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.
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.
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