A curious comment just added to the discussion page of Wikipedia’s illusion entry has really got me thinking:
the beginning of the article claims that all human senses can be fooled. I’ve yet to expirence an illusion of taste/smell. i.e. something salty tasting sweet. it may follow that consumption is the ‘truest’ of human expirences… [sic] Andrew
The only example that I could find on PubMed suggests that we experience taste in areas of the mouth without taste receptors, because we are fooled by the touch sensations of the food in our mouths. I could find no similar ‘smell illusions’.
If anyone knows of any examples of taste or smell illusions I’d be very interested to hear about them.
UPDATE: There’s some great answers on the comments page. Keep ’em coming. Thanks!
Mind Hacks 
I use the tongue tactile-taste illusion as a demonstration in Intro Psych. Some students seem not to experience the illusion… Not sure why. The students enjoy the demonstration though. Most have absorbed the myth that we have areas of our tongue which are localized for certain flavors. After demonstrating that is not correct, I do the illusion demo. Fun stuff.
It might help to consider for a second what actually _could_ constitute an illusion in each of the sensory domains.
The senses of sight, hearing and touch can and do directly interact in the brain; the inputs from each sensory system converge on polymodal regions of the brain such as the superior colliculus. And there’s a reason for this: because the data from these modalities can be used for cross-validation and mutual calibration. So I might suspect that I am experiencing an illusion when my senses of sight and touch produce contradictory information. I could be _sure_ that I was experiencing an illusion if I resorted to measurement using external tools such as light meters or what have you. But often I don’t have to do this, because my other senses operate in an intuitively homologous coordinate system (i.e. 3D space) so that sight, hearing and touch can quite reliably be used to cross-check one another.
Now, as far as I know (which is not very far!) the signals from our olfactory neurons don’t converge on polymodal regions until we’re really far up the “sensory processing stream”. Notice also that our sense of taste/smell has low spatial and temporal resolution. And it’s not clear what everyday tools we could use that would provide us with information on whether or not we were experiencing the smell/taste that we “should”.
So consider, for example, an aftertaste. Is that an illusion? Perhaps — just as a Mach band is perceived between areas of light and darkness — an aftertaste of X is perceived between regions of space/time containing and not containing the diffuse presence of the chemical producing taste X.
So maybe we experience illusions in the smell/taste modality all the time but we don’t notice because there’s no cross-validation. And perhaps there’s no cross-validation because (a) it isn’t of much use to us and (b) it would be difficult to implement since smell/taste doesn’t operate in the same spatial coordinate system as do sight, hearing and touch. (?)
Here is a great example, and one I’ve lusted after for some time. miraculin, a glycoprotein expressed by the Miracle Fruit berry, is tasteless. However, it binds to the tongue and, in a poorly understood mechanism, makes acid foods taste sweet. Attempts have been made to commercialize the protein, with little success. Proteins are notoriously unstable, adopting a “native fold” that is rapidly lost if the right solution and temperature conditions are not satisfied.
If anyone can get me some miraculin, miracle fruit berries, or a miracle fruit plant, I will be forever grateful.
I have meant to write this up on the blog for awhile now. You may have inspired the next post!
How about the classic illusion involving both smell and taste, where you hold a piece of pear under someone’s nose, then give them a slice of apple to eat, and it tastes like pear?
People respond differently to an odor in a test tube if they’re told that it is fancy cheese than if they’re told that it is feces. In Malcolm Gladwell’s Blink, there are also examples where people’s reports of how much they like the taste of some food product depend on its packaging and the brand name that labels it. I don’t know if these are properly called illusions, though, or if they’re just evidence that smell and taste (or at least evaluations of smells and tastes) are not purely bottom-up processes.
Odor is also similarly fooled. Indole, for example, smells intensely of feces. At high dilution, it takes on a pleasant odor of orange blossoms. Indole is in fact present in both feces and neroli oil. It is also an article of commerce in perfumery. It is used as a fixative (long-lasting fragrance note) because of its low volatility. Also contributing to this is its lipophilicity; indole sticks to skin like mad. I have made various indoles and can attest to what happens if you touch even a single crystal of them without gloves – the perverse fascination of sniffing your finger for DAYS and having synthesized, well, essence of crap. Soap and bleach are only marginally helpful.
Phenylacetic acid smells like urine, or honey at dilution. Present in both. While perhaps not a case of “fooling smell,” enantiomers (mirror images) of the same compound can smell different. The enantiomers of carvone smell respectively of anise and spearmint. I have blog entries on several of these which I’d link if I were allowed. If you follow the link above it is searchable (or look under the “perfumey” and “stinky” categories).
I was reading a great article on the Miracle Fruit a couple of weeks ago; they managed to get around the berry’s fragility and grow it in the US in the 1970s, but never managed to get FDA approval. It does seem that seeds are available, though.
(Sorry, didn’t realise you stripped URLs. http://quisqualis.com/mirfrtdmc1a.html)
I’m not sure whether this constitutes a true illusion of taste, but if you put salt on a fresh pineapple, it will actually taste sweeter than it did before. Try it and taste… 🙂
Another trick I know, and love doing with students, has to do with smell and habituation.
If you take a strong smell, any strong smell, for example essence of almond and mix a tiny bit of it it into a quantity of vinegar (and this may be a very large quantity of vinegar even) people, when they smell it, will smell just the vinegar. When, however, you let them smell pure vinegar for a minute or two, and then give them the vinegar with a tiny bit of essence of almond mixed in, they will smell the almond. Actually most people won;t even the smell the vinegar at all, they’ll just smell the almond.