Effective Connectivity of Mirror System Brain Areas in Autism Spectrum Disorder
Autism Spectrum Disorders (ASD) are characterized by difficulties with facial mimicry, imitation, and the understanding of mental and emotional states in others. It has been suggested that deficits in relating to the intentions and emotions of other people are associated with reduced functioning of the human mirror neuron system (MNS). In previous studies, reduced activation in brain areas related to the MNS (in particular in the inferior frontal gyrus, IFG) during imitation and observation of actions has been taken as evidence for such a mirror system deficit in ASD. However, more recent activation based fMRI results are increasingly inconclusive with regard to the mirror system deficit hypothesis of autism. Several studies report either comparable activation in the MNS or even hyperactivation. Differential connectivity patterns in the core areas of the MNS (i.e. the superior temporal sulcus (STS), inferior parietal cortex (IPC) and IFG) may better characterize impaired and spared functionality of the MNS in autism and reconcile conflicting findings.
We investigated functional connectivity patterns during observation and imitation of facial expressions in mirror system areas.
Eighteen adolescents and young adult patients with ASD and 18 matched controls were scanned with fMRI during imitation and observation of facial expressions of emotion. Brain imaging data were analyzed with SPM8, using a flexible factorial ANOVA model. Using DCM8, effective connectivity patterns of the MNS network were investigated. Bayesian model selection was used to determine the optimal model for each group, reflecting modulatory influences of task conditions (such as imitation and stimulus properties) within the network.
Both groups showed comparable magnitudes of brain activation in core areas of the MNS (e.g. STS, IPC, IFG) in response to facial stimuli (imitation and observation). However, different functional network architectures emerged with respect to the modulatory influences of the instruction to imitate. In particular, controls showed significant functional connectivity in the connections between STS->IPC->IFG, as well as in a direct STS->IFG connection, whereas patients with ASD showed functional connectivity only in the STS->IPC->IFG pathway.
Our findings are in accordance with the EP-M theory of autism that predicts intact functioning in the STS->IPC->IFG pathway (reflecting largely intact goal-directed emulation and planning [EP] of imitative actions), but reduced connectivity in the direct STS->IFG pathway (reflecting reduced direct facial mimicry [M]) for autism. Furthermore, these results highlight the importance of functional connectivity analyses to elucidate characteristics of brain network functioning and its disturbance in autism.