Historically, it was neuropsychology, i.e. the study of the consequences of brain lesions, that led to the first advances in understanding the brain mechanisms of linguistic operations. Early observations quickly led to a dissociation between Broca's aphasia with agrammatism, in which there is a massive deficit in articulation and syntactic sentence organization, and Wernicke's aphasia, in which these functions can be largely preserved. As early as Paul Broca's studies, the third inferior frontal convolution of the left hemisphere Broca's area ) appeared to be essential for the production of articulated language. However, the old idea that this area is only involved in language production has now been invalidated : patients with " Broca's aphasia " also have considerable difficulty understanding the organization of the sentences they hear, even in tasks that do not require speech production. Understanding and manipulating semantically reversible sentences, such as " the cat that the dog bites is black ", in which only the syntax makes it clear who is doing what to whom, poses great difficulties for them. Caramazza and Zurif (1976) conclude that " in Broca's aphasics, the deficit affects a general language processing mechanism, involved in syntax in both comprehension and production ".
The crucial role of the left inferior frontal region, but also of the upper regions of the left temporal lobe, was rapidly confirmed, first by direct cortical stimulation, performed during surgery, and then by the first positron emission tomography (PET) brain imaging studies. Today's functional MRI can identify these language networks in a matter of minutes, for example by contrasting listening to a known and an unknown language, or by examining which brain areas synchronize in the brains of two people listening to the same text. Within this vast set of regions, the syntactic complexity of sentences specifically modulates the activity of the left inferior frontal region (Broca's area ) and a posterior region of the left superior temporal sulcus. These two regions appear to be systematically activated as soon as a person has to mentally manipulate syntax trees of varying complexity.
