Action Observation Modulates Mirror Neuron System in Toddlers and Preschoolers with and without Autism Spectrum Disorders

Background:
Action execution and observation lead to activation of the mirror neuron system (MNS). This system is hypothesized to be fundamental to social cognition. However, little is known about the development of the MNS, especially the functioning of this system in toddlers and preschoolers, the stage when communication skills are rapidly developing. Given that children with autism spectrum disorders (ASD) start to show clear signs of social deficits in this period, an important question is whether ASD is linked to impairments in the MNS that can be detected at this age.

Objectives:
The aim of the study was to compare neurophysiological activation during action observation and execution in the MNS between young children with and without ASD. Furthermore, we investigated whether observation of an action that is more relevant in social contexts (i.e. pointing) modulates the MNS activity differently than a direct action (i.e. grasping).

Methods:
Participants
15 typically-developed (TD) young children (M = 42.3 months; range 25-59) and 6 young children with ASD (M = 37.0 months; range 24-48) participated and provided quality data. ASD diagnoses were confirmed on the ADOS-2 Toddler module or Module 1.

Stimuli and procedure
To measure activation of the MNS, electroencephalography (EEG) recordings were obtained using the EGI system during action execution and observation. Power in the 7-10 Hz frequency band at the sensorimotor areas (C3 and C4 in the 10/20 system), the mu rhythm, was used as an index for MNS activity. This was measured when the toddlers and preschoolers performed a right-handed grasping action and when they observed video clips of the same grasping action and a pointing action performed by an actor. Data were included if children provided a minimum of 10 trials for each of the three conditions, execute grasp, observe grasp and observe point.

Results:
As expected, both the TD and ASD groups demonstrated activation in left sensorimotor area when grasping an object. In the TD group, modulations of power in the mu rhythm were stronger when observing grasping actions than pointing actions. Responses were bilateral with stronger effects in the left hemisphere during the observation of a grasping action. In contrast, the young children with ASD exclusively demonstrated a significant effect in the right hemisphere during the observation of pointing but not for grasping.

Conclusions:
The findings of the present study show that toddlers and preschoolers with ASD appear to have a different activation pattern in the MNS during action observation. Since this system has been proposed to play a crucial role in social development, impairments in the MNS could explain some of the deficits in social and communication abilities. The current study shows that this relation is already present at an early age, which could lead to new approaches in early intervention designed to improve the functioning of the MNS.