the same container to prompt them to recall their newly formed memories. Once again, the mice in the odor group got a whiff of almond extract during their session, while the no-odor group was not exposed to any scent.

But this time, neither group received any new electric shocks. Consistent with the systems consolidation theory, both groups exhibited significant activation of the hippocampus during this early recall session, indicating they remembered receiving the shocks from the day before. However, during the next recall session 20 days later, the researchers were in for a shock of their own.

As expected, in the no-odor group, processing of the fear memory had shifted to the prefrontal cortex—but the odor group still had significant brain activity in the hippocampus.

If this is the case, an odor needs only to be present during memory formation for a memory to retain its vividness. Many psychotherapy- and drug-based treatments for PTSD involve trying to suppress or dampen traumatic memories, but this process can only be carried out effectively when people actively recall the memories first.

In other words, the scents that spark our memories may be more powerful than we realize. Today, they serve as the triggers for our nostalgia and our anxiety—but tomorrow, they could be our treatments. It may be a very powerful tool. Kerry Benson Profile.

Boston University moderates comments to facilitate an informed, substantive, civil conversation.