In addition to our sense of smell, do we have the ability to sense certain chemical signals emitted by people around us—without being aware of it? Many other mammals use a separate set of sensory receptor cells in their nose to receive social and sexual information from members of their own species, and there is growing suspicion that we do, too.

A whiff of airborne chemicals from a female mouse, for instance, may spur a male mouse to mate immediately. Certain chemical messages from other males may make him aggressive. Other messages may produce changes in his physiology—as well as in that of the responding female.

The effects of such messages would be far less obvious in humans. If we do receive chemical signals from people in our vicinity, these signals must compete with many other factors that influence our behavior. Yet our physiology may be just as responsive to chemical messages as that of other mammals. It is known that certain chemical messages from other mice lead to the onset of puberty in young males, while a different set of signals brings young female mice into estrus. Similarly, there are some suggestions that women may alter their hormonal cycles when exposed to chemical signals from other people.

In the past five years, scientists have become extremely interested in these signals, as well as in the "accessory olfactory system" that responds to them in many animals. This system starts with nerve cells in a pair of tiny, cigar-shaped sacs called the vomeronasal organs (VNOs), where the signals are first picked up.

"The VNO appears to be a much more primitive structure that uses a different set of molecular machinery than the main olfactory system," says Richard Axel, who has become intrigued with this system. "It seems to work in a different way—and we don't know how."

The VNOs are located just behind the nostrils, in the nose's dividing wall (they take their name from the vomer bone, where the nasal septum meets the hard palate). In rodents, at least, signals travel from the VNO to the accessory olfactory bulb (rather than to the main olfactory bulb) and then, as Sally Winans of the University of Michigan showed in 1970, to parts of the brain that control reproduction and maternal behavior.

"It's an alternate route to the brain," explains Rochelle Small, who runs the chemical senses program at the National Institute on Deafness and Other Communication Disorders in Bethesda, Maryland. If the accessory olfactory system functions in humans as it does in rodents, bypassing the cerebral cortex, there is likely to be no conscious awareness of it at all.

— Maya Pines

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The VNOs are narrow sacs, only a few millimeters long. They lie on either side of the nasal septum, quite far from the olfactory epithelium.

Illustration: Eade Creative Services, Inc./George Eade illustrator (adapted from The Journal of NIH Research Vol. 6, January 1994)

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