When I Get That Feeling, I Need Sexual Sweating

Did you know that male "sexual sweat" differs from ordinary sweat? Apparently so, according to a new paper in the Journal of Neuroscience (Zhou & Chen, 2008). Curiously, the article did not cite any references for this, nor did it specify the chemical composition of sexual sweat. Nonetheless, 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. How was such a study conducted, you might ask?


And here the fun begins...

Sweat collection. From 2 d before the experiment until the end of the experiment, 20 heterosexual male donors in a larger study refrained from using deodorant/antiperspirant/scented products, and used scent-free shampoo/conditioner, soap, and lotion provided by the experimenter. They reported to have experience with watching sexually explicit videos, and signed informed consent before participation. Subjects kept a 4" x 4" pad (rayon/polyester for maximum absorbance) in each armpit while they watched 20-min-long video segments intended to produce the emotions of sexual arousal (sexual intercourse between heterosexual couples) and neutrality (educational documentaries), respectively. ... Over the course of the 20 min video segments, donors experienced greater arousal (measured by skin conductance) while watching erotic videos than while watching neutral videos... Three healthy, male nonsmokers (aged 26, 29, and 29 years) were subsequently selected for the current study mainly because of their higher level of the self-reported sexual arousal.

How were the female participants selected?

We recruited only women for their superior sense of smell and sensitivity to emotional signals. Twenty right-handed females (mean age = 23.4 years) were selected from a group of 42 women on the basis that they reported to have no rhinal disorders or neurological diseases, and that they showed superior olfactory sensitivity to PSP [the putative sex pheromone androstadienone] and PEA [phenyl ethyl alcohol]. They either were in a heterosexual relationship or had been in one within the previous year. They were not on hormone contraceptives, and were tested during the periovulatory phase of their menstrual cycles. ... Subjects were informed that the study was on brain activations to natural compounds. They were blind to the nature of the smells used in the experiment.

The scanning was performed while the women were inhaling...

...the sweat of sexual arousal in comparison with two other social chemosensory compounds (PSP and the sweat of neutrality) and a nonsocial smell [phenyl ethyl alcohol (PEA)].

The sweat of neutrality. The sweat of sexual arousal! [plus the two others.] The subjects rated the four inhalants (presented 10 times each) on intensity and pleasantness, as shown below. And the smell of sexual sweat was not particularly pleasant...


Figure 1. Mean intensity and pleasantness ratings. There are four types of olfactory stimuli, and SE bars are shown. For intensity, 1 refers to no smell, 2 little smell, 3 moderate smell, 4 quite a bit smell, and 5 strong smell. For pleasantness, 1 refers to very unpleasant, 2 unpleasant, 3 neutral, 4 pleasant, and 5 very pleasant. Sex, Sexual sweat; Neutral, neutral sweat. Sexual sweat and PSP were perceived to be more intense than neutral sweat; PEA was perceived to be more pleasant than sexual sweat and neutral sweat.

At the end of the experiment, the participants gave verbal descriptions of the smells. Only one characterized sexual sweat as "sweaty/human." So the women were not [consciously] aware that the odor was obtained from sexually aroused men.

The right hypothalamus showed increased activity to sexual sweat relative to alcohol, but so did androstadienone and neutral sweat. The two brain regions that responded more to sexual sweat than to the other odors are illustrated below. The right orbitofrontal cortex is an olfactory region, but the right fusiform gyrus is a high-level visual region. The authors say their fusiform region¹ falls in the vicinity of the fusiform face area (FFA) and fusiform body area (FBA). Hmm.


Figure 3. a. Coronal view showing an area in the right orbitofrontal cortex (33, 40, –1) activated in the omnibus ANCOVA F test (svc, p less than 0.005). d. Sagittal view showing a region in the right fusiform gyrus (35, –51, –7) activated in the omnibus ANCOVA F test (uncorrected p less than 0.0005, cluster size = 49 mm³).

The authors took a giant leap when speculating about visual imagery of faces and bodies:

The Talairach coordinates of the fusiform region identified in our experiment fall in the range of the coordinates for FFA and FBA. Such anatomical location likely reflects a recognition of the human quality in the sexual sweat, whose emotional nature may have also contributed to the activation. Considering its functional connectivity to the right hippocampus/ parahippocampal gyrus, the recognition may arise from implicitly associating the sexual sweat with humans based on past experience. The fact that most subjects did not perceive the sexual sweat as human related suggests that the effects we observed occurred at a subconscious level.

Wholly unconscious face/body visual processing in response to a sexual chemosensory cue? In the absence of any specific activity in the hypothalamus or amygdala? That's a hard one to swallow.

Footnote

¹ The FFA and FBA have been dissociated with scanning at high resolution.

Reference

W. Zhou, D. Chen (2008). Encoding Human Sexual Chemosensory Cues in the Orbitofrontal and Fusiform Cortices Journal of Neuroscience, 28 (53), 14416-14421 DOI: 10.1523/JNEUROSCI.3148-08.2008

Chemosensory communication of affect and motivation is ubiquitous among animals. In humans, emotional expressions are naturally associated with faces and voices. Whether chemical signals play a role as well has hardly been addressed. Here, we use functional magnetic resonance imaging to show that the right orbitofrontal cortex, right fusiform cortex, and right hypothalamus respond to airborne natural human sexual sweat, indicating that this particular chemosensory compound is encoded holistically in the brain. Our findings provide neural evidence that socioemotional meanings, including the sexual ones, are conveyed in the human sweat.