Studies Using Botulinum Toxin (Botox)
Because facial expressions involve both motor (efferent) and sensory (afferent) mechanisms, it is possible that effects attributed to facial feedback are due solely to feedback mechanisms, or feed-forward mechanisms, or some combination of both. Recently, strong experimental support for a facial feedback mechanism is provided through the use of botulinum toxin (commonly known as Botox) to temporarily paralyze facial muscles. Botox selectively blocks muscle feedback by blocking presynaptic acetylcholine receptors at the neuromuscular junction. Thus, while motor efference commands to the facial muscles remain intact, sensory afference from extrafusal muscle fibers, and possibly intrafusal muscle fibers, is diminished.
Although several studies have examined the correlation of botox injections and emotion, only a few studies have used experimental control to test the hypothesis that botox affects aspects of emotional processing.
In a functional neuroimaging study, Andreas Hennenlotter and colleagues asked participants to perform a facial expression imitation task in an fMRI scanner before and two weeks after receiving botox injections in the corrugator supercilii muscle used in frowning. During imitation of angry facial expressions, botox decreased activation of brain regions implicated in emotional processing and emotional experience (namely, the amygdala and the brainstem), relative to activations before botox injection. These findings show that facial feedback modulates neural processing of emotional content, and that botox changes how the human brain responds to emotional situations.
In a study of cognitive processing of emotional content, David Havas and colleagues asked participants to read emotional (angry, sad, happy) sentences before and two weeks after botox injections in the corrugator supercilii muscle used in frowning. Reading times for angry and sad sentences were longer after botox injection than before injection, while reading times for happy sentences were unchanged. This finding shows that facial muscle paralysis has a selective effect on processing of emotional content. It also demonstrates that cosmetic use of botox affects aspects of human cognition - namely, the understanding of language.
These studies suggest that botox can dampen the ability to understand anothers' emotions, and they lend considerable support to Darwin's original notion that "...the repression, as far as this is possible, of all outward signs softens our emotions..."
Read more about this topic: Facial Feedback Hypothesis
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