Some have proposed that the core impairment in individuals with ASC is to a system which maps sensory and motor representations of action (the ‘broken mirror hypothesis’). Impairments in this system may result in those with ASC being unable to infer intentions and mental states of others, if observed actions cannot be translated into the motor codes used to perform those actions. However, evidence for this hypothesis is mixed, with some studies finding sensory-motor mapping impairments in ASC and others finding intact mapping.
The studies which have and have not demonstrated impaired sensory-motor mapping tend to differ in two respects. First, the studies which have found impairments have used simple action observation tasks, where actions are observed and incidental motor activations are recorded. In contrast, the studies which have found no impairments have implemented motor tasks dependent on observed actions, and measured the degree to which observing action primes execution of matching action. Second, the studies finding impairments have tended to use facial actions, while the studies which have found no impairments have used manual actions.
The present study investigated whether it is the task or the effector that determines whether sensory-motor mapping impairments are observed in ASC, to gain a better understanding of whether those with ASC have any impairment to sensory-motor mapping systems. Adult participants with ASC, and age-, gender-, and IQ-matched control participants, were required to perform a facial motor task dependent on observed facial actions. If impairments were found, this would suggest that the effector determines whether impairments are observed, and if impairments were not found, the task is more likely to be responsible for these differences.
Participants watched stimuli of the upper or lower half of the face. The face first appeared in a neutral posture, and after a period the eyebrows would raise or lower (upper half) or the mouth would open or close (lower half). Participants were required to execute a pre-specified response (e.g. raise their eyebrows) whenever the face moved. This generated trials where the response was compatible with the observed movement (eyebrows lifting) and trials where the response was incompatible (eyebrows lowering). The reaction time (RT) on compatible trials was subtracted from the RT on incompatible trials to obtain a measure of the degree to which the observed action primed its execution (‘automatic imitation’), and therefore, the strength of the sensory-motor mapping.
Those with ASC demonstrated ‘automatic imitation’ of both eyebrow and mouth actions, comparable with effects seen in the control group.
Intact imitation of facial actions in the present study indicates that previous findings of sensory-motor mapping impairments in ASC may be driven by the task used. Specifically, requiring participants simply to observe actions while measuring motor activations may have generated apparent impairments to this system if those with ASC were not actually observing the actions. These findings suggest that those with ASC do not have ‘broken mirrors’ mapping sensory and motor representations of action, and that their core impairments lie elsewhere.