When One Plus One Equals New

Last year, Linda B. Buck sent e-mails to fellow olfaction researchers in search of observations relevant to a discovery she and Zhihua Zou had made at the Fred Hutchinson Cancer Research Center in Seattle about how mammals sense mixtures of odorants.

A reply from a fragrance chemist stood out. Eugenol, an organic chemical that smells like clove, and phenylethyl alcohol, which smells like roses, can smell like carnations when both are sniffed at the same time. In the mind's nose, the two odors morph into a distinct third odor.

That perfumer's tidbit was thrilling to Buck, who in 2004 shared a Nobel Prize in Physiology or Medicine for discoveries about odorant receptors and the olfactory system. After all, she and Zou had just found in mice what well could be the neural basis for such perceptions (Science 2006, 311, 1477).

Using a technique for visualizing if and when cells in a mouse's olfactory system respond to odorant stimuli, the researchers found a population of olfactory cortex cells that respond only to paired odorant molecules, but not to either odorant alone. Among the pairs tested were ethyl butyrate (apple) and vanillin, and dimethylpyrazine (chocolate, nuts) and eugenol.

Each odorant generates a code of sorts when it interacts with different sets of receptors on different sets of cells in the animal's nose, Buck says. The new findings demonstrate that initially segregated "receptor codes" merge onto the same cortical neurons as part of the brain's reconstruction of the "odor image" in the environment.

"It is a logical interpretation that these cells are uniquely activated by mixtures of odorants that might underlie perceptual quality differences," such as the carnation sensation emerging from the clove-rose combo, notes Leonardo Belluscio, who studies the mammalian olfactory system at the National Institute of Neurological Disorders & Stroke in Bethesda, Md.