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Language and brain: production-perception links/Langage et cerveau: lien entre la perception et la production

Abstract/Résumé: Historically, according to a ‘localizationist’ view of the brain and based on lesion studies, speech production and perception were thought to be based on distinct brain regions, with Broca’s area (a region of the left inferior frontal cortex) underlying production, and Wernicke’s area (a cortical region situated at the junction of the temporal and parietal lobes) underlying speech perception. With the advent of modern neuroimaging methodologies, investigations of speech processing have shown that speech production and speech perception rely on partially overlapping networks of cortical regions, including but not limited to Broca and Wernicke’s areas. Increasing quantities of evidence demonstrate that not only is there a strong anatomical link between regions traditionally associated with speech perception and production, but that there is also a significant functional link between them. The motor theory of speech perception proposes that speech perception relies on identification of vocal tract gestures that would be required to generate it. Although this claim has proved to be controversial over the five decades since it was first proposed by Liberman and colleagues, some elements of the theory are supported by recent discoveries. Specifically, evidence is mounting that an auditory-motor link may play a role in speech perception under certain circumstances. For example, during speech perception under adverse conditions, there is activation in premotor brain regions. Other studies have shown that during auditory speech perception, there is increased excitability of motor regions underlying speech production, and that this activation is correlated with activity within Broca’s area. Studies of phoneme perception implicate Broca’s area and regions of pre-motor cortex, alongside auditory processing regions in phoneme categorization. Structural brain imaging has shown that cortical regions implicated in phonetic perception and production partially overlap. In addition, phonological working memory is thought to be subtended by an auditory-motor loop. Nevertheless, it must not be neglected that patients suffering from lesions to brain regions concerned with speech production do not necessarily completely lose their ability to comprehend speech. In this talk we will review evidence for convergent and also for divergent brain functional and structural correlates of speech perception and production, with a focus on the phonetic level of processing. We will also show how some of these principles extend to processing at higher levels of the language hierarchy.