Something Might Just Be in the Air

WASHINGTON — Certain chemicals similar to the male and female sex hormones trigger distinctive brain activity when sniffed by the opposite sex, providing the strongest evidence yet for the existence of human “pheromones,” scientists reported Monday.

Brain scans of two dozen volunteers in Sweden found that a part of the brain involved in regulating sexual behavior lit up when women were exposed to a substance similar to testosterone, while the same brain area in men lit up when they were exposed to a substance similar to estrogen.

The research, published in this week’s issue of the journal Neuron, convincingly demonstrated that the effect of these chemicals on the brain is not because of their odor.

While human pheromones have been embedded as real in the public imagination, spawning a bustling market of perfumes and potions, scientists have long debated whether they exist. The new research suggests at least some human behaviors may be subliminally influenced by invisible chemicals with no obvious odors.

“It’s great, it’s very exciting and very interesting,” said Noam Sobel, a neuroscientist at UC Berkeley who studies pheromones.

No visual or auditory signal that he could think of, said Sobel, has ever been known to produce so sharp a distinction between men and women. “Is it proof that these are pheromones?” asked Sobel. “No, but it is another block in the wall and it is a block in the wall that closes up the hole.”

While animal studies have shown that the part of the brain activated in the new study--the hypothalamus--is associated with reflexive sexual responses, it remains unclear whether humans necessarily respond in similarly predictable ways.

Besides sex, pheromones are widely involved in regulating other behaviors in the animal world. Animals, for example, mark territory using pheromones. Scientists also have found a piece of tissue in the nasal passageways of animals called the vomeronasal organ, or VNO, which they use to sense pheromones.

There has been some evidence that humans are similarly influenced by pheromones. Women living in groups, for example, tend to begin to menstruate at the same time--an effect that some scientists think is caused by pheromones. But scientists have been unable to identify specific chemicals that clearly act as pheromones or convincingly prove that humans have VNOs.

Researchers at the Karolinska Institute in Stockholm exposed a dozen men and a dozen women to a variety of smells. One was plain air, another vanilla. The two others were the testosterone- and estrogen-related compounds. While the subjects breathed the chemicals for a minute, the researchers conducted scans of their brains.

“One would expect that females would find the female compound more pleasant, but there was no difference,” said Ivanka Savic, a neurologist in Karolinska’s department of neuroscience. “There was no [smell] difference between males and females. This means it is not the smell component that is responsible for the sex-specific activation. It’s something else.”