Reply to Bowman et al.: Building the foundations for moving mu suppression research forward

It is clear from recent publications that methods of investigating mu suppression, and interpretations of mu suppression findings, are in need of critique, refinement and consensus. To this end, wewelcome ongoing debate about the measure, including discussion of our recent registered report. In the report, we evaluated claims that mu suppression represents a valid measure of mirror neuron system (MNS) engagement, and examined how choice of a baseline interval affected mu suppression investigations. The comment by Bowman et al. highlights several current challenges in this field. These include the issue of isolating mirror neuron activity from other cognitive processes that may be reflected in changes in the EEG, understanding variability in individual participants' mu suppression responses, the potential for developmental evidence to aid our interpretations of mu suppression, and finally the shift in the literature from examining “mirror neurons systems” to broader “mirroring”. We consider these topics here. A primary issue is how researchers can extract from suppression what is genuinely neural mirroring, and what is additional activation, driven by non-mirroring processes such as attentional fluctuations. In our report, we argued that if mu suppression was indeed a sufficient MNS measure, then suppression should be greater at central sites for conditions in which participants observed biological stimuli as opposed to non-biological stimuli. Bowman et al. correctly note however that alpha-related suppression due to attentional engagement and mu-related suppression due to sensorimotor processing are not mutually exclusive: observing an action may engage both attention and sensorimotor processes, leading to simultaneous central and occipital suppression. As they outline,