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Yes. Smell can be misleading in ways that resemble optical and audio illusions: context can change how an odor seems, and mixtures can produce percepts or neural responses that are not a simple sum of their ingredients. The key is to distinguish a perception shaped by context from a change in the odor itself—and from a smell disorder.
What counts as an olfactory illusion?
A useful definition is a mismatch between an odor stimulus and the perception it produces. That can happen when the same odor seems different under different conditions, or when different stimuli lead to a similar percept. In a 2011 review, Richard J. Stevenson describes both kinds of cases and notes why smell illusions can be difficult to notice: odors are hard to verify independently, people have less shared vocabulary for describing them, and gradual changes may not register consciously. Read the review.
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Olfactory illusions are not all the same kind of effect. A useful comparison with visual or auditory illusions asks whether the stimulus stayed constant, whether contextual information changed, whether an odorant was present, and whether the effect concerned identity, quality, intensity, or pleasantness. The evidence also matters: a reported human perception, a brain-imaging result, and a recording from animal sensory neurons answer different questions.
Can a label change how a smell seems?
Yes. In a 2001 experiment, Rachel Herz and Trygg Engen von Clef tested five odors: violet leaf, patchouli, pine oil, menthol, and a mixture of isovaleric and butyric acids. Participants encountered positive or negative verbal labels in separate sessions one week apart and rated their perceptions and interpretations. The authors reported that labels significantly influenced perception and proposed that instances in which a label reversed the perceived odor were olfactory illusions. The label changed the information participants had about the odor, not its molecules; this was a specific experimental finding, not proof that labels always override smell. Read the study abstract.
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A later study examined verbally induced olfactory illusions in early-blind, late-blind, and sighted participants. Its abstract reports more amplified illusions in both blind groups than in the sighted group in that study’s paradigm. That finding should be kept within the tested task: it does not establish a general difference in smell perception among all blind and sighted people, and the mechanism remains uncertain. Read the study abstract.
Can two smells combine into something different?
Mixtures can be more complicated than adding the perceived effects of each ingredient. In a 2020 study summarized by the National Institutes of Health, researchers imaged mouse olfactory sensory neurons responding to odor blends. One odor could change a neuron’s response to another even when the first odor alone did not activate that cell. As a result, the blend’s neural response differed from the response predicted by simply adding the individual odors. This is evidence about mouse olfactory coding, not a direct human demonstration of a named illusion. Read NIH’s summary.
A separate human fMRI study explored jasmine, indole, and a pleasant mixture of the two. Brain regions associated with pleasantness represented the mixture more like jasmine, while regions associated with unpleasantness represented it as retaining more of indole’s unpleasant component. That result suggests a mixture can seem pleasant overall while the brain still represents information associated with an unpleasant ingredient. Read the study abstract.
Why can concentration change an odor’s character?
Some odorants smell different at different concentrations. Indole, for example, is described as floral at low concentrations and putrid at higher concentrations in an NCBI Bookshelf chapter. This is a change in stimulus conditions, so it is useful to distinguish it from a context-induced illusion in which the same odor is presented but information such as a label changes. Read the chapter.
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- Author: Deveraux, Jude.
- Publisher: Pocket Books
- Pages: 432
- Publication Date: 2010-12-28
- Edition: Original
How is an illusion different from a smell disorder?
Illusion experiments examine how perception can be shaped by context or by the way odors interact. Smell-disorder terms describe changes in a person’s ability to detect or perceive odors. The National Institute on Deafness and Other Communication Disorders (NIDCD) defines:
- Hyposmia: reduced ability to detect odors.
- Anosmia: complete inability to smell.
- Parosmia: distortion of an odor that is present—for example, a familiar smell seeming different or a pleasant odor seeming foul.
- Phantosmia: sensing an odor when no odor is present.
Parosmia involves a real odor perceived in a distorted way; phantosmia is an odor sensation without an odor stimulus. These clinical descriptions are not interchangeable with a laboratory demonstration that a label or mixture can shape perception. See NIDCD’s overview of smell disorders.
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What the examples do—and do not—show
Olfactory illusions are real, but the term covers different phenomena rather than one universal mechanism. A verbal label can influence people’s ratings of an odor; mixtures can alter neural responses or yield a complex combination of pleasant and unpleasant qualities; and concentration can change odor character because the stimulus itself changes. Those findings make smell comparable to vision and hearing in one important respect: perception is not a transparent readout of the outside world. They do not mean every surprising smell is an illusion or that every illusion works the same way.
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