It's one of the hottest topics for researchers who study smell and taste: Do humans communicate through subtle airborne chemicals called pheromones? A new study adds fuel to the debate. As researchers report in the 30 August issue of Neuron, men and women react in different ways to two chemicals similar to compounds found in sweat.
In animals, pheromones affect reproductive behavior, for instance accelerating puberty or blocking pregnancy. Researchers have shown that human sweat, when dabbed on a woman's upper lip, can synchronize menstrual cycles, but they have not isolated the compound responsible, and no one has chemically identified pheromones in humans. Many animals have a small bulge inside each nostril called the vomeronasal organ, a part of the nervous system that conveys pheromone signals directly to the brain. Humans have vomeronasal ducts inside the nose, but so far no one has proven that they communicate with the brain, and some argue that without that connection, humans cannot process pheromones.
Neuroscientist Ivanka Savic and colleagues at the Karolinska Institute in Stockholm investigated male and female responses to two hormonelike compounds, a derivative of testosterone called AND and a substance resembling estrogen, known as EST. They used positron emission tomography, or PET, to study the blood flow to various parts of the brain while a person breathed in one of the two potential pheromones. They found distinct patterns of brain activity: Males activated the hypothalamus when smelling EST, but not AND; in females, it was the other way around.
While that difference is interesting, says neuroscientist Charles Wysocki of the Monell Chemical Senses Center in Philadelphia, it's premature to conclude that the compounds are pheromones, because it's unclear whether they affect behavior or reproduction. "To actually have anyone define a compound or set of compounds as a pheromone requires a functional outcome," he says, "not just a PET scan of the brain."
Savic agrees. But even if these compounds are not natural pheromones, the sex-specific activation of the hypothalamus, a brain structure known to help process pheromones in some mammals, strongly suggests that humans can react to pheromones, she says.