Women who were exposed to chemicals from fear-induced sweat performed more accurately on word-association tasks than did women exposed to chemicals from other types of sweat or no sweat at all. The study was published this month in the journal Chemical Senses.
"It is well-documented in the research literature that animals experiencing stress and fear produce chemical warning signals that can lead to behavioral, endocrinological and immunological changes in their fellow animals of the same species, but we wanted to see if this applies to humans as well," said principal investigator Denise Chen, assistant professor of psychology at Rice.
For the study, Chen collected samples of sweat from research volunteers who kept gauze pads in their armpits while they watched videos of horror movies and nonthreatening documentaries. The sweat samples were then stored in a freezer until needed for the study.
Next, Chen had 75 female students between the ages of 18 and 22 respond to 320 pairs of words that flashed for three seconds each on a computer screen. For each pair, the participants had to press a key to indicate whether the words were associated with each other (for example, arms and legs) or not (arms and wind). Some of the words were associated with threatening or fear-related topics, like weapons.
Each participant had a piece of gauze attached above their lips so that they were exposed to either chemicals from sweat or none at all during the tests. Chen compared how the chemicals from sweat impacted the speed and accuracy of participants' results on the word-association tests.
When processing meaningfully related word pairs, the participants exposed to the fear chemicals were 85 percent accurate, and those in either the neutral sweat or the control (no-sweat) condition were 80 percent accurate. "The subjects in the fear condition were six percent more accurate, which is a statistically significant difference," Chen said.
When processing word pairs that were ambiguous in threat content, such as one neutral word paired with a threatening word or a pair of neutral words, subjects in the fear condition were 15 to 16 percent slower in responding than those in the neutral sweat condition, and this difference was statistically significant. Chen's theory is that the chemicals from fear-induced sweat prompted subjects to be more cautious.
The research participants were not aware of the nature of the smells, and the smells did not differ on the intensity or pleasantness ratings.
"We demonstrated that in humans, chemical signals from fear facilitated overall accuracy in identifying word relatedness independent of the perceived qualities of the smells," Chen said. "The effect may arise from a learned association, including greater cautiousness and changes in cognitive strategies."
"Human olfaction is a young, vibrant field," Chen said, noting that the behavioral study of this subject is still in the early stage. "Olfactory receptors were discovered in the early 1990s. We now know that olfaction involves hundreds of receptors."
Results like these from Chen's behavioral research and studies from other labs form an integral part of a multipronged approach to the understanding of human olfaction.
Coauthors of Chen's study included former Rice undergraduate students Ameeta Katdare and Nadia Lucas, a Rice Century Scholar.
Chen's research was funded by the National Institutes of Health.