Bases neurophysiologiques de la mémoire de reconnaissance verbale et visuelle (étude des potentiels évoqués de surface et intra-cérébraux)

The main purpose of the current studies was to address the issue of hemispheric specialization during the multi-step process of recognition, taking into account the confounding factors of tasks (encoding vs. recognition) and processing stages. The first study of scalp evoked potentials in healthy volunteers allowed the identification of a negative wave, peaking around 270 ms. This N270 was evoked by both categorization and recognition tasks of words and abstract designs, was functionally and anatomically distinct from the N400 and was thought to reflect the processing stage of perceptivo-mnesic transition. The process reflected by the N270 was asymetric according to the material, was not modulated by the repetition effect, and was best modelled by spatially restricted generators localized in the rhinal cortex. In the second study, we used intra-cerebral evoked potentials and showed that activity recorded within ento and peri-rhinal cortices had distinct temporal dynamics, and distinct functional properties. Peri-rhinal activity was characterized by two successive potentials, the N230 and N400, modulated by a repetition-suppression effect. Ento-rhinal activity was characterized by a N250 potential, modulated by the material and by a repetition-enhancement effect which was also observed in hippocampus. These intra-cerebral data support the hypothesis that the scalp N270 would result from the summation of the peri-rhinal N230 and ento-rhinal N250. In the third study, we studied the reorganization of perceptivo-mnesic networks in patients with left or right temporal lobe epilepsy. Results suggests that this potential may be used as an index of residual functionality of medial temporal structures, ispilateral to the epileptogenic zone.L objectif de ce travail était d étudier avec les potentiels évoqués la question de la spécialisation hémisphérique au cours de la reconnaissance, en tenant compte de la tâche et surtout des différentes étapes qui jalonnent la transition perceptivo mnésique de la voie visuelle ventrale vers le lobe temporal médial. La première étude des potentiels évoqués de surface chez des volontaires sains a identifié une composante négative culminant autour de 270 ms (appelée N270) évoquée à la fois par la catégorisation et la reconnaissance des mots et des images abstraites, qui était fonctionnellement et anatomiquement distincte de la N400. La N270 reflétait un processus asymétrique en fonction du matériel, qui n était pas modulée par l effet de répétition et dont les générateurs étaient spatialement plus localisés (moins distribués) que ceux de la N400, avec une source majeure au niveau du cortex rhinal. Dans la seconde étude nous avons montré à partir d enregistrements intra cérébraux que les cortex ento et péri-rhinal se différenciaient par leur dynamique temporelle, et par des propriétés fonctionnelles distinctes. L activité du cortex péri-rhinal était caractérisée par deux potentiels successifs N230 puis N400 modulés par l effet de répétition-suppression. L activité du cortex ento-rhinal était caractérisée par un potentiel N250, modulé par la nature du matériel et par un effet de répétition-amplification qui était également observé dans l hippocampe. Ces données confortent l hypothèse selon laquelle la N270 de scalp résulte de la sommation des potentiels N230 péri-rhinal et N250 ento-rhinal. Enfin, dans la troisième étude, nous avons étudié la réorganisation des réseaux perceptivo-mnésiques induite par l épilepsie temporale en à l aide des potentiels évoqués de reconnaissance de surface. Nos résultats suggèrent que ce potentiel pourrait refléter la fonctionnalité résiduelle des structures temporales internes ipsilatérales à l épilepsie.AIX-MARSEILLE2-BU Méd/Odontol. (130552103) / SudocSudocFranceF

Organisation anatomo-fonctionnelle du langage dans l'épilepsie temporale

Quels sont les corrélats anatomo-fonctionnels sous tendant la compréhension et la production de la parole ? De la perception des indices acoustico-phonétiques à l accès au sens des mots, quel est le degré de latéralisation des réseaux mis en jeu dans le traitement du langage ? Nous avons tenté de répondre à ces questions au travers du modèle pathologique de l épilepsie temporale. En effet, dans le cadre du bilan pré-chirurgical de cette pathologie, l implantation d électrodes intracérébrales (SEEG) permet, lors de la cartographie fonctionnelle, l enregistrement in vivo de l activité corticale lors de processus linguistiques particuliers. Par ailleurs, l épilepsie temporale en raison de la potentielle désorganisation (critique et/ou inter-critique) de certaines structures impliquées dans la perception et/ou la production du langage, s avère être un modèle pathologique intéressant. Dans un premier travail, nous avons corrélé le traitement temporel d indices acousticophonétiques et la latéralisation hémisphérique pour le langage. A partir d enregistrements de surface, nous avons localisé les générateurs responsables de cette activité au niveau du cortex auditif gauche lorsque les sujets avaient une spécialisation hémisphérique gauche pour le langage. Puis nous avons mis en évidence, grâce à des enregistrements intracérébraux, le traitement parallèle de l information verbale au niveau de l hémisphère dominant pour le langage lors de tâches comportant un monitoring de mots ou pseudo-mots. Les processus lexico-sémantiques mettent en jeu préférentiellement la voie ventrale (temporale), les processus phonologiques mettent en jeu la voie dorsale. La région frontale inférieure semble contribuer à ces deux processus. Nous avons précisé le rôle de la région temporale et basale postérieure gauche dans l accès lexical. L anomie observée chez les patients dont l épilepsie concernait l hémisphère dominant pour le langage serait liée au dysfonctionnement de cette région. Notre dernière étude a confirmé le rôle clef de la région temporo-basale gauche dans l accès lexical ainsi que sa participation multi-modalitaire au traitement d un matériel verbal. L ensemble de ces travaux suggèrent (1) l existence d un traitement parallèle des informations lexico-sémantiques du langage parlé après un traitement spécifique des indices acousticophonétiques par le cortex auditif de l hémisphère dominant pour le langage (2) la participation de la région basale et postérieure temporale à l accès lexical dans la production, comme dans la perception de la parole.AIX-MARSEILLE2-BU Méd/Odontol. (130552103) / SudocSudocFranceF

Specialization of left auditory cortex for speech perception in Man depends on temporal coding

International audienceSpeech perception requires cortical mechanisms capable of analysing and encoding successive spectral (frequency) changes in the acoustic signal. To study temporal speech processing in the human auditory cortex, we recorded intracerebral evoked potentials to syllables in right and left human auditory cortices including Heschl's gyrus (HG), planum temporale (PT) and the posterior part of superior temporal gyrus (area 22). Natural voiced (/ba/, /da/, /ga/) and voiceless (/pa/, /ta/, /ka/) syllables, spoken by a native French speaker, were used to study the processing of a specific temporally based acoustico-phonetic feature, the voice onset time (VOT). This acoustic feature is present in nearly all languages, and it is the VOT that provides the basis for the perceptual distinction between voiced and voiceless consonants. The present results show a lateralized processing of acoustic elements of syllables. First, processing of voiced and voiceless syllables is distinct in the left, but not in the right HG and PT. Second, only the evoked potentials in the left HG, and to a lesser extent in PT, reflect a sequential processing of the different components of the syllables. Third, we show that this acoustic temporal processing is not limited to speech sounds but applies also to non-verbal sounds mimicking the temporal structure of the syllable. Fourth, there was no difference between responses to voiced and voiceless syllables in either left or right areas 22. Our data suggest that a single mechanism in the auditory cortex, involved in general (not only speech-specific) temporal processing, may underlie the further processing of verbal (and non-verbal) stimuli. This coding, bilaterally localized in auditory cortex in animals, takes place specifically in the left HG in man. A defect of this mechanism could account for hearing discrimination impairments associated with language disorders

Contributions of Electrophysiology for Identifying Cortical Language Systems in Patients with Epilepsy

International audienceA crucial element of the surgical treatment of medically refractory epilepsy is to delineate cortical areas that must be spared in order to avoid clinically relevant neurological and neuropsychological deficits post-operatively. For each patient, this typically necessitates determining the language lateralization between hemispheres and language localization within hemisphere. Understanding cortical language systems is complicated by two primary challenges: the extent of the neural tissue involved, and the substantial variability across individuals, especially in pathological populations. We review the contributions made through the study of electrophysiological activity to address these challenges. These contributions are based on the techniques of magnetoencephalography, intracerebral recordings, electrical cortical stimulation, and the electro-video analyses of seizures and their semiology. We highlight why no single modality alone is adequate to identify cortical language systems and suggest avenues for improving current practice

Troubles du traitement de la parole chez le dyslexique adulte

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Inter‐individual variability in dorsal stream dynamics during word production

International audienceThe current standard model of language production involves a sensorimotor dorsal stream connecting areas in the temporo-parietal junction with those in the inferior frontal gyrus and lateral premotor cortex. These regions have been linked to various aspects of word production such as phonological processing or articulatory programming, primarily through neuropsychological and functional imaging group studies. Most if not all the theoretical descriptions of this model imply that the same network should be identifiable across individual speakers. We tested this hypothesis by quantifying the variability of activation observed across individuals within each dorsal stream anatomical region. This estimate was based on electrical activity recorded directly from the cerebral cortex with millisecond accuracy in awake epileptic patients clinically implanted with intracerebral depth electrodes for pre-surgical diagnosis. Each region's activity was quantified using two different metrics-intracerebral evoked related potentials and high gamma activity-at the level of the group, the individual, and the recording contact. The two metrics show simultaneous activation of parietal and frontal regions during a picture naming task, in line with models that posit interactive processing during word retrieval. They also reveal different levels of between-patient variability across brain regions, except in core auditory and motor regions. The independence and non-uniformity of cortical activity estimated through the two metrics push the current model towards sub-second and sub-region explorations focused on individualized language speech production. Several hypotheses are considered for this within-region heterogeneity

Inter-Individual Variability in Dorsal Stream Dynamics During Word Production

The current standard model of language production involves a sensorimotor dorsal stream connecting areas in the temporo-parietal junction with those in the inferior frontal gyrus and lateral premotor cortex. These regions have been linked to various aspects of word production such as phonological processing or articulatory programming, primarily through neuropsychological and functional imaging group studies. Most if not all the theoretical descriptions of this model imply that the same network should be identifiable across individual speakers. We tested this hypothesis by quantifying the variability of activation observed across individuals within each dorsal stream anatomical region. This estimate was based on electrical activity recorded directly from the cerebral cortex with millisecond accuracy in awake epileptic patients clinically implanted with intracerebral depth electrodes for pre-surgical diagnosis. Each region's activity was quantified using two different metrics, intra-cerebral evoked related potentials and high gamma activity, at the level of the group, the individual, and the recording contact. Using picture naming task, the two metrics show simultaneous activation of parietal and frontal regions in line with models that posit interactive processing during word retrieval. They also reveal different levels of variability across brain regions and patients except in auditory and motor regions. The independence and non-uniformity of cortical activity according to the two metrics push the current model towards sub-second and sub-region explorations focused on individualized language speech production. Several hypotheses are considered for this within-region heterogeneity

Cortical Dynamics of Semantic Priming and Interference during Word Production: An Intracerebral Study

International audienc

Single-trial analysis of oddball event-related potentials in simultaneous EEG-fMRI

International audienceThere has recently been a growing interest in the use of simultaneous electroencephalography (EEG) and functional MRI (fMRI) for evoked activity in cognitive paradigms, thereby obtaining functional datasets with both high spatial and temporal resolution. The simultaneous recording permits obtaining event-related potentials (ERPs) and MR images in the same environment, conditions of stimulation, and subject state; it also enables tracing the joint fluctuations of EEG and fMRI signals. The goal of this study was to investigate the possibility of tracking the trial-to-trial changes in event-related EEG activity, and of using this information as a parameter in fMRI analysis. We used an auditory oddball paradigm and obtained single-trial amplitude and latency features from the EEG acquired during fMRI scanning. The single-trial P300 latency presented significant correlation with parameters external to the EEG (target-to-target interval and reaction time). Moreover, we obtained significant fMRI activations for the modulation by P300 amplitude and latency, both at the single-subject and at the group level. Our results indicate that, in line with other studies, the EEG can bring a new dimension to the field of fMRI analysis by providing fine temporal information on the fluctuations in brain activity