I Know What You Sweated Last Summer

From the authors who first brought you "sexual sweat" (Zhou & Chen, 2008)...

Be afraid... be very afraid and prepare yourself for the sequel: "FEARFUL SWEAT" (Zhou & Chen, 2009)!!!

In case you didn't know that "sexual sweat" (collected from men watching porn) differs from ordinary sweat, the results of an fMRI experiment suggested that the orbitofrontal cortex and the fusiform region in 20 female participants responded differently when smelling the two substances (Zhou & Chen, 2008). However, we don't know anything specific about the unique chemical composition of sweat obtained from sexually aroused men, and why it resulted in differential brain activity in women who could not identify the odor as "sweaty/human" (see When I Get That Feeling, I Need Sexual Sweating).



Nonetheless, in the present study Zhou and Chen (2009) wanted to determine the effects of another putative chemosensory signal on the perception of emotional expressions in faces. Specifically, as they explain below...

...we conducted two experiments focused on the effect of a fear-related chemosignal (sweat collected from donors viewing horror videos) in an emotion-identification task. We used the same type of olfactory stimuli (emotional sweat collected on gauze pads and gauze pads with no sweat) throughout, but varied the effectiveness of the visual input by varying the ambiguity of the facial emotions (from somewhat happy to ambiguous to somewhat fearful). Our manipulation of ambiguity was achieved through morphing between happy and fearful faces [as shown in Fig. 1a].

Fig. 1a (Zhou & Chen, 2009). Examples of the morphed faces of two actors. For each actor, we selected seven morphs, ranging from somewhat happy to somewhat fearful. These faces were judged to be fearful 20% to 80% of the time in our pilot experiment, in the absence of any olfactory stimuli. Specifically, the Level 4 morph for each actor was the most ambiguous, judged to be fearful in the pilot study 45% to 55% of the time.

And what about the olfactory stimuli obtained from the male sweat donors?

On the day of each session, they wore next to their skin a new T-shirt (provided by the experimenter), to prevent odor contamination by their regular clothes. During each session, they kept a 4- x 4-in. pad (rayon-polyester blend for maximum absorbance) under each armpit while they watched each of three 20-min video segments intended to produce the emotions of fear (horror movies), happiness (slapstick comedies), and neutrality, respectively. Different videos were shown in each session. During the videos, participants’ heart rate was recorded... After watching each video, the donors rated how angry, fearful, happy, neutral, and sad they felt during the video, using a 100-mm visual analog scale. From each donor, we selected the pads worn during the 20-min videos that elicited the highest level of self-reported happy feelings and the highest level of self-reported fearful feelings.

So the 48 young female subjects (mean age 19.6 years) viewed the various faces while exposed to different olfactory stimuli, and decided whether they were happy or fearful. Results indicated that on average they were significantly more likely to identify the most ambiguous morph as fearful when smelling the fearful sweat relative to the control condition (which, unfortunately, was a rayon-polyester pad with no sweat). Although the likelihood of identifying an ambiguous face as fearful did not differ between the happy sweat and control conditions, there was no direct statistical comparison between the two sweat conditions, which would seem to be a problem.


adapted from Fig. 2b (Zhou & Chen, 2009).

Nevertheless, there was some evidence that male horror movie sweat was able to bias the women towards viewing an ambiguous face as fearful, and this was not due to the pleasantness (or lack thereof) or intensity of the olfactory stimulus. I'd be curious to see how the "sweat of neutrality" and the "sweat of sexual arousal" [as identified by Zhou & Chen, 2008) in their earlier study] would influence emotion recognition judgments...


References

Zhou W, Chen D. (2008). Encoding Human Sexual Chemosensory Cues in the Orbitofrontal and Fusiform Cortices. Journal of Neuroscience, 28 (53), 14416-14421.

Zhou, W., & Chen, D. (2009). Fear-Related Chemosignals Modulate Recognition of Fear in Ambiguous Facial Expressions. Psychological Science, 20 (2), 177-183. DOI: 10.1111/j.1467-9280.2009.02263.x

Integrating emotional cues from different senses is critical for adaptive behavior. Much of the evidence on cross-modal perception of emotions has come from studies of vision and audition. This research has shown that an emotion signaled by one sense modulates how the same emotion is perceived in another sense, especially when the input to the latter sense is ambiguous. We tested whether olfaction causes similar sensory modulation of emotion perception. In two experiments, the chemosignal of fearful sweat biased women toward interpreting ambiguous expressions as more fearful, but had no effect when the facial emotion was more discernible. Our findings provide direct behavioral evidence that social chemosignals can communicate emotions and demonstrate that fear-related chemosignals modulate humans’ visual emotion perception in an emotion-specific way—an effect that has been hitherto unsuspected.

Bonus! See sensory psychologist and olfactory specialist Avery Gilbert's take on these two studies in Basic Instinct: The Smell of Fear and Sex.



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