Lower-Level and Top-Down Processing Underpins Motor Contagion in Autism Spectrum Disorders

Background: A person becomes significantly more variable when performing a horizontal movement if they concurrently observe another person executing a vertical (incongruent) arm movement, compared to a horizontal (congruent) arm movement. Increased movement variability is a consequence of the observed movement properties interfering with the sensorimotor processes controlling the ongoing movement (Kilner et al., 2003). The interference effect is called ‘motor contagion’ and is underpinned by a sensorimotor system directly linking perception and action (Prinz, 1997). Motor interference effects have been reported in autism and indicate intact lower-level sensorimotor processing (Sowden et al., 2016) and top-down control (Hamilton et al., 2007). In the present study, we further examined motor contagion in autism to establish if lower-level and top-down processes operate within an embedded system that leads to combined additive interference effects.

Objectives: (1) examine the contribution of end-point and trajectory information on motor contagion.

Methods: Twenty-two autistic adults and twenty-two matched control participated in the study. The autistic adults were diagnosed by a clinical assessment and ADOS, and the study was approved by the local ethics committee. Participants performed cyclical horizontal arm movements whilst simultaneously observing different stimuli. The stimuli were horizontal (congruent end-points and trajectory), vertical (incongruent end-points and trajectory), and curvilinear (congruent end-points and incongruent trajectory). Mean data for deviation and peak position were submitted to separate 2 group x 3 stimulus repeated measures ANOVA. Alpha was set at p < 0.05.

Results: Although deviation was significantly greater in the autism group than the control group (p < 0.05), both groups demonstrated motor contagion. Deviation and peak position were significantly higher in the vertical compared to the horizontal conditions (ps < 0.05), with additional significant increases in deviation and peak position between the curvilinear compared to the vertical condition (ps < 0.05).

Conclusions: The observation of vertical incongruent arm movements during motor execution led to motor contagion in autistic and control adults. Increased deviation and peak position is underpinned by sensorimotor processes representing the observed movement properties in the sensorimotor system at a similar time as the motor commands are controlling the executed horizontal movement. Importantly, increased deviation and peak position was found when viewing a curvilinear stimulus that had an incongruent trajectory, but congruent end-points. The additive contagion effects found in the curvilinear condition indicated motor interference was modulated by combined processing of the movement trajectory and end-points. These findings indicate lower-level sensorimotor, and top-down, processing is intact in autism.