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'Odotope Theory', also known as Weak-Shape Theory, is a leading neurophysiological theory of how the sense of smell functions. The model is analogous to a molecular Braille: it is propsed that any number of the 347 different protein-based smell receptors in the nose binds to only certain parts of a smellable molecule, and thus a few hundreds of different receptors can, through combinatorial explosion, theoretically recognize an infinite number of distinct smells.
Odotope theory sidesteps the issue of how a limited number of shape detectors could identify more than 10,000 smells (and counting) in the human alone.
For more information see:
• Burr, Chandler. The Emperor of Scent: A true story of perfume and obsession. Random House, New York: 2002.