Abstract
Previous research has demonstrated that eye contact between actor and observer specifically enhances the ‘mirroring’ of others’ actions, as measured by transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs). However, it remains unknown whether other markers of mirror system activation, such as suppression of the EEG mu rhythm (8–13 Hz) over the sensorimotor strip, are also susceptible to perceived eye contact. Here, both TMS-induced MEPs and EEG mu suppression indices were assessed (in separate sessions) while 32 participants (mean age: 24y; 8m) observed a simple hand movement combined with direct or averted gaze from the actor. Both measures were significantly modulated by perceived eye gaze during action observation; showing an increase in MEP amplitude and an attenuation of the mu rhythm during direct vs. averted gaze. Importantly, while absolute MEP and mu suppression scores were not related, a significant association was identified between gaze-related changes in MEPs and mu suppression, indicating that both measures are similarly affected by the modulatory impact of gaze cues. Our results suggest that although the neural substrates underlying TMS-induced MEPs and the EEG mu rhythm may differ, both are sensitive to the social relevance of the observed actions, which might reflect a similar neural gating mechanism.
Highlights
Previous research has demonstrated that eye contact between actor and observer enhances the ‘mirroring’ of others’ actions, as measured by transcranial magnetic stimulation (TMS)induced motor evoked potentials (MEPs)
A mixed-model analysis of variance (ANOVA) with within-subject factor ‘observed gaze’ and between-subject factor ‘session’ (TMS or EEG first) was performed on the MEP data to investigate the effect of observed gaze on corticospinal excitability
In accordance with previous TMS studies[29,30,31] investigating the effect of eye-to-eye contact on corticospinal excitability, MEPs recorded from the first dorsal interosseous (FDI) muscle were significantly higher when movement observation was accompanied with direct gaze from the stimulus person, compared to averted gaze
Summary
Previous research has demonstrated that eye contact between actor and observer enhances the ‘mirroring’ of others’ actions, as measured by transcranial magnetic stimulation (TMS)induced motor evoked potentials (MEPs) It remains unknown whether other markers of mirror system activation, such as suppression of the EEG mu rhythm (8–13 Hz) over the sensorimotor strip, are susceptible to perceived eye contact. Subsequent TMS studies have confirmed these observations (for a review, see ref.8), and provided evidence that the human observation-execution matching mechanism is specific to the muscles recruited in the observed a ctions[7,9,10,11], with a close temporal coupling[12,13] Another commonly adopted method for investigating mirror system activity relates to the assessment of cortical rhythms in the electroencephalogram (EEG) or magnetoencephalogram (MEG). Since the sensorimotor mu rhythm oscillates in the same 8–13 Hz frequency band and displays similar response properties as occipital alpha rhythms (i.e. dominant when at rest, suppressed by perceptual events and attentional processing), an important issue in EEG action observation studies is the potential contamination of the mu rhythm by changes in a lpha[16,22]
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