The Emotional Modulation of Facial Mimicry: A Kinematic Study

Abstract

It is well-established that the observation of emotional facial expression induces facial mimicry responses in the observers. However, how the interaction between emotional and motor components of facial expressions can modulate the motor behavior of the perceiver is still unknown. We have developed a kinematic experiment to evaluate the effect of different oro-facial expressions on perceiver's face movements. Participants were asked to perform two movements, i.e., lip stretching and lip protrusion, in response to the observation of four meaningful (i.e., smile, angry-mouth, kiss, and spit) and two meaningless mouth gestures. All the stimuli were characterized by different motor patterns (mouth aperture or mouth closure). Response Times and kinematics parameters of the movements (amplitude, duration, and mean velocity) were recorded and analyzed. Results evidenced a dissociated effect on reaction times and movement kinematics. We found shorter reaction time when a mouth movement was preceded by the observation of a meaningful and motorically congruent oro-facial gesture, in line with facial mimicry effect. On the contrary, during execution, the perception of smile was associated with the facilitation, in terms of shorter duration and higher velocity of the incongruent movement, i.e., lip protrusion. The same effect resulted in response to kiss and spit that significantly facilitated the execution of lip stretching. We called this phenomenon facial mimicry reversal effect, intended as the overturning of the effect normally observed during facial mimicry. In general, the findings show that both motor features and types of emotional oro-facial gestures (conveying positive or negative valence) affect the kinematics of subsequent mouth movements at different levels: while congruent motor features facilitate a general motor response, motor execution could be speeded by gestures that are motorically incongruent with the observed one. Moreover, valence effect depends on the specific movement required. Results are discussed in relation to the Basic Emotion Theory and embodied cognition framework.

Keywords: basic emotion theory; embodiment; emotional valence; facial expression; facial mimicry; lip kinematics; mouth gesture.

Figure 1

(A) Movement tasks. The figure depicted an example of the execution of the Lip Stretching (at the left) and Lip Protrusion (at the right) movements that participants had to perform in response to experimental stimuli. (B) Experimental Stimuli. In the upper part are presented mouth-closing orofacial gestures with positive (a) and negative (b) value. At the center, positive (c) and negative (d) mouth-opening orofacial gestures are presented. Meaningless closing-mouth (e) and opening-mouth (f) gestures are depicted at the bottom. The same stimuli executed by a male actor were presented during the experiment.

Figure 2

Experimental paradigm. Panel (A,B) reported an example of congruent (A) and Incongruent (B) trials (A). The trial started with a fixation cross (500 ms duration), followed by a cue (diverging arrows, 350 ms duration) that instructed the subject about the movement to perform. After the cue, the video stimulus was presented and the subject started the movement after the end of the video. In this case a mouth-opening gesture was followed by a lip-stretching movement. (B) The sequence is the same of that reported in (A). However, in this case, the requested movement was incongruent respect to the visual stimuli.

Figure 3

Mean values of Response Times for both lip protrusion and lip stretching movements for the stimuli categorized as meaningless, negative and positive (A) and as aperture and closure (B). Other conventions as in Figure 5.

Figure 4

Mean values of Response Times and kinematic parameters of lip protrusion (C,D) and lip stretching (E,F) movements compared with the corresponding baseline mean value depicted at the left of each graph. Other conventions as in Figure 5.

Figure 5

Mean values of kinematic parameters (Movement duration and Mean Lip velocity) plotted separately for lip protrusion (G,H) and lip stretching movements (I,J). Vertical bars represent standard errors (SE). Horizontal bars represent significant difference (p < 0.05). The legend at the bottom depicts the corresponding stimuli for each plotted experimental condition.