title: Botulinum toxin-induced facial muscle paralysis affects amygdala responses to the perception of emotional expressions: preliminary findings from an A-B-A design.
abstract: BACKGROUND
  It 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.
  
  
  RESULTS
  Consistent 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.
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: Department of Psychological and Brain Sciences, Dartmouth College, 6207 Moore Hall, Hanover, NH 03755, USA.
authors.forename: M Justin
authors.initials: MJ
authors.lastname: Kim
authors.affiliation: Department of Psychology, B84 East Stadium, University of Nebraska-Lincoln, Lincoln, NE 68588-0156, USA.
authors.forename: Maital
authors.initials: M
authors.lastname: Neta
authors.affiliation: US Army Natick Soldier Research Development and Engineering Center, Cognitive Science Research Team, 10 Kansas St., Natick, MA 01760, USA ; Department of Psychology, Tufts University, 490 Boston Avenue, Medford, MA 02155, USA.
authors.forename: F Caroline
authors.initials: FC
authors.lastname: Davis
authors.affiliation: Department of Psychology, University of Washington, 119A Guthrie Hall, Seattle, WA 98195, USA.
authors.forename: Erika J
authors.initials: EJ
authors.lastname: Ruberry
authors.affiliation: Department of Psychology, University of Virginia, 102 Gilmer Hall, Charlottesville, VA 22904, USA.
authors.forename: Diana
authors.initials: D
authors.lastname: Dinescu
authors.affiliation: Department of Psychological and Brain Sciences, Dartmouth College, 6207 Moore Hall, Hanover, NH 03755, USA.
authors.forename: Todd F
authors.initials: TF
authors.lastname: Heatherton
authors.affiliation: Department of Surgery (Plastic) and Pediatrics, Geisel School of Medicine at Dartmouth, Dartmouth-Hitchcock Medical Center, Lebanon, NH 03756, USA ; Department of Surgery (Plastic and Craniofacial), Sidra Research and Medical Center, Doha, Qatar.
authors.forename: Mitchell A
authors.initials: MA
authors.lastname: Stotland
authors.affiliation: Department of Psychological and Brain Sciences, Dartmouth College, 6207 Moore Hall, Hanover, NH 03755, USA.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Whalen
keywords: Amygdala; Botulinum toxin; Corrugator supercilii; Facial feedback hypothesis