A scent is a chemical particle that floats in through the nose and into the brain’s olfactory bulbs, where the sensation is first processed into a form that’s readable by the brain.
Brain cells then carry that information to a tiny area of the brain called the amygdala, where emotions are processed, and then to the adjoining hippocampus, where learning and memory formation take place.
Scents are the only sensations that travel such a direct path to the emotional and memory centers of the brain.
All other senses first travel to a brain region called the thalamus, which acts like a “switchboard,” relaying information about the things we see, hear or feel to the rest of the brain, said John McGann, an associate professor in the psychology department of Rutgers University in New Jersey.
But scents bypass the thalamus and reach the amygdala and the hippocampus in a “synapse or two,” he said.
That results in an intimate connection between emotions, memories and scents. This is why memories triggered by scents as opposed to other senses are “experienced as more emotional and more evocative,” said Rachel Herz, an adjunct assistant professor of psychiatry and human behavior at Brown University in Rhode Island and author of the book “The Scent of Desire” (Harper Perennial, 2018). A familiar but long-forgotten scent can even bring people to tears, she added.
Smell and memory seem to be so closely linked because of the brain’s anatomy, said Harvard’s Venkatesh Murthy, Raymond Leo Erikson Life Sciences Professor and chair of the Department of Molecular and Cellular Biology.
Murthy walked the audience through the science early in the panel discussion “Olfaction in Science and Society,” sponsored by the Harvard Museum of Natural History in collaboration with the Harvard Brain Science Initiative.
Smells are handled by the olfactory bulb, the structure in the front of the brain that sends information to the other areas of the body’s central command for further processing.
Odors take a direct route to the limbic system, including the amygdala and the hippocampus, the regions related to emotion and memory.
“The olfactory signals very quickly get to the limbic system,” Murthy said.
But, as with Proust, taste plays a role, too, said Murthy, whose lab explores the neural and algorithmic basis of odor-guided behaviors in terrestrial animals.
When you chew, molecules in the food, he said, “make their way back retro-nasally to your nasal epithelium,” meaning that essentially, “all of what you consider flavor is smell. When you are eating all the beautiful, complicated flavors … they are all smell.”
Murthy said you can test that theory by pinching your nose when eating something such as vanilla or chocolate ice cream. Instead of tasting the flavor, he said, “all you can taste is sweet.”
For decades individuals and businesses have explored ways to harness the evocative power of smell. Think of the cologne or perfume worn by a former flame. And then there was AromaRama or Smell-O-Vision, brainchildren of the film industry of the 1950s that infused movie theaters with appropriate odors in an attempt pull viewers deeper into a story — and the most recent update, the decade-old 4DX system, which incorporates special effects into movie theaters, such as shaking seats, wind, rain, as well as smells.
Several years ago, Harvard scientist David Edwards worked on a new technology that would allow iPhones to share scents as well as photos and texts.
Today, the aroma of a home or office is big business. Scent branding is in vogue across a range of industries, including hotels that often pump their signature scents into rooms and lobbies, noted the authors of 2018 Harvard Business Review article.
“In an age where it’s becoming more and more difficult to stand out in a crowded market, you must differentiate your brand emotionally and memorably,” they wrote.
“Think about your brand in a new way by considering how scent can play a role in making a more powerful impression on your customers.”
Someone who knows that lesson well is Dawn Goldworm, co-founder and nose, or scent, director of what she calls her “olfactive branding company,” 12.29, which uses the “visceral language of scent to transform brand-building” in the actual buildings where clients reside (mostly through ventilation systems or standalone units).
Among Goldworm’s high-profile customers is the sportswear giant Nike. Its signature scent, she explains in a video on her company’s website, was inspired by, among other things, the smell of a rubber basketball sneaker as it scrapes across the court and a soccer cleat in grass and dirt.
Her goal, she said, is to create “immediate and memorable connections between brands and consumers.”
Goldworm, who designed signature fragrances for celebrities for more than a decade before starting her own company, knows the science, too. She spent five years in perfumery school followed by a master’s degree at New York University where her thesis focused on olfactory branding.
During the talk she explained that smell is the only fully developed sense a fetus has in the womb, and it’s the one that is the most developed in a child through the age of around 10 when sight takes over.
And because “smell and emotion are stored as one memory,” said Goldworm, childhood tends to be the period in which you create “the basis for smells you will like and hate for the rest of your life.”
For decades individuals and businesses have explored ways to harness the evocative power of smell.
She also explained that people tend to smell in color, demonstrating the connection with pieces of paper dipped in scents that she handed to the audience.
Like most people, her listeners associated citrus-flavored mandarin with the colors orange, yellow, and green. When smelling vetiver, a grassy scent, audience members envisioned green and brown.
Be careful of your snout, both speakers cautioned the audience. The bony plate in the nose that connects to the olfactory bulb, which in turn sends signals to the brain, is particularly sensitive to injury, meaning head trauma can “shear that plate off” and cause people to lose their sense of smell entirely, making them anosmic, said Murthy. (Feb. 27 is anosmic awareness day.)
“Wear a helmet if you ride a bike or are doing extreme sports,” said Goldworm.
People do tend to lose their sense of smell as they age, she added. But not to worry. Your nose is like a muscle in the body that can be strengthened, she said, by giving it a daily workout, not with weights, but with sniffs.
“Just pay attention,” with your nose, said Goldworm. “When you are walking down the street, consciously indicate what you are smelling … the more you use [your nose], the stronger it gets.”
Autobiographical memory is an essential part of the human memory system as it allows the construction and maintenance of self-awareness, personal knowledge and self-image , and can be defined as the ability to relive past personal events.
Autobiographical memory has been found to be impaired in Alzheimer’s disease (AD) [2,3,4,5,6]. Autobiographical memory compromise in AD is characterized by an overgenerality, i.e., a reduced ability of AD patients to produce specific memories [4,7,8,9,10,11,12,13,14].
Decline of autobiographical memory in AD is also associated with changes in the strength and the quality of the sense of identity in patients . As a result of autobiographical memory impairment, AD patients demonstrate difficulty in mentally reliving past events with the perceptual, sensory and conceptual details of the original event, replaced by a general feeling of knowing or familiarity [2,16,17,18,19,20].
Studies have tried to alleviate impairment of autobiographical memory in AD by focusing on sensory cues. Some studies have shown that powerful perceptive cues such as music and odor have a beneficial impact on the involuntary retrieval of autobiographical memories in AD, principally by diminishing the time taken to retrieve these memories [2,8,21].
Involuntary autobiographical memories were described by  as conscious memories of personal events that come to mind spontaneously. These authors suggest that involuntary retrieval may promote a direct link between the cue and the memory trace, thus avoiding the complex recovery process involved in voluntary autobiographical remembering [1,8,23,24].
Involuntary retrieval may be triggered by odor and, unlike other modalities, olfactory stimuli are not relayed by the thalamus during cortical processing . Olfactory signals are directly connected with two key structures involved in memory and emotions: the amygdala and the hippocampus , which may explain why odor-evoked autobiographical memories are typically emotional and vivid in general populations .
Odor has been found to be unique in its ability to enhance the quality of autobiographical memory. Research has suggested that odor exposure has a beneficial effect on the recovery of phenomenological details associated with the evocation of autobiographical memories.
Odor-evoked autobiographical memories are characterized by significant mental time travel [28,29,30], i.e., the feeling of being brought back to the past at the moment of the original event .
Odor-evoked autobiographical memories are also more vivid than memories cued by other sensory modalities [32,33,34]. Interestingly, neuroimaging studies have shown that odor-evoked memories are characterized not only by the activation of brain areas involved in memory recovery and cortical processing of olfactory stimuli, but also by the recruitment of brain regions usually activated during visual imagery (olfactory gyrus and precuneus) and emotions (limbic and tempopolar regions) [32,35], which may explain why involuntary memories are described as more evocative.
Besides being evocative, odor-evoked autobiographical memories trigger significant emotional content [28,36,37,38]. Herz and Schooler  found that odor-evoked autobiographical memories were rated as more emotional than those evoked by verbal or visual cues.
Regarding emotional valence, research has shown that odor-evoked autobiographical memories are more “pleasant” than those elicited by other modalities in young healthy adults  and older healthy adults .
The power of odor in eliciting positive memories could be due to the neuronal processing of olfactory memories. Arshamian et al.  reported that odor-evoked autobiographical memories were associated with an increased cortical activation in the temporal gyrus and the temporal pole.
The temporal pole is known to be involved in the processing of pleasant memories. Arshamian et al.  suggested that increased activity in the temporal lobe underlies the positivity of odor-evoked autobiographical memories.
To our knowledge, two studies have already demonstrated the beneficial effects of odor on autobiographical remembering in AD. In a recent study, El Haj et al.  investigated the involuntary nature of autobiographical memory in AD triggered by music and odor.
AD patients showed improved specificity, emotional experience, retrieval time and mental time travel when memories were cued by odor compared to an odor-free condition. The authors observed shorter retrieval time for memories cued by odor than for those evoked by no cue.
This study was the first to demonstrate the beneficial effect of olfactory cueing in AD and has important clinical implications, as olfactory cues could serve as a useful tool to stimulate autobiographical memory in this pathology. In a related vein, Glachet et al.  found similar results for specificity and recovery time.
They also showed that compared with memories evoked without odor, odor-evoked autobiographical memories were characterized by a higher subjective reliving in AD. Interestingly, there was no significant effect of odor in terms of specificity and reminiscence in heathy older adults, suggesting that this type of cueing is particularly efficient when autobiographical memory is impaired.
While these studies demonstrated positive effects of odor on autobiographical in AD, they did not evaluate the impact of olfactory cueing on arousal and emotional valence of these memories. Therefore, the main aim of the present study was to assess whether odor exposure may enhance the subjective reliving and the emotional properties of autobiographical memories in AD.
In our view, it would be of interest to evaluate whether odor may help patients with AD to retrieve positive autobiographical memories, despite the prevalence of depression in AD. Generally speaking, AD is associated with several behavioral and neuropsychiatric symptoms such as apathy and depression [40,41], and the latter affects at least 50% of AD patients .
Critically, research has demonstrated that patients with emotional disorders such as depression tend to retrieve overgeneral memories with little access to specific details [43,44]. Moreover, Young et al.  demonstrated reduced activity in the left amygdala during the retrieval of positive autobiographical memories in patients with depression and considered that hypoactivity in the left amygdala might be a marker of depression.
We therefore evaluated the relationship between depression and phenomenological characteristics (e.g., emotional valence) of odor-evoked autobiographical memories in AD.
To summarize, while previous research has demonstrated positive effects of odor on autobiographical in AD [2,21], it did not evaluate the impact of olfactory cueing on arousal and emotional valence of these memories. We therefore evaluated whether odor exposure may enhance the subjective reliving and the emotional properties of autobiographical memories in AD.
We also evaluated the relationship between depression and phenomenological characteristics (emotional valence, arousal, subjective reliving and specificity) of odor-evoked autobiographical memories in AD. We expected that odor-evoked autobiographical memories would be more positive and be accompanied by more arousal and subjective reliving than memories evoked without odor in AD.
Given the prevalence of depression in AD patients  and its critical impact on autobiographical memory [43,44], we also expected a negative correlation between depression and the phenomenological characteristics of autobiographical memories (arousal, emotional valence subjective reliving and specificity) in this group.
To this end, we invited AD and control participants to retrieve autobiographical memories in an odor compared to an odor-free condition, and then evaluated them in terms of emotion (arousal and valence), subjective reliving and specificity.