You Smell!: A Look into Olfactory Hallucinations

Biology 202
2006 Second Web Paper
On Serendip

You Smell!: A Look into Olfactory Hallucinations

Carolyn Dahlgren

If you were to take a poll asking people to name which of the five senses that they could most easily live without, I am willing to bet that the sense of smell would be many people's top choice. There are, however, many people who do live without the sense of smell; just like a blind person lives without sight or a deaf person without sound. Anosmia is a condition that occurs when a person lacks the sense of smell. Dysfunctions of the sense of smell can be congenital. A number of different diseases, conditions, and medications can lead to olfactory disturbances. It may also occur for other reasons; "A number of different diseases, conditions, and medications can lead to olfactory disturbances. Major causes of olfactory dysfunction: obstructive nasal and sinus diseases, upper respiratory viral infections, head trauma, and in 22% of cases no cause is ever found (idiopathic)" 1.

Your sense of smell may seem like a trivial sense compared to something like sight or sound. The role of the nose, however, is severely under appreciated. Most people realize that the senses of smell and taste go hand and hand. When I was a child, my mother used to have me hold my nose before drinking medicine in order to diminish the taste. When you have a cold and your nose is stuffed-up, things taste bland or strange. The sense of smell enhances the flavor of the foods we consume. Lacking a sense of smell may seem like it could be a blessing, especially if one has to take some noxious medicine or, perhaps, if a skunk is nearby. The idea, however, loses some if its charm when thinking about what it would be like to have a deadened sensation of pleasant stimuli such as homemade cookies or chocolate cake. The appeal of anosmia is totally striped away if you imagine unintentionally eating spoiled food. Our sense of smell is an important self-preservation device. "It serves as an important early warning system for the detection of fire, dangerous fumes, leaking gas, and spoiled food" 1. Smell is also plays a key part in social interactions. "It enhances socialization and interpersonal relationships by protecting against objectionable body odors" 1. Smells have also been found to be a form of communication. Pheromones are chemical signals, smells that animals use to transmit messages to other organisms. "There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology" 8.

Unlike the other five senses, smell and taste are known as the chemical senses. This is because these senses are the result of external chemical stimuli acting directly upon sensory neurons. Airborne chemicals, which are actually particles from the things that are smelled, stimulate special receptors, called chemoreceptor because the chemical interact directly with these receptors. "These receptors are very small - there are at least 10 million of them in your nose - ...each with the ability to sense certain odor molecules. Research has shown that an odor can stimulate several different kinds of receptors. The brain interprets the combination of receptors to recognize any one of about 10,000 different smells" 3. When these receptors are stimulated, information is sent along the olfactory nerve to the olfactory bulb which is the part of the brain that is associated with the sense of smell. "The olfactory bulb is the most rostral (forward) part of the human brain" 7 and is located just above the nasal cavity. It sends signals to the other parts of the brain to help interpret the sensory information it receives and to translate them into the different smells we can recognize.

Thus far, this paper has discussed anosmia, the lack of ability to sense smells. There are, however, a wide range of other olfactory dysfunctions; "approximately two to five million American adults suffer from disorders of taste and smell" 2. There are two particularly intriguing disorders that I wish to highlight: parosmia and phantogeusia. Parosmia is a distortion of the olfactory sense. People with parosmia report smelling scents that are incongruent with the olfactory stimuli presented. An "affected person reports smelling something other than the scent which is present... for example, the person sniffs a banana but it smells like rotting flesh instead of a banana" 9. Phantogeusia is an 'olfactory hallucination'. A person reports a smell (and sometimes an associated taste in the back of the mouth) for which no external stimulus can be found. "There is no odorant present, but the affected person reports smelling something, usually something unpleasant" 9. Phantogeusia smells are not only noxious, they are also longer lasting than most olfactory experiences. "When a normal person smells an unpleasant scent, sensory adaptation takes place rather quickly -- within a few minutes the scent seems to have disappeared. The unpleasant scents in parosmia and phantosmia can, however, be very long-lasting" 9.

Why do some people have these olfactory misinterpretations? What is the brain doing during olfactory hallucinations? Unfortunately, there is not a very in-depth range of literature about olfactory dysfunctions. There are, however, some interesting similarities between smell 'hallucinations' and other reports of 'hallucinations', namely the phantom limb phenomenon experienced by some amputee victims. Perhaps olfactory hallucinations could be understood by extrapolating from some theories about phantom limbs. The phantom limb sensation is the experience of a feeling, usually pain, in a limb that has been amputated. Even though nerve connections have been severed and the nervous tissues have been taken away, many amputee outpatients report experiencing sensations in their amputated limbs. In a previous web paper for our course, Christy Taylor explored the etiology of phantom limb hallucination and explained that the phenomenon may be a product of inconsistencies between the sensory information received by the brain and the brain's corollary discharge signals, signals from the output side of the brain which sends feedback to the brain about motor output. "Corollary discharge signals are used to define expectation, and when the sensory input does not match this expectation, the nervous system sends out a signal that says 'there is something wrong' which may be felt as pain in the phantom limb"6. Phantom limb pains, like the unpleasant scents experienced in parosmia and phantosmia, are often intense and long-lasting.

If sensations of noxious smells are the olfactory equivalent to pain, then parosmia and phantosmia seem to be an olfactory equivalent to phantom limb pain. This observation may be a useful, new perspective on these olfactory dysfunctions, but it still does not explain why they occur. Why should a person receive sensory inputs from the nose that are incongruent with the corollary discharge messages? For phantom limbs this is a simple matter, there are no sensory inputs from the amputated limb and the brain thinks there should be, but for parosmia and phantosmia there is no reason for the signal to be incongruent. This is especially true for this 'chemical' sense of smell - olfactory sensory input is a result of direct interaction of neurons and molecules from the things that are smelled. And what about people with anosmia? Why don't they have phantom smells? There are still a lot of questions to explore but one thing is clear, the olfactory sense is a lot more complicated than we may think and we should appreciate our working senses of smell.

Bibliography

1. Anosmia Foundation. "Anosmia". http://www.anosmiafoundation.org/intro.shtml.

2. Anosmia Foundation. "Smell Disorders". http://www.anosmiafoundation.org/smell.shtml.

3. Cook, Steven P., MD and Gavin, Mary L. Gavin, MD. "What's That Smell? The Nose Knows". http://kidshealth.org/kid/body/nose_noSW.html. Updated: July 2004.

4. How Staff Works. "The Nose". http://health.howstuffworks.com/define-nose.htm.

5. "The Sense of Smell". http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/O/Olfaction.html. Updated: Oct. 22, 2005.

6. Taylor, Christy. "Phantom Limbs". http://serendip.brynmawr.edu/bb/neuro/neuro98/202s98-paper2/Taylor2.html. Updated: 1998.

7. Wikipedia. "Olfactory Bulb". http://en.wikipedia.org/wiki/Olfactory_bulb.

8. Wikipedia. "Pheromone". http://en.wikipedia.org/wiki/Pheromone.