Differential activity in Heschl's gyrus between deaf and hearing individuals is due to auditory deprivation rather than language modality

Sensory cortices undergo crossmodal reorganisation as a consequence of sensory deprivation. Congenital deafness in humans represents a particular case with respect to other types of sensory deprivation, because cortical reorganisation is not only a consequence of auditory deprivation, but also of language-driven mechanisms. Visual crossmodal plasticity has been found in secondary auditory cortices of deaf individuals, but it is still unclear if reorganisation also takes place in primary auditory areas, and how this relates to language modality and auditory deprivation.  Here, we dissociated the effects of language modality and auditory deprivation on crossmodal plasticity in Heschl's gyrus as a whole, and in cytoarchitectonic region Te1.0 (likely to contain the core auditory cortex). Using fMRI, we measured the BOLD response to viewing sign language in congenitally or early deaf individuals with and without sign language knowledge, and in hearing controls.  Results show that differences between hearing and deaf individuals are due to a reduction in activation caused by visual stimulation in the hearing group, which is more significant in Te1.0 than in Heschl's gyrus as a whole. Furthermore, differences between deaf and hearing groups are due to auditory deprivation, and there is no evidence that the modality of language used by deaf individuals contributes to crossmodal plasticity in Heschl's gyrus

The impact of early language exposure on the neural system supporting language in deaf and hearing adults

Deaf late signers provide a unique perspective on the impact of impoverished early language exposure on the neurobiology of language: insights that cannot be gained from research with hearing people alone. Here we contrast the effect of age of sign language acquisition in hearing and congenitally deaf adults to examine the potential impact of impoverished early language exposure on the neural systems supporting a language learnt later in life. We collected fMRI data from deaf and hearing proficient users (N = 52) of British Sign Language (BSL), who learnt BSL either early (native) or late (after the age of 15 years) whilst they watched BSL sentences or strings of meaningless nonsense signs. There was a main effect of age of sign language acquisition (late > early) across deaf and hearing signers in the occipital segment of the left intraparietal sulcus. This finding suggests that late learners of sign language may rely on visual processing more than early learners, when processing both linguistic and nonsense sign input - regardless of hearing status. Region-of-interest analyses in the posterior superior temporal cortices (STC) showed an effect of age of sign language acquisition that was specific to deaf signers. In the left posterior STC, activation in response to signed sentences was greater in deaf early signers than deaf late signers. Importantly, responses in the left posterior STC in hearing early and late signers did not differ, and were similar to those observed in deaf early signers. These data lend further support to the argument that robust early language experience, whether signed or spoken, is necessary for left posterior STC to show a 'native-like' response to a later learnt language

Nature of crossmodal plasticity in the blind brain and interplay with sight restoration

Thèse réalisée en cotutelle avec l'Université catholique de Louvain.Ce travail de thèse s’est intéressé à la plasticité cérébrale associée à la privation/restauration visuelle. A travers deux études transversales utilisant l’imagerie par résonance magnétique fonctionnelle auprès d’un groupe de participants présentant une cécité congénitale ou précoce (ainsi qu’auprès d’un groupe contrôle de participants voyants), nous avons tenté de caractériser la manière dont le cortex occipital - typiquement dédié au traitement de l’information visuelle - se réorganise afin de traiter différents stimuli auditifs. Nous démontrons qu’en cas de cécité précoce, différentes régions du cortex occipital présentent une préférence fonctionnelle pour certains types de stimuli non-visuels, avec une spécialisation fonctionnelle qui respecte celle de régions typiquement impliquées dans le traitement d’informations similaires en vision. Ces découvertes constituent une avancée conceptuelle concernant le rôle joué par les contraintes intrinsèques d’une part, et par l’expérience d’autre part, dans l’émergence de réponses sensorielles et fonctionnelles du cortex occipital. D’une part, l’observation de réponses occipitales à la stimulation auditive chez le non-voyant précoce (réorganisation transmodale) rend compte de la capacité du cortex occipital à réorienter sa modalité sensorielle préférentielle en fonction de l’expérience. D’autre part, l’existence de modules cognitifs spécialisés dans le cortex occipital du non-voyant précoce, semblables à ceux du cerveau voyant, démontre les contraintes intrinsèques imposées à une telle plasticité. Dans une étude de cas longitudinale, nous avons également exploré comment les changements plastiques associés à la cécité interagissent avec une récupération visuelle partielle à l’âge adulte. Nous avons réalisé des mesures pré et post-opératoires auprès d’un patient ayant récupéré la vision, en combinant les techniques comportementales ainsi que de neuroimagerie fonctionnelle et structurelle afin d’investiguer conjointement l’évolution de la réorganisation transmodale et de la récupération des fonctions visuelles à travers le temps. Nous démontrons que les changements structurels et fonctionnels caractérisant le cortex occipital du non-voyant sont partiellement réversibles suite à une récupération visuelle à l’âge adulte. De manière générale, ces recherches témoignent de l’importante adaptabilité du cortex occipital aux prises avec des changements drastiques dans l’expérience visuelle.The present Ph.D. work was dedicated to the study of experience-dependent brain plasticity associated with visual deprivation/restoration. In two cross-sectional studies involving the use of functional magnetic resonance imaging in a group of participants with congenital or early blindness (and in a control group of sighted participants), we attempted to characterize the way the occipital cortex - typically devoted to vision – reorganizes itself in order to process different auditory stimuli. We demonstrate that in case of early visual deprivation, distinct regions of the occipital cortex display a functional preference for specific non-visual attributes, maintaining a functional specialization similar to the one that characterizes the sighted brain. Such studies have shed new light on the role played by intrinsic constraints on the one side, and experience on the other, in shaping the modality- and functional tuning of the occipital cortex. On the one hand, the observation of occipital responses to auditory stimulation (crossmodal plasticity) highlights the ability of the occipital cortex to reorient its preferential tuning towards the preserved sensory modalities as a function of experience. On the other hand, the observation of specialized cognitive modules in the occipital cortex, similar to those observed in the sighted, highlights the intrinsic constraints imposed to such plasticity. In a longitudinal single-case study, we further explored how the neuroplastic changes associated with blindness may interact with the newly reacquired visual inputs following partial visual restoration in adulthood. We performed both pre- and post-surgery measurements in a sight-recovery patient combining behavioral, neurostructural and neurofunctional methods in order to jointly investigate the evolution of crossmodal reorganization and visual recovery across time. We demonstrate that functional and structural changes evidenced in the visually-deprived occipital cortex can only partially reverse following sight restoration in adulthood. Altogether, our findings demonstrate the striking adaptability of the occipital cortex facing drastic changes in visual experience

The neural basis of audiovisual integration

Our perception is continuous and unified. Yet, sensory information reaches our brains through different senses and needs to be processed in order to create that unified percept. Interactions between sensory modalities occur already at primary cortical levels. The purpose of such interactions and what kind of information they transmit is still largely unknown. The current thesis aimed to reveal the interactions between auditory pitch and visual size in polar coordinates, two modality specific stimulus features that have robust topographic representations in the human brain. In Chapter 1, I present the background of cross-modal interactions in early sensory cortices and of the pitch-size relationship. In Chapter 2, we explored the pitch-size relationship in a speeded classification task and, in Chapter 3, at the level of functional Magnetic Resonance Imaging activation patterns. In Chapter 4, we investigated the effects of actively learning a specific pitch-size mapping during one session on the speeded classification task. In Chapter 5, we extended learning over multiple sessions and examined learning effects with behavioral and neural measures. Finally, in Chapter 6, I summarize the findings of the thesis, its contributions to the literature, and outline directions for future research

Perception audiovisuelle de la parole chez le sourd postlingual implanté cochléaire et le sujet normo-entendant : étude longitudinale psychophysique et neurofonctionnelle

Nos travaux ont consisté à étudier les mécanismes perceptifs et neuronaux impliqués lors de la perception audiovisuelle de la parole chez des patients sourds postlinguaux implantés cochléaires et des sujets contrôles normo-entendants. Dans ce but nous avons testé les performances audiovisuelles des patients implantés au cours de suivis longitudinaux en comportemental et en tomographie par émission de positrons, ainsi qu'au travers d'un paradigme de parole audiovisuelle incongruente (McGurk). Pour comparaison nous avons systématiquement testé des sujets contrôles dans les mêmes conditions. Nous avons également testé ces derniers en condition de lecture labiale puis en utilisant des dégradations de la parole auditive et audiovisuelle par du bruit blanc et par une simulation acoustique d'implant cochléaire. L'ensemble de ces études concorde pour indiquer, à la suite de l'implantation cochléaire, une réorganisation rapide des réseaux corticaux multisensoriels impliqués dans l'intégration audiovisuelle de la parole, aboutissant à une combinaison optimale des indices perceptifs audiovisuelsOur present work consisted in studying perceptual and neuronal mechanisms involved during audiovisual speech perception in postlingually deaf cochlear-implant patients and normally-hearing controls. With this aim in view we tested audiovisual performance for implanted patients during longitudinal follow-ups using behavioral methods and positron emission tomography neuroimagery, as well as incongruent audiovisual speech (McGurk paradigm). In order to achieve appropriate comparisons, control subjects were tested in identical conditions. Moreover they were tested in a speechreading condition and with auditory and audiovisual speech degraded through white-noise masks and cochlear-implant computer simulations. Results from these studies agree to point out, following cochlear implantation, a fast reorganization of multisensory cortical networks involved in audiovisual speech integration, leading to an optimal combination of audiovisual perceptual cue

Pitch-related auditory skills in children with cochlear implants : The role of auditory working memory, attention and music

The cochlear implant (CI) provides a sensation of hearing for deaf-born children. However, many CI children show poor language outcomes, which may be related to the deficiency of CIs in delivering pitch. This thesis studies the development of those neural processes and behavioural skills linked to the perception of pitch which may play a role in language acquisition. We measured with event-related brain potentials (ERPs) the neural discrimination of and attention shift to changes in music, the perception of word and sentence stress and related acoustic cues, and the auditory working memory (forward digit span) in 4̶ 13-year-old normally hearing (NH) and early-implanted children. We studied how the development of these aspects is related to musical activities known to advance brain development and perceptual skills in the NH population, and whether the perception of music is connected to word stress or visuospatial perception in NH adults. With regard to the development of neural responses, we found for the CI children usually well-formed ERP waveforms resembling those found for the NH children. However, some brain responses implied impoverished processing for the CI children, especially for timbre and pitch. The CI children who sang regularly at home were advantaged over the other CI children for the development of attention shift, which was linked to improved auditory working memory, implying better neural discrimination, an advantaged development of neural networks for attention and better updating of auditory working memory for the CI singers. For the CI children perception of word and sentence stress improved with improving discrimination of pitch (f0) and intensity and auditory working memory. Only the CI children participating in supervised musical activities performed and developed similarly to the NH children in these skills. The perception of musical rhythm improved with improving word stress and visuospatial perception for the NH adults. The results indicate that (i) perception of music and speech are connected not only via pitch and timbre, but also via rhythm, and (ii) the combination of singing at home and taking part in supervised musical activities, using also rhythmic exercises and visual cues, might be the best way to optimize pitch-related abilities, underlying cognitive functions, spoken language skills and quality of life for early-implanted children.Sisäkorvaistutteella (SI) kuulevien, kuurona syntyneiden lasten puhekielen taidot vaihtelevat paljon ja ovat usein heikompia kuin kuulevilla lapsilla. Tämä saattaa liittyä SI:n heikkoon kykyyn välittää äänten korkeuksia. Tässä väitöskirjassa tarkasteltiin puhekielen kehitystä tukevien, äänen korkeuksien havaitsemiseen liittyvien hermostollisten mekanismien, kuulomuistin ja kuuntelutaitojen kehitystä 4 13-vuotiailla kuulevilla ja SI-lapsilla. Musiikillisten äänten hermostollista erottelua ja kuulotarkkaavuutta tarkasteltiin mittaamalla kuuloherätevasteita aivosähkökäyrällä (EEG). Sana- ja lausepainon havaitsemista ja näihin liittyvien akustisten vihjeiden erottelukykyä tutkittiin kuuntelukokeilla ja kuulomuistia numerotoistotestillä. Musiikin harrastamisen tiedetään parantavan kuulevien lasten kuulohavaintotaitoja, kuulomuistia ja kuulotarkkaavuutta. Siksi tarkastelimme SI-lasten ryhmässä musiikkiaktiviteettien yhteyksiä mittaustuloksiimme. Tutkimme myös, onko musiikin havaitseminen yhteydessä sanapainon ja suuntien havaitsemiseen kuulevilla aikuisilla. SI-lasten kuuloherätevasteet olivat samantapaisia kuin kuulevien lasten, mutta vasteet erityisesti muutoksille musiikillisten äänten laadussa ja korkeudessa heijastivat SI-lasten heikkoa kuuloerottelukykyä ja -tarkkaavuutta. Säännöllisesti kotona laulavien SI-lasten kuulotarkkaavuusvasteet kehittyivät voimakkaammiksi ja nopeammiksi kuin muiden SI-lasten. Nämä vasteet olivat nopeampia paremman kuulomuistin myötä. Tulokset viittaavat laulavien SI-lasten hyvään hermostolliseen erottelukykyyn, kuulotarkkaavuuden aivoverkostojen kehitykseen ja kuulomuistin päivitykseen. Sana- ja lausepainon havaitseminen parantui hyvän äänen korkeuden (f0) ja voimakkuuden erottelun sekä kuulomuistin myötä, joissa vain ohjattuihin musiikkiaktiviteetteihin osallistuneet SI-lapset kehittyivät yhtä hyvin kuin kuulevat lapset. Musiikin rytmien havaitseminen parantui hyvän sanapainon ja suuntien havaitsemisen myötä. Tulokset korostavat musiikin havaitsemisen olevan yhteydessä musiikin rytmien havaitsemiseen, ei pelkästään äänen korkeuksien ja laatujen havaitsemiseen. Ne korostavat kotona laulamisen sekä äänen korkeuden, myös rytmin havaitsemisen harjoituksia ja suuntavihjeitä (kuten laululeikkejä), sisältävän ohjatun musiikkitoiminnan tärkeyttä SI lasten puhekielen ja elämänlaadun parantamisessa