title: Determining the threshold for asymmetry detection in facial expressions.
abstract: OBJECTIVE
  To quantify the threshold for human perception of asymmetry for eyebrow elevation, eye closure, and smile, and to ascertain whether asymmetry detection thresholds and perceived severity of asymmetry differ in distinct facial zones.
  
  
  METHODS
  Online survey.
  
  
  METHODS
  Photographs of a female volunteer performing eyebrow elevation, eye closure, and smile were digitally manipulated to introduce left-to-right asymmetry in 1-mm increments from 0 mm to 6 mm. One hundred and forty-five participants viewed these photographs using an online survey, measuring accuracy of asymmetry detection and perceived expression unnaturalness (on a scale of 1-5).
  
  
  RESULTS
  Photographs of facial asymmetries were correctly judged as asymmetrical over 90% of the time for 2 mm or more of asymmetry in eyelid closure, and 3 mm or more of asymmetry during smiling. Identification of eyebrow elevation asymmetry gradually rose from 23% correct to 97% correct across the range of 1 mm to 6 mm of asymmetry. Greater degrees of asymmetry were ranked as significantly more unnatural across all expressions (3 tests; X(2) (6, N = 145) = 405.52 to 656.27, all P <0.001).
  
  
  CONCLUSIONS
  Thresholds for asymmetry detection vary across different zones of the face; once detected, asymmetry in eyelid position is perceived as more unnatural than asymmetries in either brow elevation or smile. These data will inform counseling of patients with segmental facial weakness and may provide more objective goals for facial reanimation procedures.
  
  
  METHODS
  4.
authors.affiliation: Department of Otology and Laryngology, Harvard Medical School, Boston, Massachusetts, U.S.A; Facial Nerve Center, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, U.S.A.
authors.forename: Marc H
authors.initials: MH
authors.lastname: Hohman
authors.forename: Sang W
authors.initials: SW
authors.lastname: Kim
authors.forename: Elizabeth S
authors.initials: ES
authors.lastname: Heller
authors.forename: Alice
authors.initials: A
authors.lastname: Frigerio
authors.forename: James T
authors.initials: JT
authors.lastname: Heaton
authors.forename: Tessa A
authors.initials: TA
authors.lastname: Hadlock
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D005146:Facial Asymmetry / Q000175:diagnosis* / Q000209:etiology / Q000523:psychology; D005149:Facial Expression; D005158:Facial Paralysis / Q000150:complications / Q000175:diagnosis* / Q000523:psychology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012904:Smiling; D011795:Surveys and Questionnaires; D055815:Young Adult
keywords: Facial paralysis; facial asymmetry
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