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    • As well as being the first

    2018-10-25

    As well as being the first study to show mu desynchronization during observation of facial expressions early in childhood, the present study extends previous EEG studies of the facial MNS by comparing emotional and non-emotional facial expressions. Additionally, most studies of mu rhythm activity use either observation of static stimuli or non-biological movement as control conditions (e.g. Ferrari et al., 2012; Moore et al., 2012), and thus do not address the specificity of the EEG response to biological movements (Cuevas et al., 2014). A recent meta-analysis on the mu rhythm has strongly recommended the use of multiple control conditions in order to assess EEG response specificity for the investigation of the MNS (Fox et al., 2016). Our use of a static neutral face baseline period controlled for observation of a face alone, and as the movement of low-level facial features was still visible in the scrambled condition, this controlled for observation of meaningless biological movement. The lack of significant mu desynchronization in response to the observation of scrambled facial expressions demonstrates that the significant mu ERD seen in the other conditions is not simply due to observation of a moving face-like stimulus or other attentional factors. Additionally, the lack of mu ERD in occipital regions during facial expression observation demonstrates that the effect seen in central clusters is not a result of alpha desynchronization in visual cortex, but is specific to somatomotor cortical regions. Our finding that mu desynchronization was right lateralized during observation of emotional expressions is in line with many studies showing right hemisphere buspirone hcl for emotional facial processing (Adolphs et al., 1996; Calvo and Beltrán, 2014; De Haan and Nelson, 1998; Moreno et al., 1990). Bilateral activation of human MNS areas during action-observation has often been reported (for a review see Rizzolatti et al., 2014), however most MNS studies have investigated observation of hand actions, and therefore may not be directly comparable with our study. In fact, other EEG studies of the facial MNS have demonstrated differential mu responses to emotional facial expressions (Moore et al., 2012), and to faces associated with reward performing happy expressions (Gros et al., 2015) in the right hemisphere. Right lateralized ERPs have also been found during emotional facial expression discrimination in the somatosensory cortex, which is where the alpha mu rhythm is thought to be generated (Sel et al., 2014). In infants, EEG studies have shown the right hemisphere to be more sensitive to early emotional experience with caregivers (Bowers and Heilman, 1984; De Haan et al., 2004), including exposure to maternal depression (Dawson et al., 1992; Jones et al., 2009), and consistent with our results, right lateralized ERPs have been found in children during observation of static facial expressions (Batty and Taylor, 2006; De Haan et al., 2004; Field et al., 1998). Our results suggest that right lateralized sensorimotor activity during observation of emotional faces is in place by 30 months of age. It could be that an MNS for facial expressions is active in even younger children and infants, and phylogenetic would interesting to investigate whether a lateralized response to emotional faces develops over time as infants form and strengthen associations between motor and emotional representations. Changes in mu rhythm activity during observation of facial expressions might also, at least in part, be explained by covert imitation. In adults, the observation of facial expressions leads to subtle, measurable effects at the muscle level, similar to covert facial responses (i.e. facial mimicry; Dimberg et al., 2002; Dimberg, 1982; Lundqvist and Dimberg, 1995). It is possible that in our study children displayed such responses, but they were not detectable at the behavioural level. In other words, although our fine-grained behavioural analysis allowed us to remove any trials containing overt movements, the EEG responses described during observation trials may still partly reflect the synergy between observing and imitating facial expressions. Results from a very recent electromyography (EMG) study (Geangu et al., 2016) do suggest that the primary muscle involved in smiling (the zygomaticus major) is activated during observation of happy faces in three-year-old children. The authors interpret this as evidence for a perception-action matching mechanism facilitated by an MNS for facial expressions. However, one MEG study has shown that mu rhythm modulation can occur without significant facial EMG activity, and therefore decreases in mu power may not necessarily reflect covert imitation (Nishitani and Hari, 2002). Further research is clearly required to explore any relationship between mu rhythm responses in children and imitative covert responses.