The scent of fresh-cut grass, your mom’s chicken soup or burning leaves may instantly transport you to a distant memory, one you can suddenly recall with razor-sharp clarity.
So-called odor-evoked memories often come along with powerful emotions and are known to activate the “neurolobiological substrates of emotional processing,” according to neuroscientist Rachel S. Herz, an adjunct assistant professor of psychiatry and human behavior at Brown University.1
Research published in Learning and Memory2 added to this, suggesting that odors may modulate the dynamics of memory consolidation, including memories linked to fear, and could potentially be used therapeutically to help people recall memories or treat memory-related mood disorders, such as post-traumatic stress disorder (PTSD).3
“If odor could be used to elicit the rich recollection of a memory — even of a traumatic experience — we could take advantage of that [therapeutically],” study author Steve Ramirez, assistant professor of psychology and brain sciences at Boston University, said in a news release.4
The Systems Consolidation Theory of Memory
The prevailing theory of memory formation — the systems consolidation theory — suggests that memories are initially processed in the brain’s hippocampus, giving the memory rich details. Over time, however, as memories become older or more remote, they become less dependent on the hippocampus and more dependent on processing by the front of the brain — the prefrontal cortex.
During this transition, which occurs particularly during sleep, memories may lose many of their rich contextual details. Further, memories that are retrieved shortly after an experience involve reactivation of cells in the hippocampus whereas retrieval of a memory later is thought to involve reactivation of cells in the prefrontal cortex.5
The systems consolidation theory explains why memories tend to become less detailed or clear as time goes by, as well as why people with a damaged hippocampus have trouble forming new memories while those with prefrontal cortex damage may have trouble remembering memories from the past.6
The theory, however, is just that — a theory — and it has some challenges as well. Researchers noted in Learning and Memory, “[P]eople often retrieve remote memories that are vivid and highly detailed. Likewise, several studies have shown that there is activity in both structures during both recent and remote memory recall … and that damage to the HPC [hippocampus] sometimes affects remote memory as well as recent memory.”7
Particularly in PTSD, people may recall intense memories years after the trauma occurred, which seemingly contradicts the systems consolidation theory, leading the researchers to conduct a study to clarify some of these contradictions.
Scent Alters the Way Memories Are Processed
In a study on mice, researchers from Boston University's Center for Systems Neuroscience startled mice with electric shocks while in a special container to induce fear memories. Half the mice were exposed to almond extract scent during the shocks while the other half received no scent exposure.
The next day, the same experiment took place, but without any electric shocks given. The odor group was exposed to the scent of almond extract while in the special container while the other group had no scent exposure. The idea was to prompt the mice to recall the fearful memory of being shocked — something all the animals did, as evidenced by significant activation of the hippocampus.
The memory recall session was repeated again 20 days later. In the no-odor group, the prefrontal cortex was activated, suggesting it was responsible for processing the fear memory, as researchers had expected. In the odor group, however, significant activity was still observed in the hippocampus. Ramirez noted:8
“[This finding suggests] that we can bias the hippocampus to come back online at a timepoint when we wouldn't expect it to be online anymore because the memory is too old. Odor can act as a cue to reinvigorate or reenergize that memory with detail.”
It’s possible that odors may delay a memory being processed in the prefrontal cortex, or that an odor reactivates the hippocampus to restore details of a memory that’s already shifted to the prefrontal cortex.
While the specifics aren’t yet known, it’s possible that odors could be harnessed to help people with PTSD actively recall traumatic memories, so that they could then be suppressed or dampened using behavioral interventions.
“Now that we know that odor can shift memories to become more hippocampus dependent, we could potentially develop strategies that engage or disengage the hippocampus,” Ramirez said, adding, “We can potentially view memory as its own kind of drug — as an antidepressant or [anxiety reducer]. And [odor] could be an experimentally controllable factor that we could deliver to people. It may be a very powerful tool.”9
Memories Induced by Scent Are More Emotional
Odor-evoked memory is dubbed the “Proust phenomenon” after a literary anecdote where Marcel Proust took a bite of a biscuit dipped in Linden tea and was transported to a moment in his childhood that he had long forgotten. Odor-evoked memories, Herz explained, are unique from memories evoked by other stimuli, such as verbal or visual stimuli.
Not only are they more rare and less frequently thought about, but they tend to come from early in life, typically within the first decade. What’s more, autobiographical memories triggered by odors tend to be much more emotional and have the perception of bringing people back to the original time and place that their memories occurred.
Part of the unique intensity of odor-evoked memories has to do with the way scent is processed by your brain. Smells get routed through your olfactory bulb, which the smell-analyzing region in your brain. It’s closely connected to your amygdala and hippocampus, brain regions that handle memory and emotion. According to Herz:10
“Odor-evoked memories are exceptionally viscerally involving because the neuroanatomy of olfaction has a privileged and unique connection to the neural substrates of emotion and associative learning.
The primary olfactory cortex includes the amygdala, which processes emotional experience and emotional memory, as well as the hippocampus, which is involved in associative learning. Thus, the mere act of smelling activates the amydala-hippocampal complex.”
Odor-Evoked Memories Influence Psychological, Physical Health
By boosting mood, lowering stress and reducing inflammation, it’s likely that the powerful emotions elicited by positive odor-evoked memories can influence psychological and physiological 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 well-being,” Herz wrote,11 noting that individuals may also experience unique effects like improvements in self-confidence, motivation and vigor depending on the emotions a specific memory invokes.
For example, if an odor triggers a memory of winning a sporting event, it could energize your behavior and trigger beneficial physiological effects, making it a reliable therapeutic agent. Likewise, odors that evoke pleasant memories could be used as reminders of safe or happy places and events, making them useful for the treatment of psychiatric conditions and reducing stress and anxiety.12
The unique anatomy of the olfactory pathways hints at the importance of odors in emotions, learning, memory and more. While other sensory systems pass through the thalamus to reach the cortex, odors get relayed directly to the limbic system in the brain, which is associated with memory and emotional processes.
This is why odors have such a powerful influence on mood, information acquisition and even social interactions. As noted in Frontiers in Behavioral Neurosciences, memory is but one crucial behavior influenced by odors:13
“Indeed, olfaction is crucially involved in behaviors essential for survival of the individual and species, including identification of predators, recognition of individuals for procreation or social hierarchy, location of food, as well as attachment between mating pairs and infant-caretaker dyads.”
Aromatherapy: Another Way Scents Influence Mood and More
Aromatherapy — the use of plant-based aromas to influence mood and wellness — is not the same as odor-evoked memory. In the case of the psychological and physiological effects induced by odor-evoked memories, the odor has such effects because of the memories and emotions attached to it as a result of past personal experience.14
This isn’t to say that aromatherapy isn’t also beneficial for health and wellness, just that it works in a different, more generalized way. Many ancient cultures, including the Chinese, Indians, Egyptians, Greeks and Romans used essential oils in cosmetics, perfumes and drugs for purposes ranging from spiritual to therapeutic.15
In the modern day, aromatherapy is used in health care settings, health spas and homes, both by professional aromatherapists and amateurs, while accumulating research backs up its many potential uses and benefits.
For instance, the two primary terpenoid constituents of lavender essential oil, linalool and linalyl acetate, have anxiety-reducing effects, and lavender essential oil is known to induce a calming effect without side effects such as sedation.16
Lavender also has an effect on the nervous system, and when inhaled it’s believed to act via the limbic system, particularly the amygdala and hippocampus,17 although it may have effects even beyond mood and anxiety.
In a study on patients undergoing open heart surgery, a cotton swab containing lavender essential oil was placed in the patients’ oxygen mask for 10 minutes. The aromatherapy led to significant reductions in blood pressure and heart rate, with researchers concluding it could be “used as an independent nursing intervention in stabilizing mentioned vital signs.”18
Beyond lavender, a study on rats showed that inhaling Roman chamomile essential oil for two weeks reduced depressive-like behaviors,19 while essential oils placed nightly on towels around dementia patients’ pillows resulted in significantly longer total sleep time, increased sustained sleep and reduced early morning awakening.20
While it’s best to consult an experienced aromatherapist before delving into essential oils, generally speaking if you’re looking to feel energized and uplifted, consider essential oils such as peppermint, grapefruit, lemon, neroli and wild orange while more calming oils include lavender, chamomile, bergamot, ylang ylang and vetiver. For more information, consult our Ultimate Guide to Herbal Oils.
Loss of Sense of Smell Also Gives Clues About Health
Paying attention to any changes in your sense of smell, including a loss of it, is also important. In a study of 3,005 community-dwelling adults, those who had a dysfunctional sense of smell were more likely to die in the next five years than those with a good sense of smell.
In fact, olfactory function was deemed to be one of the strongest predictors of five-year mortality and researchers suggested it may “serve as a bellwether for slowed cellular regeneration or as a marker of cumulative toxic environmental exposures.”21
An inability to identify odors is also an early symptom of neurological disorders, including Alzheimer’s disease and Parkinson’s disease.22 So, make a conscious effort to keep track of your sense of smell, using it for memory recall, mood boosts and as a gauge for your health should it dissipate.
While odor-evoked memories can be powerful tools for health and behavior as well, they depend largely on happenstance, in that a scent must be introduced at the precise moment that a memorable event is occurring — then that scent must later be introduced to evoke the memory.
It happens often enough that virtually everyone reading this has experienced it, but if you’re looking for a more direct way to harness the power of odors over your health, aromatherapy can be very useful.
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