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Original Investigation |

Detection and Perceptual Impact of Side-to-Side Facial Movement Asymmetry

Sang W. Kim, MD1; Elizabeth S. Heller, BA2; Marc H. Hohman, MD1; Tessa A. Hadlock, MD1; James T. Heaton, PhD2,3
[+] Author Affiliations
1Department of Otolaryngology, Massachusetts Eye and Ear Infirmary, Boston
2MGH Institute of Health Professions, Boston, Massachusetts
3Department of Surgery, Massachusetts General Hospital, Boston
JAMA Facial Plast Surg. 2013;15(6):411-416. doi:10.1001/jamafacial.2013.1227.
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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.

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Figure 1.
Facial Movement Durations

Durations shown as the time from the initiation of movement to maximal excursion (light green), the time that maximal excursion was maintained (dark green), and the return from maximal excursion to a neutral position (green). The lowest bars represent the delay increments introduced to one side of the face in the perceptually assessed video clips. Inset provides an example of blink delayed on the left by 3 increments (100 milliseconds) relative to the right.

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Figure 2.
Video Still Frames Representing Facial Movement Examined in the Perceptual Assessment Test

Examples are from the symmetrical (baseline or nondelayed) video version, including a representative neutral position (A), eyebrow raise (B), blink (C), smile (D), and lip depression (E). Movements B-E represent maximal movement excursions, represented by the black bars in Figure 1. Slow and fast eyebrow raisings had similar maximal excursions and are represented by a single still frame (B).

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Figure 3.
Reported Symmetry of Facial Movement

Movement reported as a percentage of participants (N = 58) for 5 movements across 7 levels of potential side-to-side difference in movement timing, ranging from 0- to 264-millisecond delay. For all movements, video clips with 0-millisecond side-to-side delay were typically correctly reported as being symmetrical, and 99 milliseconds of timing difference was typically identified as not symmetrical. The perception of movement symmetry differed by movement at delays of 33 milliseconds and 66 milliseconds. The dashed line at 50% represents the detection threshold.

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Figure 4.
Median Ranks of Naturalness Scores Averaged Across 58 Participants

For each of the 5 perceptually assessed movements, naturalness scores consistently ranked as less natural (higher value) with increases in side-to-side asymmetry of movement.

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Figure 5.
Median Naturalness Scores for All Participant Responses Plotted for Each Movement at 7 Levels of Asymmetry in Side-to-Side Movement Timing

Note how blink is perceived as relatively unnatural within the first several levels of delay compared with the other movements and that slower movements, such as smile and slow eyebrow raising, are generally perceived as more natural across multiple levels of asymmetry compared with faster movements.

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