What the Nose Knows - Avery Gilbert [56]
How to Create an Odor Phobia
Omer Van den Bergh makes people sick. He is a researcher at the University of Leuven in Belgium who has developed a surefire way to induce temporary (yet harmless) physiological distress. He does it by increasing the carbon dioxide level in the air, a simple move with unpleasant consequences. Within twenty seconds of breathing CO2-enriched air, a person experiences tightness in the chest, a feeling of choking or smothering, a pounding heart, sweating, hot flushes, and anxiety. The symptoms disappear quickly when the CO2 is reduced to normal levels.
Van den Bergh uses CO2 to explore the psychological mechanisms of odor aversion. In the basic setup, a volunteer breathes scented CO2-enchanced air and experiences the usual unpleasant symptoms. When the volunteer returns to the lab the next day and breathes normal air with the same scent, he feels ill again—though there is no physical basis for the reaction. Van den Bergh has conditioned his subjects to feel sick in the presence of an odor, just as Pavlov conditioned his dogs to salivate at the sound of a bell. Remarkably, all it takes is a single episode of physical distress to turn an odor into a trigger for illness. Van den Bergh calls this process “symptom learning,” to reflect the fact that it’s a form of associative learning, a basic process by which organisms respond to their environments. Symptom learning works better with malodors, such as ammonia and butyric acid, than with a pleasant, fresh scent like eucalyptus.
Another hallmark of learned aversion is that it spreads from one odor to another in a process known as stimulus generalization. For example, when Van den Bergh conditioned people to become ill at the smell of ammonia, he found they would experience symptoms in a later test when the air was scented with another unpleasant odor, such as butyric acid (smelly feet) or acetic acid (vinegar). The subjects would not become ill to an entirely different smell such as citrus, however. Generalization to related odors can happen as long as a week after the initial event. One consequence of this is that a brief exposure to a sickness-inducing smell leaves a person psychologically vulnerable for days to acquiring additional odor triggers.
If an odor aversion can form after one exposure, and if the aversion can generalize to similar smells, what stops it from becoming a psychological chain reaction? Why isn’t everyone gagging all the time? The answer is a phenomenon called extinction. When an illness-associated odor is repeatedly presented without elevated CO2, the Pavlovian response eventually fades away as the brain unlearns its conditioned response. When the odor no longer triggers symptoms, the response is said to have been extinguished. Therapists use the extinction phenomenon to help people overcome phobias to spiders, closed spaces, and so on; they call it systematic desensitization therapy.
Bad smells are natural candidates for Pavlovian conditioning, but even pleasant ones can trigger symptoms, as we saw in the example of cologne-wearing soldiers on body retrieval duty. Pleasant scents can become triggers under less dramatic circumstances, if they are given the proper psychological “spin.” In another of his experiments, Van den Bergh had test subjects read a leaflet beforehand, which discussed chemical pollution and described a patient with MCS. (The text was lifted from an environmentalist website.) In the experiment, the pamphlet’s negative spin increased CO2-induced illness to pleasant as well as unpleasant odors. Thus, even a nice fragrance becomes a trigger for acquired illness if one believes, for example, that its chemical composition is harmful. Van den Bergh sees an irony in this: “warnings and campaigns against environmental pollution, while having important beneficial effects for the environment, may inadvertently facilitate acquiring symptoms to chemicals in the environment