Mu Suppression during Action Execution and Observation in Low-Functioning, Young Children with Autism Spectrum Disorder

Background: There has been a discrepancy in literature whether the dysfunctional mirror neuron system (MNS) underlies the imitation difficulties in autism spectrum disorder (ASD). The attenuation of the EEG mu rhythm is considered to reflect the activation of the MNS. The suppression of the mu rhythm occurs not only when individuals execute actions but also when they observe actions. It has been suggested that differences in age, developmental level and symptom severity of a sample with ASD might explain why some studies have found a lack of suppression during the action observation in individuals with ASD whereas other studies have not.

Objectives: The aim of the study was to investigate mu suppression during the execution and observation of hand actions in low-functioning, young children with severe ASD. In order to study the effects of developmental level, we also had an IQ-matched comparison group of children with intellectual disability who did not show marked autistic behaviour. In addition there was a normative comparison group of age-matched typically developing children.

Methods: The original sample consisted of 58 children aged 2.5-6.5 years. The final sample included 12 children with ASD, 11 children with intellectual disability (ID) and 14 typically developing children (TD) who had a sufficient number of artefact-free trials. The EEG mu rhythm (7-11Hz) was measured using The EGI Geodesic 128-channel dense-array system whilst the children were shown 6-second video clips of object-directed hand actions (e.g., pressing a button), meaningless, non-object-directed hand actions (e.g., tapping a table with fingers), a non-biological movement (a slightly bouncing ball), or a white screen. In the action execution conditions, the children imitated the preceding object-directed action. The suppression was calculated as a ratio of mu band power between three action conditions (execute, observation of object-directed and non-object-directed actions) and the baseline conditions (non-biological movement and white screen).

Results: The repeated-measures ANOVA showed a significant main effect of condition (F (2, 34) = 25.644, p < .001), reflecting greater mu suppression over central region in the action execution condition as compared to other conditions. There was no main effects of hemisphere, group or any interaction effects involving these factors on mu suppression. Separate analyses of condition effects within groups revealed, however, that mu suppression was significantly different from baseline in the observation condition of object-directed action in the TD group only. The execution condition differed significantly from baseline in all groups. None of the groups showed significant suppression in non-object-directed condition (Figure 1).

Conclusions: The findings of the study provided no evidence for an ASD-specific deficit in MNS function, as assessed by group by condition interaction effect on mu suppression. Separate within-group comparisons were consistent with prior results showing significant mu suppression during observation in typically developing children but not in young, low-functioning children with ASD. This result was not, however, specific to children with ASD as a similar pattern was also found in children with intellectual disability, raising the possibility that the lack of mu suppression during action observation could relate to more general developmental difficulties.