The memory of scent is one of those everyday phenomena we all experience, yet rarely pay attention to. A simple smell of freshly brewed coffee, rain on asphalt, or the perfume of someone special can transport our minds back years in a matter of seconds. No conscious effort is required: the scent arrives, the memory is triggered, and the emotion creeps in unbidden.
This ability is not merely poetic or romantic; it has a powerful neurobiological basis : the sense of smell is uniquely connected to the areas of the brain that manage emotions, learning, and memory. Therefore, smells not only please or displease us; they also influence our behavior, affect our well-being, and have been key to the survival of mammals, including humans.
What exactly is olfactory memory?
When we talk about scent memory, we're referring to the brain's ability to store and retrieve smells, associating them with specific experiences, contexts, and emotions. Just as there is visual or auditory memory, there is olfactory memory, but with a key difference: memories linked to smells tend to be more intense, longer-lasting, and emotionally charged.
In practice, olfactory memory functions as an internal catalog of fragrances stored in the hippocampus and other structures of the limbic system. Every time we detect a new smell, the brain registers it along with what's happening around us: where we are, who we're with, and what we're feeling. Later, when we smell something similar again, this entire memory package—scent, image, sensation, and emotion—is reactivated.
This process is so powerful that a scent can trigger highly detailed autobiographical memories , sometimes more vivid than those generated by images or sounds. It is no coincidence that the literary example of Proust's madeleine is used to illustrate how a specific smell can evoke entire scenes from the past with surprising clarity.
How we perceive smells: from the nose to the brain
The perception of smells is both a chemical and electrical process. Odorant molecules , tiny particles, circulate in the air and, when we inhale, enter the nasal cavity. There they encounter the olfactory epithelium, an area located in the upper part of the nasal passages filled with specialized neurons.
An odorant is, in technical terms, a physicochemical molecule capable of binding to a specific olfactory receptor . Each sensory neuron expresses a single type of odor receptor gene, and in mammals, around a thousand different types of olfactory receptors have been identified. This diversity is what allows us to distinguish thousands of varied and complex odors.
When an odorant molecule binds to its receptor, an electrical signal is generated that travels to the olfactory bulb , a structure located at the base of the brain. In the olfactory bulb, the receptor neurons synapse in small clusters called glomeruli, which connect with mitral cells and other interneurons responsible for refining and organizing the information.
The identity of an odor depends on its molecular structure and complexity . A small chemical change in a molecule can significantly alter the quality of the smell we perceive. The olfactory bulb not only receives but also modulates the signal through the frequency and timing of its activation, influencing how the aroma is encoded and the likelihood of it being remembered later.
From the olfactory bulb, information is sent through the olfactory tract to various brain regions, especially the limbic system , which acts as a major integration center between olfactory sensations, emotions, and memory.
Neuromodulation and plasticity in the olfactory system
The olfactory system is particularly plastic, meaning it can be modified based on experience . This plasticity depends largely on the action of neuromodulators such as norepinephrine and acetylcholine, which regulate how olfactory memories are stored and how behavior is adjusted in response to certain smells.
Studies in mice have shown that lesions in areas related to the noradrenergic system eliminate typical olfactory learning abilities. When noradrenaline is injected into the olfactory bulb, these abilities are restored, indicating the key role of this neurotransmitter in consolidating olfactory memories.
Similarly, acetylcholine has been linked to the early stages of olfactory learning and to reducing interference between memories. Experiments with rats and drugs such as scopolamine show that altering the cholinergic system impacts the ability to differentiate and remember odors, especially when distinguishing between similar stimuli.
Plasticity is also reflected in changes in cortical and subcortical circuits following significant olfactory experiences. Neurogenesis in regions such as the olfactory bulb helps renew and adapt these circuits to integrate new smells or give different meaning to familiar fragrances.
Implicit and implicit memory of smells
When we talk about scent memory, it's important to distinguish between implicit (non-conscious) and explicit (conscious) memory. Both are present in the olfactory system, although they may involve different processes and brain areas.
Implicit olfactory memory
Implicit memory refers to memories that influence behavior without our conscious awareness of having learned them. In the realm of smell, this means that it's not necessary to deliberately recall an experience with a smell for that aroma to have left a trace in the brain.
This form of memory is studied through tests of habituation, sensitization, perceptual learning, and classical conditioning . In habituation, for example, it is observed how the response to an odor decreases with prolonged or repeated exposure, reflecting an adaptation of the receptor cells, mitral cells, and piriform cortical neurons.
Piriform neurons exhibit rapid, complete, and selective adaptations to novel odors and are considered to play a central role in habituation to olfactory stimuli. Both norepinephrine and acetylcholine participate in these processes, modulating the sensitivity and dynamics of the neuronal networks.
Implicit memory tests allow researchers to assess the impact of previous experiences with smells without requiring conscious recall. This is particularly useful for studying deficits following brain injury or in cognitive disorders , as it can provide information about the state of the olfactory system and non-declarative memory.
Explicit olfactory memory
Explicit odor memory involves the conscious retrieval of a scent and its meaning . Some authors suggest that this explicit aspect, as we understand it, may be unique to humans, since it requires verbal labeling, categorization, and voluntary retrieval.
In the olfactory context, explicit memory manifests itself by assigning associative meaning to smells : linking a fragrance to a person, a place, a brand, or a life event. Once this association is created, the smell can be used to process and compare other smells, and to make conscious decisions (for example, choosing one perfume over another).
The most common tasks used to study this type of memory are odor recognition and identification . In recognition tasks, the participant is presented with an odor and, after a short interval, is asked to decide whether a second odor is the same or not. The percentage of correct answers indicates the accuracy of the memory.
One problem with these tests is that people tend to generate verbal labels ("it smells like lemon," "it tastes like mint"), which can improve stimulus retention and blend purely olfactory memory with verbal memory. Therefore, odor recognition is considered to combine the memory of sensory information with the influence of language.
In identification tasks, the subject is asked to name or select the correct label from several options for the presented odor. This involves the neural encoding of the stimulus's identity, concentration, and hedonic value (pleasant or unpleasant), represented in patterns of action potentials that travel from the olfactory bulb to the cortex.
Genetics and neuronal coding of smells
Olfactory receptors belong to a large family of genes linked to G protein-coupled receptors . Each receptor neuron expresses only one type of receptor gene, and it has been shown that a limited subset of these genes are active in the neurons of the olfactory epithelium, despite the enormous number of genes available.
The specificity in odor recognition stems from the molecular diversity of receptor proteins and how each one binds to certain odor molecules. Although the exact binding mechanism is still not fully understood, it is known that different odors activate specific combinations of receptors, generating a unique "code" for each scent.
This combinatorial coding system, along with the genetic regulation of receptor expression, allows the olfactory system to detect and discriminate a vast array of complex and novel odors in the environment. It is one of the reasons why humans and other mammals can adapt to changing environments and recognize chemical cues crucial for survival.
Cerebral laterality and networks involved in aroma memory
Olfactory stimulation, even when performed unilaterally (presenting the odor to only one nostril), triggers bilateral brain activation . However, this activity is not identical in both hemispheres, and relevant differences are observed depending on the type of memory task.
Neuroimaging studies show that the left hemisphere is more activated during verbal semantic retrieval linked to smells (e.g., finding the name of a scent), while the right hemisphere appears to be especially involved in non-verbal retrieval: smell quality, subjective sensation, and memory of the previous encounter with that stimulus.
Despite these differences, there is significant overlap between the activated regions : semantic information about smells is distributed across both sides of the brain and integrated with other sensory modalities. Regions such as the primary olfactory cortex, the piriform cortex, the hippocampus, the amygdala, and the medial prefrontal cortex form a network that supports both the emotional and cognitive aspects of olfactory memory.
Neuronal plasticity is essential in these networks. New experiences, hormonal changes, or different emotional contexts can cause lasting alterations in cortical and subcortical circuits , modifying the way certain smells are represented and remembered.
The amygdala, the hippocampus, and the emotional value of smells
The amygdala, located in the anterior temporal lobe and just below the primary olfactory cortex, is a key nucleus in the formation of emotional memories , especially those related to fear, flight, and defensive behaviors. It maintains very dense connections with the hippocampus and the basal forebrain, allowing it to integrate olfactory sensations into complex memories.
The primary olfactory cortex and the hippocampus have direct and indirect projections to the amygdala . This is vital for an animal to create memories of odors that pose a threat to its survival. Without a functioning amygdala, olfactory memories linked to danger would be greatly diminished, leaving the animal more vulnerable to harmful stimuli that it would not recognize as such.
In humans, the close relationship between the amygdala, hippocampus, and olfactory system explains why a smell can trigger intense emotions (positive or negative) and, at the same time, bring to consciousness very specific memories of one's personal history.
Smells, emotions, and psychological well-being
Scents have a unique ability to modulate mood . Research has shown that certain fragrances can evoke positive autobiographical memories, increase pleasant emotions, reduce negative moods, decrease the subjective feeling of stress, and even influence physiological markers such as levels of systemic inflammation.
Aromacology, the discipline that studies the effect of aromas on the mind and body, uses indicators such as brain electrical activity, heart rate, and skin conductance to assess the impact of olfactory stimuli. Depending on whether the smell is pleasant or aversive, these parameters shift toward patterns of calm, alertness, relaxation, or stress.
All of this reinforces the idea that smell is not a minor sense: it is a direct way to subtly but powerfully influence subjective experience . From the feeling of comfort at home to the perception of a commercial space, aromas can color our emotions without us always realizing it.
Olfactory memory, motherhood and mother-child bond
In mammals, the olfactory system plays a crucial role in the mother-offspring bond and maternal behavior . Following birth, profound changes occur in the olfactory bulb and the main olfactory system, driven in part by a heightened state of plasticity and neurogenesis.
During pregnancy and childbirth, the mother's olfactory system enters a period of heightened sensitivity and learning capacity . This allows her to quickly learn the scents of her offspring and associate them with nurturing and protective behaviors. It is common for scents that were previously aversive (such as amniotic fluid) to become attractive immediately after birth.
Studies with sheep show that mothers are able to form an olfactory recognition memory of their lambs between two and four hours after birth. From that moment on, they actively reject other lambs and unfamiliar odors, highlighting the importance of smell in individual identification.
Amniotic fluid acts as a key olfactory cue after birth . Ewes are attracted to lambs covered in this fluid, and when the lambs are washed with water or soap, licking and other acceptance behaviors are drastically reduced. This demonstrates that the primary olfactory system is essential for the development of appropriate maternal behaviors.
In parallel, the medial prefrontal cortex is activated during olfactory memory tasks and receives extensive projections from the olfactory system , which are activated immediately after birth. Changes in synaptic circuits, especially in the olfactory bulb, contribute to the level of maternal response and the consolidation of the infants' odor memories.
Intrauterine olfactory learning and the first days of life
Physiological, anatomical, and behavioral evidence indicates that some mammal species already possess a functional olfactory system in utero . The fetus becomes familiar with the chemical signals of the intrauterine environment, and this early exposure prepares the newborn to recognize key odors immediately after birth.
Newborns have been shown to respond positively to the smell of their own amniotic fluid , which is interpreted as evidence of intrauterine learning. In rats, for example, pups avoid odors after birth that they associated with harmful stimuli while still in the womb.
After birth, human babies show a clear attraction to the olfactory cues associated with the mother's breast . They are able to discriminate and prefer the scent of their own mother's breast over that of other breastfeeding women, although in other contexts they may also be attracted to the scents of unfamiliar breasts. This unique maternal scent is known as the "olfactory signature."
In addition to the nipple and areola area, babies can recognize the scent of their mother's armpit and react with familiarity and preference. Olfactory cues are widespread in parental care and help coordinate mother-baby interaction and facilitate adaptation to the new environment outside the womb.
In experiments, babies suckled more frequently from a breast treated with a small amount of their own amniotic fluid than from an untreated alternative breast. Over time, as exposure to amniotic fluid diminished and contact with breast odors continued, the positive association transferred to the smell of the breasts, which became the central olfactory reference point linked to nourishment, warmth, and protection.
Smell, survival and evolution
Throughout mammalian evolution, the sense of smell has been a crucial factor for survival . It has been observed that the excess of brain neurons in many species has been preferentially integrated into increasingly sophisticated olfactory systems, especially in animals that had to search for and capture food.
An extreme example is the vulture, which has a large part of its brain dedicated to its sense of smell , allowing it to detect food from great distances, even without seeing it. Having an olfactory memory for different types of food helps it remember which aromas are associated with safe food and which with dangerous or inedible substances.
Scent memory has also evolved to recognize other individuals . In many species, odors allow offspring to identify their mothers, adults to distinguish between males and females, and group members to recognize mates or intruders. Furthermore, chemical signals are used to mark territory and warn of the presence or absence of predators.
Although other sensory systems such as vision and hearing have become more important in some species, the olfactory system continues to have a significant impact on social interactions . Thanks to the memory of specific odors, animals can communicate with members of their own species and, at the same time, maintain "chemical barriers" against other species that lack the receptors necessary to interpret those signals.
The sense of smell also plays a role in sexual reproduction . Pheromones, as specific chemical signals, allow us to perceive the reproductive availability of other individuals, synchronize menstrual cycles among females, and modulate sexual attraction. In many cases, these olfactory memories operate unconsciously, guiding behaviors that have been favored by natural selection.
Smell as an alarm system and odor-induced anxiety
Beyond food and reproduction, the sense of smell has evolved as an early warning system . In humans, odors such as gas, smoke, or spoiled food can trigger rapid responses even before we consciously process the threat.
This response is usually largely implicit: the olfactory system can trigger pre-attentive patterns similar to those of other senses, generating motor and autonomic changes (increased heart rate, muscle tension, feeling of restlessness) that are part of the defense reaction to danger.
In animals, odor-induced anxiety has been studied, for example, by exposing rats to the scent of cats. Under these conditions, a clear increase in anxiety-related behaviors is observed . It has been suggested that this effect is due, among other mechanisms, to the inhibition of the endocannabinoid system in the amygdala, which promotes heightened fear and alert responses to the chemical signal of the predator.
Olfactory memory deficits and neurological disorders
Alterations in the sense of smell and olfactory memory are highly sensitive markers of brain pathology . In fact, in some neurological and psychiatric disorders, olfactory deficits appear before other, more obvious symptoms.
Olfactory alterations have been found in schizophrenia, Parkinson's disease, Huntington's disease, alcoholic Korsakoff syndrome, and Alzheimer's disease , among others. In some cases, these alterations are considered possible early predictors of neurodegeneration or cognitive decline.
Animal research has shown that certain drugs that act on the brain, such as some antidepressants, can cause temporary deficits in olfactory memory . In a study with mice administered citalopram or clomipramine for three weeks, their behavior was evaluated in a Y-maze where they could choose between an odorant (butanol) or distilled water. Compared to a control group that received only saline solution, the treated mice showed significant deficits in their olfactory performance.
Clinical tests to evaluate scent memory
To assess the state of the olfactory system and olfactory memory, several standardized batteries have been developed . Among the best known are the University of Pennsylvania Scent Identification Test (UPSIT) with 40 items and its abbreviated 12-item version.
Both tests use a "scratch and sniff" format : the patient scratches an area impregnated with a fragrance and must identify the scent by choosing from several options. These tests focus on identification, but also involve memory, as they require recognizing and differentiating previously stored odors.
Another widely used battery of tests is the Sniffin' Sticks test , which uses pens impregnated with different scents and dilutions. It assesses three domains: threshold (minimum sensitivity to detect an odor), discrimination (ability to differentiate between distinct odors), and identification (naming or categorizing an odor). The combined scores provide a fairly comprehensive profile of olfactory function.
In clinical practice, the deficits detected through these tests are used as potential early indicators of depression, dementia, or neurodegenerative processes . Each disorder tends to show a characteristic pattern of impairment (for example, more identification problems than threshold problems), which helps guide diagnosis and monitoring.
Scent memory, olfactory marketing, and brand experiences
The ability of scents to evoke memories and intense emotions has not gone unnoticed by the marketing world. This has led to the emergence of olfactory marketing as a branch of sensory marketing, focused on using fragrances to create memorable experiences in stores, hotels, clinics, gyms, and all kinds of spaces.
Scent marketing works directly with olfactory memory: it creates associations between a scent and a specific brand or environment. Over time, that scent becomes an "olfactory logo" or signature that identifies the business just like a visual logo or jingle.
Biologist Linda Buck demonstrated how olfactory receptors organize signals in the brain to generate different perceptions, supporting the idea that it's possible to design specific fragrances associated with certain emotions and contexts. It's estimated that our olfactory system can distinguish around ten thousand different odors thanks to combinations of receptors.
In practice, by scenting a space with a fragrance that aligns with the brand's values (warmth, luxury, freshness, energy), the aim is for the limbic system to associate that scent with pleasant sensations. Thus, each visit reinforces olfactory memory and consolidates an emotional and almost instinctive connection with the brand. Various studies indicate that the use of appropriate aromas can increase sales, improve customer dwell time in stores, and reduce errors in work environments.
Implicit memory plays a crucial role here: the customer may not consciously recall the scent or be able to describe it, but their brain has registered it and associates it with the experience. This olfactory imprint facilitates subsequent recall and recommendations, enhancing the brand's reputation in an increasingly saturated market.
The memory of scent, in short, acts as an intimate sensory diary where all the smells that have marked us are effortlessly recorded: from the scent of our mother's skin at birth to the fragrance of a shop or the perfume of a loved one. This invisible archive accompanies us throughout our lives, influencing emotions, decisions, and relationships, and has also become a powerful tool for better understanding the human brain and for designing experiences that truly leave a lasting impression.
