Research on the neurological structures that are responsible for olfaction and emotion are discussed. The investigation focuses on studies that measure emotional physiological response to olfactory stimulation. Instruments mentioned are the electroencephalography (EEG), positron emission tomography scan (PET scan), and electrocardiogram (ECG). Basic findings include an increase of activity for odorants associated with arousal and a decrease of activity for relaxing scents. Other physiological associations include systolic blood pressure, microvibration, peripheral vasal constriction, heart rate, respiration, and electro-dermal activity. Some inconsistencies are noted in the research. Some studies suggest interplay of perception as well as personality in the effects of the odor stimulation. The concepts of aromatherapy and osmotherapy are also briefly discussed.

Cortical structures responsible for processing olfactory stimuli and emotion are located within the Limbic System (King, 1988). The Limbic System represents the hippocampus, fornic, cingulate gyrus, thalamus, mamillary bodies, amygdala and olfactory bulb. Science has accepted the role of the Limbic System in memory, learning, and emotion (Rosenzweig, Leiman, and Breedlove, 1999). Prior to the coining of the term Limbic System, this group of structures was referred to as the rhinencephalon or "smell brain" (King, 1988).

Olfaction begins when scents enter the nasal cavities and stimulate over 50 million receptor neurons (Travis, 1999). Specialized receptors react to distinct odors that allow for human recognition of over 10,000 scents (Axel, 1995). The site of the odorant receptor cells is referred to the olfactory epithelium and is located along the dorsal region of the nasal cavity, including the septum, which divides the cavity (Rosenzweig, Leiman, and Breedlove, 1999). Two types of receptor cells, the signaling cells and the axons, have been observe through the use of a scanning electron micrograph (Axel, 1995). The signaling cells, known as cilia extend to the mucous barrier of the nasal cavity and detect initial odor stimuli from the air (Rosenzweig, Leiman, and Breedlove). The information gathered by the cilia is transmitted to the axon, which transmits the incoming information to the olfactory bulb (Travis, 1999). This is the site where odor is first processed (Axel, 1995). The extent of processing being performed by the olfactory bulb defines it as a sorter of smells, distributing information yielded by the odor to the proper Limbic structures (Travis, 1999). The olfactory bulb directs information to the prepyriform cortex, amygdala, hypothalamus and other areas of the brain. These structures send information to the terminal points of the medial dorsal thalamus, orbitofrontal cortex, and lateral posterior orbitofrontal cortex (Rosenzweig, Leiman, and Breedlove, 1999). These areas of the brain are noted as being responsible for behavior and the processing of thoughts (Axel, 1995).

The sense of smell has been noted as being more emotional than intellectual (King, 1988). Emotion could be described as consisting of three aspects; subjective feelings, states of physiological arousal, and actions in response to specific stimuli (Rosenzweig, Leiman, and Breedlove, 1999). An accepted theory of neurological processing of emotion is Papez's circuit of emotion. This involves the limbic structures; hypothalamus, mamillary bodies, anterior thalamus, amygdala and the cingulate cortex (Rosenzweig, Leiman, and Breedlove, 1999).

Emotions have been observed as being processed by brain structures through the use of physiological measures such as the positron emission tomography, magnetic resonance imaging, and electroencephalography. Increased brain activity in the areas of the bilateral interior and orbitofrontal cortex has been associated with sadness. On the other hand, happiness seems to deactivate the right prefrontal and temporal-parietal regions of the brain suggesting that separate neural networks govern the opposite emotions (Lane, Reiman, Ahern, Schwartz, and Davidson, 1997). It has been suggested that the differentiation of brain response to particular emotions may be categorized as evaluative, experiential, and expressive (Lane, Reiman, Ahern, et al., 1997).

Positive emotions have been observed as occurring juxtaposition increased activity in the entorhinal cortex (a Limbic structure) while negative emotions have been associated with activation of the medial thalamus and left orbital frontal cortex (Paradiso, Robinson, Andreasen, Downhill, Kirschner, Watkins, Ponto, & Hichwa, 1997). Overall, emotions studied under laboratory conditions have been measured by increased activity bilaterally in the occipitotemporoparietal cortex, lateral cerebellum, hypothalamus, anterior temporal cortex, amygdala, and hippocampal formation (Reiman, Lane, Ahern, et al., 1997).

An interesting study that supports the findings of Ekman (as cited in Rosenzweig, Leiman, and Breedlove, 1999) was done on the change of brain activity through manipulating facial expressions. In this study, magnetic resonance imaging recorded an increase in amygdala activity when subjects were presented with a masked fearful facial expression. The difference was significant in respect to neutral and happy masked facial presentations (Whalen, Rauch, Etcoff, McInerney, Lee, & Jenike, 1998). This finding infers that visual stimulation may enhance or depress a person's mood.

As noted earlier, the olfactory bulb distributes odorant information to other areas of the brain that processes behavior and basic thought. For example, research has shown that 40% of neurons associated with the amygdala react to the odor presence (Zald and Pardo, 1997). This would implicate the sense of smell as having an influence on human emotion (Lawless, 1991). It has been suggested that study the effects of olfactory stimulation on emotion has not been readily studied due to the difficulty in reporting and recreating scents experienced by individuals (King, 1988).

Despite this difficulty, researchers have attempted to study the physiology of odor and mood and have found some interesting results. A concept that seems to be key in the examination of odor and mood interdependence is the notion of hedonics. This is the degree of pleasantness an odor is perceived as possessing (Jellinek, 1994). Hedonics has been noted as eliciting emotional responses such as fear, withdrawal, and happiness (Zald and Pardo, 1997). Such reactions to olfactory stimulation imply that there is an interplay of neurological structures responsible for the processing of olfaction and emotion (King, 1988). Structurally it has been found that the primary olfactory cortex is located at the anterior of the amygdala and extends to the amygdala and posterior orbitofrontal cortex along with the perirhinal, entorhinal, and insular cortices (Zald and Pardo, 1997).

One method of measuring the neurological activity of brain structures is through the use of electroencephalography (EEG). There are two types of encephalogram scores. One measures distinct brain wave patterns that result from being in an aroused state. This is known as the contingent negative variation. The other examines and records different brain wave patterns during relaxed and stressed states, termed as period analysis. Olfactory stimulation is also measured under this second procedure (Lawless, 1991).

As measured by the EEG, alpha waves, which are associated with relaxation, have been observed as increasing in the presence of odors that are assumed to be relaxants, such as lavender. Frontal alpha waves have been subsequently reported as being reduced during states of alertness (Diego, Jones, Field, Hernandez-Reif, Schanberg, Kuhn, McAdam, Galamaga, & Galamaga, 1998). This difference of brain activity response to specific odors has been noted as being hemisphere specific (Lorig and Schwartz, 1988). It has also been inferred that the degree to which a subject responds to an odor depends on the degree of arousal the subject currently exhibits. This theory was tested through the use of caffeine stimulation, resulting in significant differences for alert and relaxed subjects when tested with the EEG under the same odor stimulation (Lawless, 1991).

EEG results indicate that olfactory stimulation does influence the physiological response of the central nervous system (Lorig and Schwartz, 1988). It has also been suggested that the concentration of odor does not have to be great enough to be noticed consciously to have an effect on the central nervous system (Lorig, Herman, Schwartz, and Cain, 1990). This has been noted as a possibility to the difficulty associated with the examination of odor and mood (Lorig, Herman, Schwartz, and Cain, 1990). This may also be the root of the controversy that the effect of odor on mood is a matter of perception (Jellinek, 1994).

Lorig, Herman, Schwartz, and Cain (1990) attempted to probe into the question of perception by examining undetected odors and their effect on the central nervous system as measured by the EEG. They found that EEG readings were similar to those noted under detectable odor presence. As seen in the work of Lorig and Schwartz (1988) there were differences in the response of the right and left hemispheres under certain odor stimulation. It was also reported that the intensity of the response did increase parallel to the increase of odor presented. The greatest difference noted was between low to medium concentrations. This may suggest that there is a reduction or extinction of odor detection after a concentration increases as a function of time (Travis, 1999). Desensitization of odor presence may also be a primitive brain activity that depends on innate perception of odors, a concept not readily explored (Axel, 1995). EEG testing also suggests that odor's effect on mood may remain for some time after the presence of the odor is removed (Lorig and Schwartz, 1988).

Jellinek (1994) reports that contingent negative variation measurement of the EEG find that odorant inhalation seems to produce a reaction of various activity in the frontal and left central areas of the cortex. Significant findings implicate a relaxing effect, defined as a decrease of cortical function, during exposure to the scent of lavender. Subsequently, alerting odorants, such as jasmine, were observed as increasing brain activity. The scores yielded by the inhalation of the two odors were comparable and similar to the alerting effect of caffeine and the relaxing effect of sleep. Jellinek (1994) also noted that odorants perceived as being related to happiness or excitement, according to self-report, resulted in an increased contingent negative variation. This supports the notion that olfactory stimulation can alter mood states (Diego, et al., 1998).

Another way to measure physiological response to olfactory stimulation is through the use of a positron emission tomography scan (PET scan). The PET scan records regional cerebral blood flow (rCBF) (Zald and Pardo, 1997). This procedure involves injecting radioactive substances into living subjects allowing for the measurement of brain activity (Rosenzweig, Leiman, and Breedlove, 1999).

PET scan research conducted by Zald and Pardo (1997) found that distinct brain activity at the amygdala was evident under a condition of adverse olfactory stimulation. It was observed that amygdala activation was accompanied by increased activity in the left posterior-lateral orbitofrontal cortex. This supports the findings that examine the basic information gathered and distributed by the olfactory bulb (Rosenzweig, Leiman, and Breedlove, 1999). Further implication of the interaction of these structures comes from the finding that rCBF significantly changed in the amygdala under a pleasant odor condition (Zald and Pardo, 1997).

On the other hand, aversive odorants seemed to increase the activity of the amygdala bilaterally along with surrounding neurological structures that include the inferior insula (Zald and Pardo, 1997). This finding suggests that differentiation of neurological reactions are sympathetic to perceptions of hedonics (Jellinek, 1994). Zald and Pardo (1997) also found that there was an increase of activity during aversive odor stimulation that is consistent with the structure of the primary olfactory cortex, with responses from the frontal and temporal lobes.

Besides the results produced by the PET scan, subject's self reports implicate changes in respiration and muscle tension during the presence of aversive odor. In regards to hedonics, the subject's perception of the pleasantness of the condition odor effected the cerebral blood flow as detected by the PET scan (Zald and Pardo, 1997). This is supportive of the notion of hedonics as presented by Jellinek (1994).

A more unique way to study physiological changes as a result is through the use of an electrocardiogram (ECT). This is a unique instrument to be used in the investigation of odor and mood and is not often seen. In a study done by Sudakov and Uryvaev (1999), subjects were exposed to either perceptible or imperceptible concentrations of oxytocin or distilled water. ECG recordings suggested that sympathetic and parasympathetic activation were stimulated by odor. Subjects were observed, after being pricked in the finger, that the rate of blood clotting differed significantly under the various conditions. This was measured between and within subjects. The greatest difference noted was the increase of blood clotting when the sympathetic nervous system was activated by olfactory stimulation.

Physiologically, the interaction of odor and mood has been observed as seemingly causing a reaction in structures and systems that extend beyond the central nervous system. Microvibration, defined as a tremor that results from changes in muscle tension, has been observed as changing as a result of odorant presence (Jellinek, 1994). Muscle tension has been reported as increasing positively in relation to the degree of adversity that the odor is perceived as possessing (Zald and Pardo, 1994). Averse odorants may produce an arousing effect measured by an increase of microvibration as observed with odors that are regarded as being excitatory such as jasmine, chamomile, and musk (Jellinek, 1994). This observation of physiological changes occurring under odorant stimulus further implicates the interaction of the Limbic structures that are responsible for olfaction and emotion (Lorig and Schwartz, 1988).

Sudakov and Uryvaev (1999) found that the sympathetic and parasympathetic nervous systems were effected by odorant stimulation, as measured by the electrocardiogram. This is further supported by research that reports a peripheral vasal constriction as a result of olfactory stimulation (Jellinek, 1994). This constriction of the vascular structures outside the heart and central nervous system results as a response of the sympathetic nervous system to odorants that spur physiological stress, such as peppermint (Jellinek, 1994).

Systolic blood pressure has also been noted as being reduced by relaxing scents such as lavender (Romine, Bush, and Geist, 1999). This relaxing effect has been described as being effective substantially to the point that sleep has been induced by lavender olfactory stimuli (Hirsh, 1995). Other odorants that have been observed as decreasing systolic blood pressure and reduced stress include nutmeg, mace extract, and valerian oil (Jellinek, 1994). Some researchers believe in the findings of relaxing effect on systolic blood pressure to the point that applications to patent certain odorants as stress reducers have been filed (Lawless, 1991). The premise of its use is as a preventative measure of counteracting the effects stress has on increasing blood pressure (Jellinek, 1994). This has been supported by reduced brain activity when in the presence of lavender as measured by the EEG (Diego, et al., 1998). Physiologically, such findings have been useful in calming the body after stressful exercise (Romine, Bush, and Geist, 1999). This implicates practical use of the knowledge that olfactory stimulation has a physiological effect that influences emotion (King, 1988).

As a result of the functions of the peripheral nervous system, it has been observed that respiration changes in response to aversive olfactory stimulation (Jellinek, 1994). In cases where respiration differentiation was the result of odorant presence, similar reactions were recorded in the amygdala (Zald and Pardo, 1994). The research indicates that respiration increases as a result of inhaling odorants that are thought to stimulate a person psychologically and vice versa (Lawless, 1991). This reaction has been suggested to be experientially learned (Rosenzweig, Leiman, and Breedlove, 1999).

Heart rate has also been found to change under olfactory stimulation (Romine, Bush, and Geist, 1999). Specifically, it was found that pleasant odors that activate the central nervous system also increase heart rate, while sedative odors decrease heart rate (Jellinek, 1994). This decrease or increase of heart rate mimicked the reaction that systolic blood pressure had to odorant presence (Lawless, 1991). This reaction was noted as being enhanced substantially by the degree of preference or hedonics associated with the condition odorant (Jellinek, 1994). This physiological interpretation of the interaction of odor and mood is supportive of the claims of that propagate the effects of essential oils (Cerrato, 1998). Regarding cardiovascular reaction to the relaxing effect of lavender essential oil, it was also observed that pulse decreased as a physiological relaxing effect of the scent (Romine, Bush, and Geist, 1999).

Jellinek (1994) also reports that electro-dermal activity (EDA) increase during arousal. EDA, a process where the electrical current between two points of the skin is measured, has been noted as varying dependent on the odor present. Slower currents were observed as a result of the inhalation of relaxing odorants such as lavender and bergamot. This combination of odor had a more dramatic effect on EDA than either scent presented singly, supporting the assumption that there are specialized receptor neurons in the olfactory epithelium (Axel, 1995). This in turn would enhance the stimulation of structures that receive information from the olfactory bulb such as the prepyriform cortex, hypothalamus, and amygdala (Rosenzweig, Leiman, and Breedlove, 1999).

Unfortunately, this emotional reaction to the singular or combination of olfactory stimulants has been observed as being temporal and becoming undetectable as a function of time (Jellinek, 1994). This may be why individuals become used to an odor or need increased concentrations to achieve the same emotional effects or for mere detection. This assumption would be supportive of the findings that undetectable odors have an effect on emotion but are enhanced, as higher concentrations are introduced (Lorig, Herman, and Schwartz, 1990).

Physiological consequences of the inhalation of scents that are thought to enhance or depress an individual's mood have spurred the introduction of aromatherapy (Jellinek, 1994). This is the practice of using plant extracts in the form of essential oils to alter mood states (Lawless, 1991). This practice is based on the physiological reactions noted to occur as a result of olfactory stimulation, with or without support of cognitive processes (Jellinek, 1994). Research that investigates the relationship of odor and mood, whether physiologically or cognitively, uses the techniques instituted in aromatherapy, as in research by Romine, Bush, and Geist (1999).

Although the structures responsible for olfaction and emotion have been extensively studied and replicated independently, there has been little research regarding the interaction of the two systems (Zald and Pardo, 1997). Aroma-Chology, the study of the olfactory system's role in producing emotional response using a holistic approach that considers physiology as well as cognition, attempts to remedy this lack of empirical research (Jellinek, 1994).

Furthering this effort, Dodd and Van Toller (as cited in King, 1988) have reported corresponding physiological responses of odorant stimulation and the effects of neuroleptics, which act with cortical receptor sites similarly. This has lead to the creation of the term osmotherapy, an attempt to replace the less respected term of aromatherapy. This research has also acknowledged that there is a potential benefit of olfactory stimulation as an alternative to psychotropic drugs. This would eliminate the side effects produced by changing the chemical make-up of the organism.

Conversely, it has been suggested by Gellhorn and Loofbourrow (as cited in King, 1988) that an individual's emotional state may actually influence the hedonics of the presented scent, enhancing pleasantness through the influence of the trigeminal nerve. Part of the peripheral nervous system, this cranial nerve is responsible for some motor functions as well as processing facial sensation (Rosenzweig, Leiman, and Breedlove, 1999). This supports the notion that facial expressions influence emotions (Whalen, et al., 1998). This may also be the reason that individuals express different facial expression in the presence of either pleasant or aversive odorants. This facial muscle reaction may be a more simplistic method for observing odor's influence on mood.

In general, the studies suggest that there is an interaction of odor and mood. Physiologically, it would make sense that the different structures within the Limbic System would react together to some degree (Zald and Pardo, 1997). Trends between research regarding this subject suggest that olfactory stimulation may be arousing or calming depending on the source of olfaction (Lorig and Schwartz, 1988). Some researchers have accepted that essential oils have certain therapeutic effects on the human organism. Diego, et al. (1998) states that the oils, derived from plant extracts, have a "predictable and reproducible effect on the user" when its fragrance is inhaled" (Diego, et, al, 1998, p. 218).

Kikutchi et al. (as cited in Jellinek, 1994) found that the concentrated scent of rose caused an increase of heart rate. On the other hand, when measuring the effects of olfactory stimulation on EDA, the scent rose was found to be a relaxant (Jellinek, 1994). Inconsistencies of this sort give rise to further investigation in order to pinpoint the specific scents that stimulate reliable physiological responses. Lorig and Schwartz (1988) stated that the interaction of odor and mood might be dependent on perception. If this is the case, experiments should be constructed that examine the types of perceptions or perhaps personality, an aspect not prevalently studied, are associated to emotional alteration dependent on a certain odorant. Being able to introduce this method of therapy to the public would require reliable and valid measures. Odor and mood research, studied physiologically could provide the empirical evidence necessary to give justification to such concepts as aromatherapy or osmotherapy as replacements of psychotropic medications such as anti-depressants (King, 1988).

Axel, R. (1995). The molecular logic of smell. Scientific American, 273 (4), 154-160.

Cerrato, P. L. (1998). Aromatherapy: is it for real? RN, 61 (6), 51-53.

Hirsh, A.(1995). Aromatherapy: Lavender fragrance is sleep inducing. Brown University Long-Term Care Letter, 7 (19), 6-9.

Jellinek, S. J. (1994). Aroma-chology: A status review. Cosmetics and Toiletries, 109 (10), 1-28.

King, J. R. (1988). Anxiety reduction using fragrances. In S. V. Toller & G. H. Dodd (Eds.), Perfumery: The psychology and biology of fragrance (pp. 154-158). New York, NY: Chapman & Hull.

Lane, R. D., Reiman, E. M., Ahern, G. L., Schwartz, G. E., & Davidson, R. J. (1997). Neuroanatomical correlates of happiness, sadness, and disgust. The American Journal of Psychiatry, 154, 926-933.

Lawless, H. (1991). Effects of odors on mood and behavior: Aromatherapy and related effects. In R. G. Laing, R. L. Doty, & W. Breifohl (Eds.), The human sense of smell (pp. 361-386). Berlin: Springer-Verlog.

Lorig, T. S., Herman, K. B., & Schwartz, G. E. (1990). EEG activity during administration of low-concentration odor. Bulletin of the Psychonomic Society, 28 (5), 405-408.

Lorig, T. S., & Schwartz, G. E. (1988). Brain and odor: I. Alteration of human EEG by odor administration. Psychobiology, 16 (3), 281-284.

Paradiso, S., Robinson, R. G., Andreasen, N.C., Downhill, J. E., Davidson, R. J., Kirschner, P. T., Watkins, G. L., Ponto, L. L., & Hichwa, R. D. (1997). Emotional activation of limbic circuitry in elderly normal subjects in a PET study. The American Journal of Psychiatry, 154, 384 - 389.

Reiman, E. M., Lane, R. D., Ahern, G. L., Schwartz, G. E., Davidson, R. J., Friston, K. J., Yun, L. S., & Chen, K. (1997). Neuroanatomical correlates of externally and internally generated human emotion. The American Journal of Psychiatry, 154, 918-925.

Romine, I. J., Bush, A. M., & Geist, C. R. (1998). Lavender aromatherapy in recovery from exercise. Perceptual and Motor Skills, 88, 756-758.

Rosenzweig, M. R., Leiman, A. L., & Breedlove, S. M. (1999). Biological Psychology (2nd ed.). Sunderland, MA: Sinaur Associates.

Sudakov, K. V. & Uryvaev, Y. V. (1999). Odogenic change of blood coagulation in humans. Integrative Physiological & Behavioral Science, 34 (3), 150-155.

Travis, J. (1999). Making sense of scents. Science News, 155 (15), 236-239.

Whalen, P. J., Rauch, S. L., Etcoff, N. L., McInerney, S. C., Lee, M. B., & Jenike, M. A. (1998). Masked presentations of emotional facial expressions modulate amygdala activity without explicit knowledge. Journal of Neuroscience, 18, 411-418.

Zald, D. H., & Pardo, J. V. (1997). Emotion, olfaction, and the human amygdala: Amygdala activation during aversive olfactory stimulation. Proceedings of the National Academy of Sciences of the United States, 94 (8), 4119 - 4125.

Olfactory Stimulation and Emotion are processed in close proximity within the Limbic System.

Olfactory System

· Olfactory epithelium - site of reception

· Olfactory bulb - distributor of odor information

· Olfactory cortex - processes information

Structures active during emotional processing

· Bilateral interior and orbitofrontal cortex - sad

· Right prefrontal and temporal-parietal regions - happy

· Entorhinal cortex - positive emotions

· Medial thalamus, left orbitofrontal cortex - negative emotions

· Lateral cerebellum, hypothalamus, anterior temporal cortex, amygdala, and hippocampal formation - researched for emotional involvement

· Facial expressions - emotional response

Measurement of Emotional Response to Olfactory Stimulation

· PET scan(positron emission tomography) - records blood flow in the brain

· EEG (electroencephalography) - records electrical activity in the brain

· ECG (electrocardiogram)

Physiological Reactions (beyond neurological)

· Systolic blood pressure

· Muscle tension (microvibration)

· Peripheral vasal constriction

· Heart Rate

· Respiration

· EDA

Where researchers are trying to go with this information

· Aromatherapy

· Osmotherapy

Where should the researchers be going with this information?

· Psychotropic substitution

· Defining the specific scents

· Clarifying the role of personality and preference in effects & make them work toward the solution

· Move beyond preliminary research

· Test the existing research