Facial Muscle Activity: High-Sensitivity Noncontact Measurement Using Laser Doppler Vibrometry

Facial expression is regulated by facial muscles and it is an important communication modality, generally held to convey signs of emotion, stress, pain, and social engagement. The assessment of facial expression has a role in a variety of scientific areas, as well as applied situations, including affective computing, marketing, and clinical evaluation, and is now an area of intensive development using computerized image processing approaches. Assessment has conventionally involved the analysis of facial images by trained coders, relying on the parsing of facial movements into standardized elemental actions. Direct measurement of the electromyographic (EMG) activity from the engaged muscles offers theoretical advantages, especially in terms of sensitivity, but involves burdensome procedures, including application of electrodes. In this article, we describe an alternative measurement method for the direct assessment from facial muscles using the noncontact technique of laser Doppler vibrometry (LDV). The application of LDV to measuring muscle activity relies on the detection of the small lateral muscle vibration signals that accompany contraction along the longitudinal axis. The sensitivity of the LDV method was demonstrated in a study involving a posed facial expression that is generally associated with the emotion of disgust. Ascending sequences of expression strength were anchored by a condition entailing imagery of a disgusting odor while avoiding visible facial expression. LDV signals were shown to compare favorably with the simultaneous EMG, in disclosing signs of activation even at levels that were not sufficient to produce visible facial actions.