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Final Report Summary - MIRRORING AND TOM (Control of imitation and social cognition: the interaction of motor mirroring and Theory of Mind)

The discovery of the mirror neuron system has suggested that the observation of an action is associated with the activation of the corresponding motor representation in the observer. In accordance with this hypothesis, it has been demonstrated that observing an action interferes with one’s own actions when these are different from those being observed. This, however, raises the question how we avoid automatic imitation? More specifically, how can we distinguish the motor program activated by our intention from that triggered by observation? Neuroimaging studies show an increase of activation of the temporo-parietal junction (TPJ) and anterio-medial prefrontal cortex (aMPFC) when self-other distinction is needed. Interestingly, these areas are core brain regions of a network involved in high-order social-cognitive functions, such as Theory of Mind (ToM) (i.e., the ability to reflect on other people’s mental states. It has been hypothesized that these "social" areas exert a regulative function over shared representations.
This project focused on how the brain implements the control of shared representations, both in the motor domain and for higher level socio-cognitive functions, such as ToM. This issue is of paramount importance as imitation and ToM are crucial abilities for social interaction, learning and communication. We adopted different methods like brain stimulation techniques (transcranial magnetic stimulation-TMS and direct current stimulation-tDCS) and functional magnetic resonance (fMRI). Study 1. Previous studies suggest that mirror tendencies can be suppressed after extensive practice or in complementary joint action situations revealing that mirror responses are more flexible than previously thought. We have show that the mere instructions of a counter-imitative mapping changes mirror responses as indexed by motor-evoked potentials (MEPs) enhancement induced by TMS. This result suggests that the implementation of task instructions activates stimulus-response association that can overwrite the mirror system. Our outcome reveals one of the crucial mechanisms that might allow flexible adjustments of mirror responses in different contexts. Study 2. The aim of study 2 was to clarify the physiological mechanisms of response activation and inhibition of imitative behavior. With this end in mind, during the execution of an interference task, the imitation-inhibition task, we applied single-pulse TMS at various intervals after stimulus onset. TMS pulses, applied to the motor cortex, triggered motor evoked potential (MEP) and silent period (SP), providing sensitive indices of cortico-spinal excitation and inhibition. MEPs analysis allowed us to look into the dynamic of response priming (mirror effects) and response activation during the observation of congruent and incongruent movement. Our study has parsed action interference during movement observation into response activation and inhibition components. This result will have a strong impact on the study of how self-other distinction is implemented in the brain. In study 3, TDCS was used as a neuromodulatory technique to produce an off-line conditioning of the right TPJ activity. After tDCS, corticospinal output was indexed through the analysis of MEPs evoked by single pulse TMS during action observation in the context of the imitation-inhibition task. The results showed that, although an imitation tendency is always present after action observation, our system is able to overcome it by enforcing motor intention. Study 4. The forth study focused on ToM. Although ToM abilities have been widely studied in adults and children almost exclusively through tasks with explicit instructions, it has been recently argued that people also mentalize spontaneously, or at an implicit level. To date, there is a extensive discussion on whether implicit and explicit forms of ToM are based on the same cognitive/neural mechanisms or rather reflect qualitatively different processes. We directly compared implicit and explicit tasks. During the belief formation phase, greater activity was found for false vs. true belief trials in the TPJ irrespective of the task and aMPFC suggesting that implicit ToM processing activates the same brain network that has long been considered as critical for ToM in more traditional explicit tasks. Moreover, this network overlaps with brain areas that have been found to be critical for the control of imitation.
We have also extended the project in different directions. First, we developed a new ToM task were we can compare behavioral performance in believe tracking under implicit and explicit instructions. Second, we used this ToM task to investigate ToM processing in people with high-functioning autism. Implicit measures allowed us to detect subtle deficits in this population that commonly do not emerge with more standard tasks. Third, we investigate, by the use of tDCS, the role of TPJ in self-other distinction during the perception of touch and pain. Forth, we investigated how implicit ToM processing changes when we interact with other non-human living beings. Overall, the project allowed us to make an important step forwards in understanding the neurocognitive bases of social cognition processes, such as the control of shared motor representations and ToM. Our outcome is an important contribution in the study of developmental disorders with core social processes deficits, such as autism.

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