Olfaction: Not The Fifth Sense, But The First Sense

Humans have 5 senses, one of them being the sense of smell. The sense of smell is very highly evolved and recent estimates are that the human olfactory system is capable of distinguishing over 1 trillion unique odors, which makes the human sense of the smell by far the most sensitive of the human senses in terms of how many physically different stimuli it can detect. This is not surprising as the sense of smell was highly developed in prehistoric man, smell is considered to be our first developed sense, imperative in the survival of our species, the sense of smell allowed man to detect food sources, dangerto life before it was even seen, finding potential mates and even in recognition of the smell of our own kin to avoid human incest. The molecules of an odorant pass to the epithelium of the olfactory system, into the superior nasal concha, these molecules then interact with the mucopolysaccharides (enzymes and antibodies in our mucous) which acts as a solvent for these odors. The resulting reaction initiates a complicated cascade of neurologic signals leading to stimulation or inhibition of axons through the olfactory bulbs, which are then distributed via different pathways to various areas of the brain.

aromatherapy and smell, aromatherapy and olfaction

Smell is an ancient primordial sense, olfaction is the vertebrate version of chemical sensitivity. All our other senses first pass through the thalarnus of our brain, the thalarnus is considered to be the switchboard of the brain, from which the information is then passed to various primary sensory cortices of the brain. In contrast, the information from the primordial sense of smell is passed directly to the primordial portion of our brain known as the limbic system. The direct passage of olfactory information to our limbic system results in immediate basic processing of this information that evokes memory and emotion even before relaying that information to the thalarnus and therefore conscious awareness of the odorant. In other words, memory and emotions are activated by smell through our limbic system, before we are even conscious that the odor exists.

What We Know About The Olfactory System

The process by which olfactory information is coded in the brain is still not well understood. Receptors break the odor down and then the brain reconstructs that odor, therefore the chemical nature, or constituents, of an odor are so important. There are multiple cortices of the brain involved, odor memory is stored in long term memory and is strongly connected with emotional memory. It is felt that the emotional memory is due to the olfactory system's close anatomical ties to the limbic system, an area involved with emotion. The limbic system includes the amygdala, hippocampus, basal ganglia and cingulate gyrus. These areas play an essential role in emotions, new and past memories, and these areas are located in the forefront of the brain, this portion of our brain allows rapid response to stimuli and relay of that information to the higher areas of the brain for cognitive processing. It is not surprising therefore that our most sensitive and primordial sense of smell connects to the primitive portion of our brain, as we evolved both areas developed early in our evolution and were essential to the survival and propagation of our species.

The sense of smell has been linked to our memory more than any of the other of our senses. A scent can conjure up a memory unexpectedly, both positive and negative. Odors are linked with early life experience memory regions of the brain in specific neural networks. These memories are brought back into our consciousness with the original intensity of when originally exposed to that odor. This phenomenon is known as the "Proustian memory effect". Primary memories linked to a scent stay with the individual throughout life. Significant experiences throughout our lives can still imprint a strong memory to a particular odor. An example of this would be people who were in combat, many have a strong aversion to the smell of meat burning on the grill or the chemical smell of cordite. Exposure to these odors can evoke memories tied to extreme emotional responses leading to flashbacks and PTSD exacerbations.

In addition to our own childhood and adult experiences that determine our own smell perception, cultural and geographic variances contribute greatly as well. Different plants grow in different geographic areas dependent on a variety of factors. The climate of an area, the soil composition, rainfall to that area etc. all dictate which plants grow in that location and their constituents, and even the same species of plant from different geographical areas will contain different constituents, all based on the exposure of those plants to the various elements during their lifetime. Cultures will utilize these plants based on the availability and utility of those plants, and therefore their odors will provoke different memories for people from different regions and cultures.

Throughout evolution and propagation of our species body odor memory plays a vital part in our detection of a potential mate, exclusion of kin as a potential mate, and the detection of friend or foe. PET scan (positron emission tomography) studies have shown that true body odor scents fire up dedicated different neural pathways in our olfactory system independent of other odors or fake body odor scents. Genuine body odors actually deactivated areas near the secondary olfactory cortex and instead lit up areas not typically used in olfaction, but used in recognizing familiar and fearful stimuli, therefore body odors are processed by a subnetwork of neurons in the brain separate from the main olfactory neurons. Studies have shown that females participants preferred the body odor of unrelated male participants, and these women found the body odors of men with the same MHC (major histocompatibility complex, genes that make up our immune system) genotype to be displeasing and repulsive. The sense of smell activates memories that can detect microbial risk, environmental threats, and MHC genotypes through body odors that play a critical role in mate selection, detecting fit partners and avoiding inbreeding. Kissing behavior is thought to have originally developed from sniffing, a primal behavior during which we smell and taste potential partners to see if they are a match.

Aromatherapy and The Olfactory System

Smell is highly emotive, it provokes an emotional response in an individual. The basis of the perfume industry is built around tapping into the emotions of the customers, provoking desire, sexuality, power, vitality or relaxation. Smells can evoke positive or negative psychological states of the mind and reactions in milliseconds. The emotional response to a smell varies widely from person to person, because the individual response to a scent is based on that individuals personal experience with that smell. A particular odor may be positive to one person based on pleasing childhood memories attached to that odor, while another person may find that same odor displeasing or even disgusting as in that individual the Proustian memories associated with that odor are distressing. The scents that one chooses to surround themselves with can drive personal behavior in positive or negative ways. The scents that one surrounds themselves with within their own environment allow that person to be in control, or the master of, an odor or fragrance that can be used as a tool to create a particular mindset, provoking motivation and/or relaxation. Senses are intertwined and not in a vacuum, we experience input from our physical world through all our senses congruently. Studies have shown that smell can affect how we perceive something in our visual field, and visual input can influence our sense of smell. The bond between taste and smell is widely known and proven for quite some time. Hunger can increase one's sensitivity to odors but not one's accuracy to those odors, while being full decreases sensitivity to an odor but increases accuracy to an odor. Studies have also shown that people that are overweight are more sensitive to odors than their thin counterparts.

aromatherapy and olfaction, aromatherapy and smell

Olfactory perception appears to be linked to personality traits as well. Test of subjects sniffing 3 shirts, two with stranger's body odor and one of their roommate's body odorfound that women were more accurate in detecting the roommates body odor than men, and women who consistently scored higher in body odor accuracy scored consistently higher on tests of emotional sensitivity, making them more sensitive to social smells and emotional signals. Multiple studies have shown that female participants were more dependent than male participants on olfactory clues used for daily decision making. Participants that evaluated their own olfactory functions more positively also relied more on olfactory information in everyday life. These studies have shown a correlation between neuroticism (individuals who are more emotional and anxious) and the ability to identify odors more consistently. Not only do neurotic individuals more accurately identify various odors, they are also more sensitive to them. In contrast, participants with high degrees of impulsiveness and assertiveness identify odors less accurately. In addition, it was found that regardless of whether odors were pleasant or unpleasant highly anxious individuals detected odors more quickly than did less anxious ones.

People with olfactory impairments tend to complain more strongly about decreased quality of life than people with normal olfactory function, and this was more pronounced in women than men. Males without a sense of smell reported significantly fewer sexual relationships compared to age-matched healthy males. Women without a sense of smell felt less secure about sexual partnerships compared with healthy women in the control group. The loss of smell (anosmia) has an enormous impact on humans, as it is the loss of our most sensitive and most primordial of our senses that allows us to interact and connect with the world around us. Losing one's sense of smell can result in the loss of important sentimental pathways to our memories, which of course is not exclusive of our emotions. The loss of smell can occur with disease states as well such as Parkinson's disease, multiple sclerosis and Alzheimer's disease, and in fact studies have shown that anosmia may be an early sign of these diseases, occurring several years before the motor skill problems arise. It has been found that personality traits modulate perceptions such as odor sensitivity, discrimination and identification. Neurodegenerative disorders, schizophrenia, multiple sclerosis, Parkinson's disease, Huntington's disease and myasthenia gravis are all conditions that have been shown to decrease or eliminate individuals sense of smell.

Study by Han-SeokSeo et al was performed to see if there was a potential connection between personality traits and attitudes towards the sense of smell. The study used the EPQ-R 48 self-reporting questionnaire examining four major dimensions of personality traits. Psychoticism- a subscale assessing behavior patterns used to characterize psychotic individuals or psychosis. Extravertsubscale measures the extentto which individuals are sociable and active. Neuroticism-subscale assesses the extent to which individuals are predisposed to experience negative emotion. Finally, the Lie-scale-subscale that reflects an individuals socially conforming behaviors or their tendency to ''fake good". Participants that judged their olfactory function to be more positive correlated with the fact that they more frequently used their sense of smell in everyday life and for decision making. Participants that showed socially conforming behaviors (scored high on the Lie-scale) where found to be more dependent on olfactory cues for daily decision making and socially communicating as well.

Psycho-olfactory processing is known to involve the orbitofronta! cortex (OFC) of the brain, which functions less effectively in individuals scoring higher in psychopathic personality traits. Studies show a relationship between psychopathy and olfactory discrimination and identification but not in odor threshold. The finding suggests that the brain areas subservicing higher olfactory processes- identification and discrimination- are somehow less efficient in individuals who scored higher on the psychopathic traits, related most likely to their processing in the OFC. (Chemosensory perception, Dec 2012, vo!5, issue 3-4, pp300-307,Mahmut M K et al).

The fact that the sense of smell interacts and communicates with the other senses is well known and documented, but the exact mechanism of action remains unclear. Scents convey a lot of information that help us interpret and interact with our environment, the world around us, the sense of smell impacts us all the time whether we recognize it or not. The olfactory system has connections to emotion, memory and social behavior dictating personality traits. Smell is key to our survival, we can detect millions of smells, these are the first warnings of danger and foe, of friend and mate. Smell drives our behavior on an instinctive primordial level, a subconscious level. The olfactory system displays robust and functional neuropathways throughout or entire lives, it contains cells with very broad developmental potential. The band cells of the olfactory epithelium are stem cells, capable of continuously regenerating olfactory receptor neurons throughout or lifespan. The complex processing of odorants by the olfactory system, the transfer of that information to the limbic system for primal interpretation, then the relay of that information to the higher cognitive cortex areas explains why there are definitive therapeutic benefit to aromatherapy by causing a cascade of physical and psychological events.


Dr. Tonda Bradshaw CPA

certified aromatherapist, student aromatherapist, aromatherapy certification

Dr. Bradshaw has been a practicing physician for over 25 years, board certified in Internal Medicine by both the American Osteopathic Association and the American Medical Association, and also carries board certification in Hospice and Palliative Care by the American Osteopathic Association. Seeing patients in her clinic, hospital setting, skilled nursing facilities as well as patients on hospice, Dr. Bradshaw understands that having knowledge into complementary treatment options would allow her to provide patients interested in alternative approaches a compressive integrative health plan. Patients need to be cautious of individuals claiming knowledge in the use of therapeutic grade essential oils without the proper education, as these essential oils contain powerful complicated constituents that are the foundation of medicines used in today’s pharmaceuticals. As a certified professional Aromatherapist, receiving her certification from the Institute of Holistic Phyto-Aromatherapy, Dr. Bradshaw is able to incorporate this aspect of alternative health care into her practice, for those patients desiring this option, in an individualized process with safety and health considerations being of the utmost importance. Dr. Bradshaw’s philosophy is to provide patients with the knowledge and support that they need to optimize their overall health physically, mentally, and spiritually within the framework of their lives. 



1. Han-Seok Seo et a I, 2013. Relationships between personality traits and attitudes toward the sense of smell, frontiers in psychology Nov 2013, vol. 4 article 901pp 1-8.

2. Weir, K 2011. Scents and sensibility, American Psychological Association Science Watch, Feb 2011, vol. 42, No.2

3. Bergland, C 2015. How does scent drive human behavior?, www.psychologvtoday.com/basics/unconcious/201506

4. Psychology and smell, 2015. www.fifthsense.org.uk/wpcontent/uploads/2015/06/Nasal-Olfactionl.jpg