The mirror neuron system (MNS) consists of interconnected brain regions, namely the inferior frontal gyrus (IFG) and the inferior parietal lobe (IPL). First discovered in monkeys, the MNS contains neurons that respond similarly when an individual observes an action, such as reaching for a cup, or executes the same action. One difference between the monkey and human MNS is that the monkey MNS only responds when an object is present as the goal of the action, while the human MNS will respond to meaningless or pantomimed gestures that do not have objects present. It has been hypothesized that this system underlies the ability to understand the actions of others, including their goals and intentions. These are skills that individuals with autism often have demonstrated difficulty, leading to the broken mirror theory of autism (Williams et al., 2001). However, it appears that some skills that rely on the MNS are not compromised in ASD. For instance, the ability to imitate actions performed on objects (transitive actions) is less affected than the ability to imitate gestures without objects (intransitive actions), such as opening and closing the hand (Rogers et al., 1996), suggesting there may not be a global MNS impairment in ASD.
The goal was to examine the MNS in ASD and typical development using fMRI as related to the understanding of transitive and intransitive actions.
Methods: Functional neuroimaging was obtained from age and gender matched children (aged 8-17 years) who were either typically developing or diagnosed with ASD. Participants passively viewed 5-second videos of transitive and intransitive hand actions (adapted from Umiltà et al., 2001; Colombi et al., 2009) while functional images were acquired. For half of the stimuli, the end of the hand movement was occluded by a screen to assess the effect of visibility during observation.
Results: Our results indicate that both TD and ASD participants demonstrate neural activation of the MNS, in the IFG and IPL, although to a lesser extent in the ASD group. The TD group shows an expected effect of transitivity, in that the MNS responds more to transitive than intransitive actions. In contrast, the ASD group only demonstrates a transitivity effect in the IPL. IFG activity was notably absent during transitive actions in the ASD group. Consistent with Umiltà’s results is that in the TD group, occluding the end of the action did not significantly affect the IFG response.
The fMRI results in the TD group are consistent with previous findings of the MNS in monkeys, as well as what has been seen in TD adults (Umiltà et al., 2001; Rizzolatti & Craighero, 2004). While the MNS responds in the ASD group, differences were observed, particularly with regards to IFG activation during transitive actions on objects. These findings suggest that children with autism do not exhibit a MNS deficit in the IPL, which may be consistent with behavioral data showing more typical imitation responses in children with ASD when executing transitive actions than when executing intransitive actions (Rogers et al., 1996).
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