Sunday, April 5, 2015

We can’t really smell fear, but our brains can.

The brain is such a fascinating organ. Despite having spent a decade studying it, I learn more about the brain, its capabilities, and complexities every day. For example, I recently went to a fascinating talk where I learned that the brain recognizes others’ fear through chemicals in sweat. This post is about how we learned that. 

Dr. Lilliane Mujica-Parodi, a professor at Stony Brook University, is interested in how emotions are communicated between individuals. We have 5 senses: visual (sight), tactile (touch), auditory (sound), gustatory (taste), and olfactory (smell). In particular, Dr. Mujica-Parodi is interested in how emotions are communicated through smell. We know that facial expressions help us visually communicate with one another and language helps us communicate with auditory information, but what communicative purpose does smell have?

To answer this question she collected two different types of sweat: fear-induced sweat and physical exertion induced sweat. The fear-induced sweat was collected from 144 individuals who went tandem skydiving for the first time, including a 1 minute free fall from 13,000 feet.. The physical exertion induced sweat was collected while these individuals ran on a treadmill. Their hypothesis was that the fear-induced sweat would contain different olfactory information

They then recruited a group of 16 individuals (50% female) and had them smell 40 different sweat samples (20 of each type) for 2.5 seconds each. Participants smelled each of these samples from inside an fMRI scanner, while the research team recorded activation of their amygdala, the fear recognition center of the brain. They found that participants had great activation in the amygdala when they were smelling the samples taken from people while sky-diving compared with when smelling the samples taken during exercise.

They then recruited 16 new participants (50% female), and repeated the experiment with 40 new sweat samples, half collected during skydiving and the other half during exercise. They found the same thing: participants demonstrated greater amygdala activation, or recognition of fear, when smelling the sweat collected from people while skydiving.

But were the participants aware that they were exposed to two different types of sweat? It doesn’t appear that way.

To test whether the two types of sweat were noticeably different to the participants, they recruited 16 more participants (50% female), and asked them to smell 16 pairs of sweat samples. After smelling each pair of samples, they were asked to decide whether the samples were both the same or different from one another. The results of this experiment suggest that the participants were unable to accurately report any differences between the two types of sweat.

Finally, they conducted a 4th experiment to test whether exposure to fear sweat influenced perception of emotions in others. Because, after all, what good would it be for our brains to recognize fear in others through chemical signaling, if that different give us any behavioral or perceptual benefits? To do this, they recruited 14 more participants and had them complete a computer task where they saw faces with different expressions ranging from neutral to angry and had to report whether the face was “neutral” or “threatening.” The participants completed half of the trials while smelling the fear-induced sweat and half while smelling the exercise-induced threat. They found that after smelling the fear-induced sweat, participants were more accurate at identifying whether a face was threatening or neutral than when they smelled the exercise induced threat.

I am always impressed by all of the things our brain is constantly doing outside of our awareness, and these findings are no exception. Basically, this study demonstrates that while we aren’t aware of it, we can smell fear in others, the fear circuits in our brain are activated, and that activation leads us to more accurately perceive threat in our environment. 

As a person who would never go skydiving, and who barely rides the kiddie-roller coaster at amusement parks, this makes me wonder about the role of these olfactory signals in amusement parks and haunted houses. Part of our anticipation of the ride could be driven by our brains picking up on sensory information that people have been scared here before. But what about other emotions? Might there be chemical signals for happiness, desire, sadness that communicate our experiences with people who will come after us? Are some people more sensitive to olfactory emotional signals? Is that a strength or a vulnerability? This study controlled for potential differences between men and women, but I would imagine that men and women differ quite meaningfully in how receptive they are to olfactory signals of emotions in others.

As is the case with many studies using brain imaging, there were very few people in each of the experiments I described so the findings warrant replication. As is the case with all cutting edge research, we are left with more questions than answers, but that’s what is so exciting about the field of psychological science. Stay tuned!


Mujica-Parodi, L. R., Strey, H. H., Frederick, B., Savoy, R., Cox, D., Botanov, Y., ... & Weber, J. (2009). Chemosensory cues to conspecific emotional stress activate amygdala in humans. PLoS One, 4(7), e6415.

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