Facial expressions selectively modulate P300 somatosensory evoked-potential, but emotional scenes do not: Electrophysiological evidence for sensorimotor simulation

Abstract

The role of the sensorimotor cortices in processing facial expressions remains a topic of debate. While substantial evidence supports their involvement via simulation and mirroring mechanisms, an alternative view argues that sensorimotor activation reflects a general emotional tuning to affective content. To clarify these competing hypotheses, we examined sensorimotor responses to emotional (disgusting) scenes—which evoke affect without requiring simulation—and emotional (disgusted) facial expressions. In one-third of trials, gentle tactile stimulation was applied to the left levator labii superioris muscle at two time points to elicit somatosensory evoked potentials (SEPs). A subtraction approach was used to isolate pure somatosensory activity by removing visual-only responses (VEP) from combined visual-tactile responses (SEP+VEP), with a blank-screen condition as an additional baseline.

We observed a small but significant reduction in P300 SEP amplitude at right central, centro-frontal, and centro-parietal electrodes when tactile stimulation followed disgusted facial expressions compared to disgusting scenes. This effect was independent of subjective ratings of arousal and valence. Importantly, only SEPs following facial expressions differed significantly from those following tactile stimulation alone, suggesting a specific modulation by facial expression processing. Despite the relatively small amplitude of the observed effects, and the somewhat preliminary nature of the results, these findings provide novel evidence that facial expressions engage the sensorimotor system in a specific and privileged manner, consistent with the simulation hypothesis.