Natural Attraction

Study Finds Brain Reacts To
Sex-Specific Chemicals

By Shankar Vedaniam

Certain chemicals similar to the male and female sex hormones trigger distinctive brain activity when sniffed by the opposite gender, providing the strongest evidence yet for the existence ofhuman “pheromones,” scientists reported yesterday.

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, which convincingly demonstrated that the effect of these chemicals on the brain is not because of their odor, will be of interest to romantics, pharmaceutical companies and savants of armpit chemistry.

Although human pheromones have long been embedded as real in the public imagination, spawning a bustling market of perfumes and potions for suitors seeking to turn on the opposite sex, scientists have long debated whether they existed.While those scientific questions persist, the new research suggests that 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 Noarn Sobel, a neuroscientist at the University of California at 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.

“I’m leaving open the possibility that these and other compounds may be human pheromones but one should not walk away after reading this paper that these two compounds are the [only] human pheromones, and that one affects females and the other affects males,” said Charles J. Wysocki, a neuroscientist at the Monell Chemical Senses Center in Philadelphia. “There’s much much more work that needs to be done.”

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

There has been some evidence that humans are similarly influenced by pheromones. Women living in groups, for example, begin to tend to menstruate together -- an effect that some scientists believe is caused by pheromones. But scientists have been unable to identify any specific chemicals that clearly act like pheromones, or convincingly prove that humans have active 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.

None of the chemicals seemed to be particularly striking in terms of odor. Participants rated all about average. Men did not show any striking. brain response to the testosterone-related chemical and women’s brains were likewise blase to the estrogen-related compound.

“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.”

Whatever the something else, it caused an activation in the part of the brains of both men and women that is involved in regulating sex — the hypothalamus, Wysocki said.

Activation of the hypothalamus in monkeys, according to published research, elicits penile erections in male monkeys and copulatory behavior in females, said Savic, in a telephone interview.

“In rodents, the females start ovulating as soon as they smell a male pheromone,” she said. “There is no way that pheromones are going to induce such a reflex in humans because we are so much more complicated.”

Among other differences, humans have a much larger frontal cortex than most animals, and it is the part of the brain that is involved in many higher cognitive functions, including inhibition and self-control.

Of the two chemicals studied, said Bernard Grosser, chairman of the department of psychiatry at the University of Utah School of Medicine, androstadienone, the testosterone-related compound, was shown to reduce pulse rate, increase temperature and produce relaxation inwomen in other studies. No obvious sexual effects were observed.

The estrogen-like compound produced similar, but not identical, effects in men. Feelings of well-being and relief from tension could be part of humans’ sexual responses, of course, and Grosser pointed out that other chemicals may also be involved in regulating human sexual behavio~.

Savic, who published her research in this week’s issue ofthe journal Neuron, said that it was interesting that the men and women in her study chose “animalistic” associations when asked to describe the two chemicals. Some said it smelled “sweaty” or like “an animal.”

“One of my female subjects said this smells like my sister’s old sanitary napkins,” said Savic. “To me, that’s very illustrative.”

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