Enhanced mirroring upon mutual gaze: multimodal evidence from TMS-assessed corticospinal excitability and the EEG mu rhythm

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.

Broken or socially-mistuned mirror neurons in autism? An investigation via transcranial magnetic stimulation

“…Recently, a series of TMS studies extended this behavioral work by showing that activity within the MNS is enhanced when observed movements are accompanied by the actor's direct compared to averted gaze (Prinsen & Alaerts, 2019, 2020Prinsen et al, 2017;Prinsen, Brams, & Alaerts, 2018), thereby providing support to the notion that the MNS underlies the encountered gaze-related modulations of mimicry in the Wang experiments.…”

“…Next, movement observation trials were presented through the transparent LC screen via a female stimulus person seated behind the panel. Adopted movements were similar to those used in previous studies (Prinsen & Alaerts, 2019, 2020, involving index finger abduction movements of a horizontally held right hand (dorsal view) in three different contexts. To investigate basic motor mirroring, hand movements were presented against a white background, without the stimulus person being visible to the observing participant.…”

“…To address socially modulated mirroring, movements were presented in combination with the stimulus person gazing directly towards the observing participants vs. with the stimulus person averting her face and gazing 30° to the right. Previous studies in neurotypical participants have consistently demonstrated enhanced MNS activity upon observation of actions combined with direct vs. averted eye gaze from the acting model (Prinsen & Alaerts, 2019, 2020Prinsen et al, 2017). Each trial was of 4 seconds duration.…”

“…Coinciding with the TMS pulse on the third second of stimulus presentation, the stimulus person performed the index finger abduction movement (similar set-up as in Prinsen & Alaerts, 2019, 2020. In order to ensure that all participants viewed and attended the stimuli properly, they were asked once at a random time point during the TMS assessment to verbally report the type of stimulus that was presented in the previous trial.…”

“…Indirectly, this seems to be in line with an intact but socially mistuned MNS as proposed by the STORM model, instead of a generally broken MNS as advanced by the Broken Mirror Theory. A principal aim of the current study was to investigate the extent of putative MNS deficits in adults with ASD and assess the relevance of both accounts, as well as to extend prior TMS work in neurotypical individuals (Prinsen & Alaerts, 2019, 2020Prinsen et al, 2017) and behavioral work in individuals with ASD (Forbes et al, 2017) to obtain a neurophysiological investigation of socially modulated mirroring in ASD. To do so, we used TMS for probing potential group differences in MNS activity during observation of simple hand movements.…”