Smiling Muscle Stimulation Alters Neural Processing During Emotional Face Comparison

Within embodied and predictive-processing frameworks, emotional face perception is informed by bodily states, such as proprioceptive facial feedback. In line with this, our earlier work showed that activation of smiling muscles with facial neuromuscular electrical stimulation (fNMES) can induce a happiness bias in emotional face perception, and reduce visual face processing demands. Here, we extended this research to examine whether the same facial muscle stimulation biases the emotional representation of a face when it serves as a perceptual template (prime) for comparison with a following target face. In a delayed matching-to-sample task, 53 participants judged which of two sequentially presented faces appeared happier (or more female, in a control sex task). On half of trials, fNMES was applied bilaterally to the zygomaticus major during encoding of the prime face. Performance was held constant using an adaptive staircase. fNMES altered comparison judgments only when emotion was task-relevant, flattening the relation between emotion difference and choice. Across tasks, fNMES reduced N170 amplitudes and increased Early Posterior Negativity (EPN) differences between happy and sad faces. The prime EPN predicted choice only when emotion was task-relevant, whereas fNMES modulated emotion effects on the target N170 only in the control task. Facial feedback may thus increase the perceptual salience of emotion-relevant information, with task-dependent consequences for decisions.