Brain Activity Is Influenced By Chemical Signals Undetectable As Odors, Researchers Find

A team of scientists led by the University’s Martha McClintock, one of the nation’s leading experts on chemosignals and pheromones, and Suma Jacob, a University researcher, found marked differences in the brain activity among women exposed to the naturally produced male steroid androstadienone when compared to themselves when they had not been exposed to the substance.

A scan of the women’s brains after exposure to the steroid showed increased activity in regions associated with smell as well as those areas associated with vision, attention and emotion, the researchers report in a paper, “Sustained Human Chemosignal Unconsciously Alters Brain Function,” published in the current issue of the journal NeuroReport

Androstadienone is found in sweat, auxiliary hair, blood and semen. It is a common ingredient used in the manufacture of perfumes and colognes.

The results of the University of Chicago study suggest that chemosignals may be more influential than previously thought, said McClintock, the David Lee Shillinglaw Distinguished Service Professor in Psychology at the University.

“The widespread neural effects of this compound open the possibility that other olfactory signals may also have a significant impact on the whole human brain, even in the absence of conscious detection as an odor,” she writes with four co-authors in the paper. The lead author is University researcher Suma Jacob. Other authors are Leann Kinnunen, a University researcher; John Metz, a Research Associate in Psychiatry at the University; and Malcomb Cooper, Associate Professor of Radiology at the University.

In earlier work, Jacob and McClintock established that androstadienone moderates a woman’s mood. When women were exposed to a tiny amount of the steroid, they were able to maintain a positive attitude even after two hours of completing a tedious questionnaire.

In their current work, McClintock and Jacob sought to take the research to the next step to see how the steroid affects the brain. They used PET scan equipment to determine which areas of the brain were impacted by the androstadienone.

For the experiment, they told 10 women, aged 20 to 35, that they were conducting a study related to olfaction, but did not explain that they were studying the impact of the steroid.

Researchers placed a minute amount of androstadienone in propylene glycol. They used a small trace of clove oil in the solution to mask any possible odor. They tested each woman in separate sessions two days apart by swiping a sample of the clove-propylene glycol solution with the androstadienone under the women’s noses in one test and a sample of the same solution without the steroid in another.

In order to avoid the women responding to external emotional experiences and to create a similar testing situation on the two days of the tests, they had the women sit at a computer and perform simple, nonstressful tasks as they received a glucose solution intravenously.

The women then lay with their heads in an open ring device that is part of the PET equipment, which measures the amount of the glucose just used by their brain. The equipment produced a chart of brain activity that highlighted which areas of the brain became more engaged and which became less active.

The scan showed that while areas of the brain associated with olfactory processing were more engaged when the steroid was present other areas changed as well, including regions associated with attention, emotion and vision processing.

“During PET neuroimaging, subjects’ attention was focused on the visual discrimination task they had just performed on the computer. Our strong effects in this area support the hypothesis that androstadienone modulates ongoing behavior, or, more specifically, processing the task at hand,” said McClintock.

McClintock added that an alternative explanation, just as exciting, is the possibility that the steroid plays a previously unknown role in processing visual information, as areas of the brain associated with vision were particularly active. Further study is needed to determine more precisely the role of androstadienone on brain function, she said.