Congruency Effects and Developmental Trajectories In Simultaneously Perceiving and Producing Facial Expressions and Hand Movements - An EMG Study

Background:

It has been suggested that patients with autism spectrum disorders (ASD) have a fundamental impairment of the mirror neuron system (MNS). Several studies have reported deficits for the imitation and the automatic mimicry of facial expressions. Findings for hand- and finger movements are inconsistent. Thus, it remains unclear at which level an impairment in the MNS may occur. Furthermore, the developmental trajectories of these abilities in typically developing controls (TDC) are poorly understood.

Objectives:

We aimed at investigating the developmental trajectories of automatic mirroring of observed movements (hand movements and facial expressions) using electromyography (EMG) in patients with ASD and TDC.    

Methods:

39 children and adolescents (aged 6 to 19 years) were investigated using EMG. We employed a paradigm that allowed testing for the effect of automatic mirroring effects (likely mediated by the mirror neuron systems) by using an observation/execution interference task. The participants were asked to execute pre-specified finger movements (lifting of middle or index finger) or facial expressions (smiling or frowning), as indicated by a color cue. Simultaneously, photos were presented that depicted either congruent, incongruent or non-moving/neutral finger movements or facial expressions. EMG signals were acquired at the musculus corrugator supercilii, musculus zygomaticus major (facial expressions), musculus extensor indicis and musculus extensor digitorum (finger movements). For each trial, the latency of muscle activity (as indicated by the EMG-signal) was determined, relative to the onset of the movement cue. Mean error rates and reaction times for congruent, incongruent and neutral conditions were calculated. To avoid a speed/accuracy trade-off effect, only correct trials were used to calculate reaction times. An additional sample comprising 20 adolescents with ASD is currently being collected.

Results:

Irrespective of age, a significant congruency effect could be observed for the TDC. In congruent conditions (i.e. observed and executed movements were identical) participants had shorter reaction times and made less errors than in incongruent and neutral conditions. Furthermore, we observed that the size of the congruency effect was modulated by age. Both reaction time and error rate effects decreased with age. Further analyses will show whether congruency effects can also be observed in patients with ASD and whether these are similarly modulated by age.

Conclusions:

The observed congruency effect is likely mediated by the mirror neuron system. Automatic mirroring of observed movements seems to be present from early childhood on and is subject to developmental change. The decrease of congruency effects with age suggests that in TDC, automatic mirroring can be overridden more effectively at older ages due to maturing control processes. In TDC, comparable results were observed for finger movements and emotional facial expressions. The paradigm is thus well designed to investigate differential impairments of the MNS in ASD for both modalities. Observed differences will give important insights into the nature of such impairments, whereas an intact congruency effect and comparable developmental changes would be a strong argument against a core impairment of the MNS in ASD.