Effects of STN-DBS on voluntary facial mimicry in Parkinson's patients: a pilot study

Parkinson disease (PD) causes hypomimia, an early manifestation driven by motor, limbic and basal ganglia dysfunction that impairs nonverbal communication. Traditional clinical rating scales used to assess facial expressivity are subjective, traditional clinical scales are subjective, whereas automated video analysis based on the Facial Action Coding System (FACS) provides an objective, fine-grained measurement of specific facial Action Units (AUs). Subthalamic Deep Brain Stimulation (STN-DBS), a widely adopted therapy for PD, modulates basal ganglia circuits, yet its precise effect on facial mimicry remains poorly characterized. This study evaluates how STN-DBS influences voluntary facial expressivity by comparing Stim-ON/Med-ON and Stim-OFF/Med-ON states.

18 PD-patients with STN-DBS were evaluated in two experimental conditions (Stim-ON/Med-ON VS Stim-OFF/Med-ON). Video recordings captured participants reproducing seven voluntary facial expressions (six basic emotions plus neutral). Facial mimicry was quantified using OpenFace 2.0 to extracts AUs intensities according to the FACS system.

MDS-UPDRS-III scores improved significantly in the DBS-ON state (p < 0.001), confirming the expected improvement in motor symptoms. Automated FACS analysis revealed emotion-specific differences in AUs, showing reduced mimicry in the Stim-OFF/Med-ON condition, for neutral (AU06, p = 0.042), happiness (AU06, p = 0.034), fear (AU25, p = 0.043), and anger (AU23, p = 0.036). For sadness, AU09 activation was higher in the OFF state (p = 0.021). No significant differences emerged for disgust or surprise.

STN-DBS appears to selectively modulate voluntary facial mimicry in PD, enhancing activation of emotion-relevant AUs, and reducing non-pertinent activations for sadness. These findings indicate that STN-DBS influences emotional motor output beyond classical motor control, potentially benefiting nonverbal social interaction.