Role of Broca's area in motor cognition

Processing hierarchy is crucial in humans, allowing us to perform complex behaviours from expert activities to everyday-life actions, including linguistic skills. A common feature of many of these behaviours is that they rely on structured sequences, which must obey certain syntactic rules. Because of its crucial role in linguistic syntax, Broca’s area, located in the left inferior frontal cortex of the human brain, has been considered as a possible candidate to support syntactic function across multiple domains; accordingly, this area has been named the “supramodal syntactic processor”. Despite a constantly growing number of studies trying to gain further insight into this issue, experimental evidence supporting this view remains scarce. In the present thesis, a series of six experiments was conducted, taking advantage of the TMS technique, to determine the causal implication of Broca’s area in various “motor syntax” related tasks. We started our investigations by testing the involvement of Broca’s area in sequencing complex human actions. To this end, we used an action observation paradigm and a sequencing task, showing the subjects different types of videos and pictures depicting biological (or “human”) movements either syntactic or not. Then, in order to investigate further the syntactic component of action sequences, in the subsequent studies, we used key-presses sequences, more easily manageable in laboratory conditions. We therefore investigated the involvement of Broca’s area in the online processing of pre-learned sequences characterized by different levels of syntactic complexity. Also, because syntactic rules are known to play an important role while learning complex sequences by structuring them, in the last three studies, we investigated the role of Broca’s area in sequence learning either by observation or by physical practice. Noteworthy, in a last study, we went a step further by investigating more precisely the implication of Broca’s area in “high-level chunking” i.e. in establishing higher-order relationships between the elements composing a sequence during its learning. Even if those studies differed methodologically, a common finding is that a virtual lesion of Broca’s area invariably delays the response to the stimuli that initiate the pre-learned or learnt sequences, especially for structurally complex sequences. Overall, our results converge to the view that Broca’s area is crucially involved in learning and processing structured sequences whatever their type: abstract (key-presses sequences) or more ecological (action sequences), motor or cognitive, and thus emphasize its role as a possible “supramodal syntactic processor”. Besides this general conclusion, the present work highlights a particular facet of Broca’s area role: ensuring an accurate overall structure and in fine a well-formed sequence. Further investigations are needed to determine the parameters of this contribution, especially the critical time window during which such process takes place as well as the minimal level of structural complexity required to elicit Broca’s area involvement. This could help understanding more precisely the manifold implication of Broca’s area into syntax-related tasks.