The interpretation of mu suppression as an index of mirror neuron activity: past, present and future

Abstract

Mu suppression studies have been widely used to infer the activity of the human mirror neuron system (MNS) in a number of processes, ranging from action understanding, language, empathy and the development of autism spectrum disorders (ASDs). Although mu suppression is enjoying a resurgence of interest, it has a long history. This review aimed to revisit mu's past, and examine its recent use to investigate MNS involvement in language, social processes and ASDs. Mu suppression studies have largely failed to produce robust evidence for the role of the MNS in these domains. Several key potential shortcomings with the use and interpretation of mu suppression, documented in the older literature and highlighted by more recent reports, are explored here.

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

“…In other words; MEP recordings of mirror system functioning during TMS are obtained at the level of the muscle, reflecting corticospinal processes, whereas the EEG mu rhythm mainly reflects central cortical activity. Although both techniques have been shown to reliably capture mirror system activation (see reviews by Fadiga, Carighero & Olivier, 2005;Hobson & Bishop, 2017), it has been suggested thatconsidering these substantial differences in neurophysiological underpinnings -both techniques might target different aspects of the mirror system. In this respect, the neural processes triggered by action observation have been proposed to be layered in several hierarchically organized functional levels (Grafton & Hamilton, 2007;Kilner, Friston, & Frith, 2007).…”

“…There are several methodological considerations to be taken into account when evaluating the EEG mu rhythm. For an in-depth discussion, the interested reader is referred to reviews by Hobson & Bishop (2017) and Cuevas, Cannon, Yoo, & Fox (2014). Here, we briefly touch upon some relevant issues relevant that motivated our adopted design.…”

“…In order to investigate eye contact-induced changes in M1 excitability, a repeated measures analysis of variance (RM-ANOVA) with within-subject factor 'observed gaze' (averted, direct) was performed on the MEP peak-to-peak amplitude data. For the EEG data; it was first tested whether all movement observation conditions elicited a significant suppression relative to the pre-trial baseline segments (as recommended by Hobson & Bishop, 2017); i.e. ratio values were tested using single-sample t-tests against a value of 0, separately for each gaze condition (table 1).…”

“…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 alpha. In line with recent guidelines by Fox et al (2016) and Hobson and Bishop (2017), occipital alpha suppression was also taken into account.…”

Electroencephalographic evidence for the involvement of mirror neuron and error monitoring related processes in virtual body ownership

“…The fourth theoretically-relevant EEG index is Mu suppression. The is measured at central electrodes, and its suppression assumingly reflects the activation of sensorimotor areas (Fox et al, 2016;Hobson and Bishop, 2017). Mu suppression occurs when an individual either performs or observes motor actions, and is considered as a valid measure of MNS activity (Fox et al, 2016).…”