Role of the human mirror system in automatic processing of musical emotion: Evidence from EEG

The human mirror system (HMS) consists of a core parietofrontal network of regions in the inferior parietal lobule and inferior frontal gyrus/premotor cortex, which can be activated by action observation and execution. Mu rhythm suppression is considered an electrophysiological indicator of the HMS given their similarity in reaction to action observation and execution. Mu rhythm comprises alpha (8-13 Hz) and beta (15-25 Hz) frequency bands, which are typically measured at the power change of midline electrode sites. The beta frequency band is related to the movement preparation, whereas the alpha frequency band is suppressed during the execution of movement. Consistent with the role of the HMS in social cognition, such as emotion understanding, theory of mind, and empathy, mu rhythm suppression is modulated by the processing of social information, such as facial emotional information. Emotion is an important component of social communication. In addition to the emotional facial expression, music is an effective means of expressing emotions through imitation, and for most of people, the main purpose of listening to music is to process musical emotions. However, information on whether mu rhythm suppression is involved in the processing of musical emotions is limited. The aims of the present study were to examine whether mu rhythm suppression is modulated by the processing of musical emotions using Electroencephalogram (EEG). Given that the HMS is involved in the automatic processing of musical emotions, the present study focused on this point by using a cross-modal affective priming paradigm with an SOA of 200 ms. Fifteen musically untrained normal individuals participated in the experiment. Target faces with pleasant and unpleasant emotions were primed by affectively congruous or incongruous chords. Forty-eight congruous and 48 incongruous trials were included in the present study. The participants were instructed to decide as fast and accurately as possible whether the emotion of the face was pleasant or unpleasant. Behavioral results showed that the affectively congruous target faces (M = 575.17 ms, SD = 75.34) were judged faster than affectively incongruous target faces (M = 605.38 ms, SD = 87.74). However, no difference was observed in the percentages of correct responses to the affectively congruous (M = 98%, SD = 2.4%) and incongruous (M = 97%, SD = 2.5%) target faces. Electrophysiological results revealed that the beta frequency band (18-24 Hz) oscillations were less strong for incongruous than for congruous target faces at a time window of 500-650 ms after the onset of chords. A significant desynchronization of the a frequency band was observed for both the congruous and incongruous target stimuli at a time window of 300-450 ms after the onset of chords. Moreover, source analysis exhibited the central-frontal area responsible for automatic musical emotion processing, where the HMS is located. Overall, the present study showed that mu rhythm suppression was involved in the automatic processing of chord emotions, as shown in the alpha and beta frequency bands. The results extend the role of the mu rhythm and provide electrophysiological support for the role of the HMS in the processing of musical emotions.