Abstract
BackgroundIt has long been suggested that feedback signals from facial muscles influence emotional experience. The recent surge in use of botulinum toxin (BTX) to induce temporary muscle paralysis offers a unique opportunity to directly test this “facial feedback hypothesis.” Previous research shows that the lack of facial muscle feedback due to BTX-induced paralysis influences subjective reports of emotional experience, as well as brain activity associated with the imitation of emotional facial expressions. However, it remains to be seen whether facial muscle paralysis affects brain activity, especially the amygdala, which is known to be responsive to the perception of emotion in others. Further, it is unknown whether these neural changes are permanent or whether they revert to their original state after the effects of BTX have subsided. The present study sought to address these questions by using functional magnetic resonance imaging to measure neural responses to angry and happy facial expressions in the presence or absence of facial paralysis.ResultsConsistent with previous research, amygdala activity was greater in response to angry compared to happy faces before BTX treatment. As predicted, amygdala activity in response to angry faces was attenuated when the corrugator/procerus muscles were paralyzed via BTX injection but then returned to its original state after the effects of BTX subsided. This preliminary study comprises a small sample size and no placebo condition; however, the A-B-A design affords the present sample to serve as its own control.ConclusionsThe current demonstration that amygdala responses to facial expressions were influenced by facial muscle paralysis offers direct neural support for the facial feedback hypothesis. Specifically, the present findings offer preliminary causal evidence that amygdala activity is sensitive to facial feedback during the perception of the facial expressions of others. More broadly, these data confirm the utility of using BTX to address the effect of facial feedback on neural responses associated with the perception, in addition to the experience or expression of emotion.
Highlights
It has long been suggested that feedback signals from facial muscles influence emotional experience
When the corrugator and procerus muscles were paralyzed via botulinum toxin (BTX) injection, amygdala activity in response to angry vs. happy faces was attenuated
Since we did not explicitly instruct the participants to either imitate or not imitate the facial expressions, they might have been less aware of their facial muscular movements and less resistant to spontaneous elicitations of their own facial expressions when viewing other people’s faces. This may have influenced the difference in the observed laterality of the amygdala— that is, whereas Hennenlotter and colleagues report that BTX affected the left amygdala [24], we found the right amygdala to be modulated by BTX-induced paralysis of the corrugator supercilii
Summary
It has long been suggested that feedback signals from facial muscles influence emotional experience. Previous research shows that the lack of facial muscle feedback due to BTX-induced paralysis influences subjective reports of emotional experience, as well as brain activity associated with the imitation of emotional facial expressions. It remains to be seen whether facial muscle paralysis affects brain activity, especially the amygdala, which is known to be responsive to the perception of emotion in others. It has long been suggested that feedback signals from facial muscle activity associated with forming facial expressions are critical to emotional experience [1,2] This notion has developed into what we know as the facial feedback hypothesis [3]. This study offered clear behavioral evidence that feedback signals from the facial muscles influence our emotional experiences
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