The idea that the mere act of smiling can cheer us up is traceable to Darwin's "facial feedback hypothesis" and also to the work of famous American psychologist William James, who believed that muscular as well as more visceral bodily feedback influences our emotional state.
Modern research showing the influence of facial expression on mood has supported these classic theories, but an aspect that remains unclear is whether it is the neural command to pull a given facial expression that influences emotion centres in the brain, or if instead or additionally, it is feedback from the skin and muscles regarding the position of our facial features.
Now Andreas Hennenlotter and colleagues have devised an ingenious way to test these possibilities, by conducting experiments on women who'd received botox (botulinum toxin) injections to their face for cosmetic reasons, thus rendering them unable to flex the frown muscles of their face (corrugator supercilii).
Thirty-eight women had their brains scanned while they imitated pictures of sad or angry facial expressions. Crucially, half of them were tested prior to receiving botox injections to their frown muscles, while the other half were tested two weeks afterwards when the effects of botox are greatest.
As expected, imitating an angry or sad facial expression provoked increased activity bilaterally in the amygdala region of both groups of participants - this is a structure of the brain known to be critically involved in emotions.
However, for angry expressions, activity in the left amygdala was lower in the women who'd already received botox compared with those who hadn't yet had it. This suggests that pulling an angry expression has an effect on the amygdala, via both the neural command to flex the face muscles and via feedback from the positioning of the facial muscles and movement of the skin (the latter being absent for the botox-injected women).
There was no difference in amygdala activation between the two groups when they pulled sad facial expressions, but there were differences in orbitofrontal cortex activity - this is a region involved in evaluating the emotional value of touch signals.
"Our findings provide evidence that peripheral feedback from face muscles and skin during imitation modulates neural activity within central circuitries that are known to be involved in the representation of emotional states," the researchers concluded.
Hennenlotter, A., Dresel, C., Castrop, F., Ceballos Baumann, A., Wohlschlager, A., & Haslinger, B. (2008). The Link between Facial Feedback and Neural Activity within Central Circuitries of Emotion--New Insights from Botulinum Toxin-Induced Denervation of Frown Muscles Cerebral Cortex, 19 (3), 537-542 DOI: 10.1093/cercor/bhn104