Determining the threshold for asymmetry detection in facial expressions.

OBJECTIVES/HYPOTHESIS: 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. STUDY DESIGN: 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). CONCLUSION: 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. LEVEL OF EVIDENCE: 4.

Detection and perceptual impact of side-to-side facial movement asymmetry.

IMPORTANCE: In this study, we examined whether specific facial movements have different time-delay detection thresholds, and to what extent such side-to-side facial movement asymmetry affects subjective ratings of movement naturalness. Ratings of dynamic asymmetry in experimentally manipulated video recordings demonstrate that there are different side-to-side time-delay thresholds for distinct regions of the face, with a strong inverse correlation between naturalness rating and the length-of-time delay. These findings will be helpful for counseling patients with unilateral facial paralysis and guide the design of neural interfaces for facial reanimation. OBJECTIVE: To determine the detection threshold of side-to-side facial movement timing asymmetry and measure its effect on perceived movement naturalness. DESIGN, SETTING, AND PARTICIPANTS: Videos of 5 symmetrical facial movements (eye blink, rapid eyebrow raising, slow eyebrow raising, smiling, and lip depression) were edited to introduce 6 levels of side-to-side timing asymmetry, ranging from 33 to 267 milliseconds. Participants (N = 58) viewed video clips through an online survey service, indicating whether they noticed side-to-side asymmetry and judging movement naturalness on a 5-point scale. RESULTS: There was a significant difference among facial movements in asymmetry detection threshold. There was a strong correlation between naturalness ratings and amount of delay across movements (R = 0.823), with greater asymmetry being judged as progressively less natural. Blink was judged as less natural at 33, 67, 100, and 133 milliseconds of side-to-side delay compared with all other movements (P < .005). CONCLUSIONS AND RELEVANCE: Side-to-side asymmetry in blink timing is detected sooner and viewed as less natural compared with asymmetry of the eyebrow and lips. At 100 milliseconds of delay, nearly all movements are detected as asymmetric, although blink is judged as the least natural. These findings will help set timing goals for facial pacing technologies treating unilateral paralysis. LEVEL OF EVIDENCE: NA.