Wednesday, 30 March 2011
Wen Zhou and colleagues scanned the brains of nineteen women while they were exposed to the smell of two types of men's sweat, a floral scent, and the human steroid (and putative pheromone) androstadienone. One of the male sweat types was sexual, the other was neutral, and they were collected from men's armpits as they watched either a sexual film or an educational documentary. The women weren't told what the different smells were or where they came from.
Human sweat is known to convey social signals. For example, it's been shown that people can tell a person's emotional state purely from the smell of their sweat. The key findings in this new study are that the two types of sweat, compared to the other odours, led to increased activation in the orbitofrontal cortex (OFC) of the women's brains, and that the level of this activation was related to the women's amount of self-reported trait social anxiety. The women didn't have any psychiatric diagnoses but the higher they scored on a measure of trait social anxiety (e.g. they said they felt uncomfortable in large groups), the less activation they exhibited in their OFC when exposed to the men's sweat.
It's important to emphasise that most of the women (nearly 90 per cent) didn't realise the smells were from humans, and the smells had no effect on their in-the-moment mood or anxiety levels. Consistent with this, the different smells didn't differentially affect the amygdala, a bilateral subcortical structure associated with fear processing. What the study appears to be showing is that subconscious social signals trigger increased OFC activity compared with nonsocial smells, and that the level of this activity is moderated by trait social anxiety.
Why the OFC? The OFC is heavily interconnected with the amygdala and is known to be involved in the learning of rewards and punishments and in decision-making. Another brain imaging study found that public speaking was associated with increased activation in the amygdala and reduced activation in the OFC. So it makes sense that people with a predisposition for social anxiety may have an OFC cortex that functions differently from those without such a disposition.
'Whether such inherent variations can be directly mapped onto genetic differences or personality traits in both normal and clinical populations, is an important open question and this deserves serious studies in the future,' the researchers said.
Zhou, W., Hou, P., Zhou, Y., and Chen, D. (2010). Reduced recruitment of orbitofrontal cortex to human social chemosensory cues in social anxiety. NeuroImage DOI: 10.1016/j.neuroimage.2010.12.064