Biomedical subjects

J F Démonet

Publications and source records attributed to J F Démonet.

At least 19 recordsLinked to original sources

PubMed2005

Auditory evoked potential patterns to voiced and voiceless speech sounds in adult developmental dyslexics with persistent deficits.

Auditory evoked potentials (AEPs) were recorded from eight developmental dyslexic adults with persistent reading, spelling and phonological deficits, and 10 non-dyslexic controls to voiced (/ba/) and voiceless (/pa/) consonant-vowel syllables. Consistent with previous data, non-dyslexics coded these stimuli differentially according to the temporal cues that form the basis of the voiced/voiceless contrast: AEPs had time-locked components with latencies that were determined by the temporal structure of the stimuli. Dyslexics were characterized by one of two electrophysiological patterns: AEP pattern I dyslexics demonstrated a differential coding of stimuli on the basis of some temporal cues, but with an atypically large number of components and a considerable delay in AEP termination time; AEP pattern II dyslexics demonstrated no clear differential coding of stimuli on the basis of temporal cues. These data reveal the presence of anomalies in cortical auditory processing which could underlie persistent perceptual and linguistic impairments in some developmental dyslexics. Furthermore, scalp AEP distribution maps showing the difference observed between /ba/ and /pa/ activity over time suggest that the regions implicated in the processing of crucial time-related acoustic cues were not systematically lateralized to the left hemisphere like they were for non-dyslexics. These findings may be conducive to a better understanding and treatment of perceptual dysfunctions in developmental language disorders.

Acoustic Stimulation↗

PubMed2004

Anatomy and time course of discrimination and categorization processes in vision: an fMRI study.

Studies investigating the cerebral areas involved in visual processes generally oppose either different tasks or different stimulus types. This work addresses, by fMRI, the interaction between the type of task (discrimination vs. categorization) and the type of stimulus (Latin letters, well-known geometrical figures, and Korean letters). Behavioral data revealed that the two tasks did not differ in term of percentage of errors or correct responses, but a delay of 185 ms was observed for the categorization task in comparison with the discrimination task. All conditions activated a common neural network that includes both striate and extrastriate areas, especially the fusiform gyri, the precunei, the insulae, and the dorsolateral frontal cortex. In addition, interaction analysis revealed that the right insula was sensitive to both tasks and stimuli, and that stimulus type induced several significant signal variations for the categorization task in right frontal cortex, the right middle occipital gyrus, the right cuneus, and the left and right fusiform gyri, whereas for the discrimination task, significant signal variations were observed in the right occipito-parietal junction only. Finally, analyzing the latency of the BOLD signal also revealed a differential neural dynamics according to tasks but not to stimulus type. These temporal differences suggest a parallel hemisphere processing in the discrimination task vs. a cooperative interhemisphere processing in the categorization task that may reflect the observed differences in reaction time.

Adult↗

PubMed2002

Lexical therapy and episodic word learning in dementia of the Alzheimer type.

The effect of a language therapy in a group of eight anomic mild patients (the Lexical Therapy group) was assessed by using a 5-month long Lexical Therapy in comparison with an occupational program used in a matched control group (AD; n = 8). The Lexical Therapy group benefited significantly from a language therapy as shown by the naming improvement postintervention. The improvement reached significance only for items that were included in the language therapy protocol and no significant generalization to untreated items was observed. In mild AD patients with anomia and no severe semantic impairment, a reinforcement of the relationship between the form of the object and the corresponding lexical label in episodic long term memory during language therapy may account for the observed lexical improvement.

Aged↗

PubMed2002

Neural substrates of spoken language rehabilitation in an aphasic patient: an fMRI study.

Little is known about the neural counterparts of speech therapy in aphasic patients. An fMRI experiment was performed before and after a specific and intensive speech output therapy in RC, a patient with long-lasting speech output deficit following a left-sided ischemic lesion. Overt picture naming and picture/word rhyming were used as activation tasks in RC and 6 control subjects. The naming task concerned the output lexicon deficit to be rehabilitated while rhyming referred to preserved levels of processing and was used to control for repetition effect. The speech therapy program improved naming performance. By comparison to the pattern observed before therapy, the naming task after therapy induced a pattern of activation close to that observed in control subjects, involving left-sided language areas surrounding the lesion. Speech therapy effect was associated with activations in Broca's area and the left supra-marginal gyrus, which might reflect a therapy-induced phonological compensatory strategy for naming.

Adult↗

PubMed2001

Dyslexia: cultural diversity and biological unity.

The recognition of dyslexia as a neurodevelopmental disorder has been hampered by the belief that it is not a specific diagnostic entity because it has variable and culture-specific manifestations. In line with this belief, we found that Italian dyslexics, using a shallow orthography which facilitates reading, performed better on reading tasks than did English and French dyslexics. However, all dyslexics were equally impaired relative to their controls on reading and phonological tasks. Positron emission tomography scans during explicit and implicit reading showed the same reduced activity in a region of the left hemisphere in dyslexics from all three countries, with the maximum peak in the middle temporal gyrus and additional peaks in the inferior and superior temporal gyri and middle occipital gyrus. We conclude that there is a universal neurocognitive basis for dyslexia and that differences in reading performance among dyslexics of different countries are due to different orthographies.

Adult↗

PubMed2001

Language and brain: what is up? What is coming up?

The classical aphasiological model of brain/language relationships is nowadays complemented by independent results from functional neuroimaging studies using techniques such as Positron Emission Tomography, functional Magnetic Resonance Imaging, or Event-Related Electro-Encephalography and Magneto-Encephalography mapping. Although brain mapping of language is still hampered by many methodological pitfalls, these methods now appear reliable and provide a renewed description of the temporal spatial dynamics of neural ensembles subserving language functions. Moreover, neuroimaging techniques should also shed a new light on remaining difficult issues such as neural and functional plasticity in developmental or post-lesional contexts.

Aphasia↗

PubMed2001

[Developmental dyslexia: contribution of modern neuropsychology].

In France about 1 million children are thought to present learning disabilities for reading that in most cases correspond to developmental dyslexias. These are specific and constitutional deficits that prevent rapid and automatic reading abilities from developing, in spite of a normal intelligence and normal visual and auditory acuity. The most frequent form of dyslexia is called "phonological dyslexia" as it results from a deficit of perception and manipulation of phonemes (i.e. "phonological awareness"). Functional neuro-imaging has great potential for helping understand such functional deficits that are likely to be caused by microscopic abnormalities such as micro-polygyria localised in the perisylvian cortex. Tomographic methods have shown deficits of activation of these cortical areas. Electrophysiological methods revealed abnormalities of various events such as MMN or N400, and more recently, delayed latencies of earlier markers suggesting deficits of encoding short-lived features of speech signal.

Awareness↗

PubMed2000

Information processing in large-scale cerebral networks: the causal connectivity approach.

Today, cognitive functions are considered to be the offspring of the activity of large-scale networks of functionally interconnected cerebral regions. The interpretation of cerebral activation data provided by functional imaging has therefore recently moved to the search for the effective connectivity of activated regions, which aims at understanding the role of anatomical links in the activation propagation. Our assumption is that only causal connectivity can offer a real understanding of the links between brain and mind. Causal connectivity is based on the anatomical connection pattern, the information processing within cerebral regions and the causal influences that connected regions exert on each other. In our approach, the information processing within a region is implemented by a causal network of functional primitives, which are the interpretation of integrated biological properties. Our choice of a qualitative representation of information reflects the fact that cerebral activation data are only the approximate view, provided by imaging techniques, of the real cerebral activity. This explicit modeling approach allows the formulation and the simulation of functional and physiological assumptions about activation data. Two alternative models explaining results of the striate cortex activation described by Fox and Raichle (Fox PT, Raichle ME (1984) J. Neurophysiol 51:1109-1120; Fox PT, Raichle ME (1985) Ann Neurol 17:303-305) are provided as an example of our approach.

Animals↗

PubMed2000

[Generation of isolated words in normal subjects].

This study aims at analyzing the performance according to age, sex, and educational level in 112 French normal subjects on word generation tests, i.e., time-limited production of a noun or a verb semantically linked to a target "noun" or "verb". The nature of hits (exclusively production of nouns) has moreover been analyzed according to different features of the targets: noun vs verb, names of natural objects vs handmade objects, transitive vs intransitive verbs. Results show 1) a significant effect of age and educational level on performance, 2) a degree of difficulty, the verb/verb production test being significantly more difficult than the three other tests, 3) a convergency effect according to the natural vs handmade dichotomy, as subjects tend to produce, without specific instruction, an item belonging to the same category as that of the stimulus, and 4) object nouns tend to be produced in response to transitive verbs whereas subject nouns are not predominantly produced in response to intransitive verbs.

Adult↗

PubMed1999

Temporal sorting of neural components underlying phonological processing.

Event-related haemodynamic responses (EHRs) were recorded in subjects performing phonological tasks to test whether distinguishable temporal involvement of corresponding neural components would show through. A sequence of activation leading from primary auditory cortices to premotor regions emerged in the fast repetition and the phoneme monitoring tasks used. EHRs peaked significantly earlier in Wernicke's area (phonological decoding) than in Broca's area, the left supramarginal gyrus and the precentral gyrus (phonological rehearsal). Moreover, the sensitivity of within cluster temporal gradients to the nature of the tasks indicated either sensory to association cortex synchronization for fast repetition or delayed analysis for phoneme monitoring. These results are consistent with previous findings on working memory and show that fMRI permits temporal tracking of cognitive activations.

Adult↗

PubMed1999

ERP correlates of phoneme perception in speech and sound contexts.

To address the question of the existence of a phonetic module for speech perception, event-related potentials were recorded using a 32 channel system in subjects performing a detection task where the target was the ambiguous, noise-like phoneme /f/ presented either among syllables (speech context) or among environmental sounds (non-speech context). Significant context effects were observed on the N2/P3 complex elicited by the target. In particular, a well localized N2b (250-280 ms) appeared at the left temporoparietal sites on the difference wave between contexts as the result of an enhanced negativity when the target was presented among non-speech stimuli. These findings suggest the involvement of the left temporoparietal region in autonomous, modular processes of speech perception.

Acoustic Stimulation↗

PubMed1999

Hemispheric preponderance in categorical and coordinate visual processes.

Two experiments were conducted to address whether a left hemispheric bias would be observed for categorical processing in both 'what' and 'where' systems (experiment 1) while a reverse bias would characterize coordinate processing whatever the systems (experiment 2). Young normal subjects were tested using divided visual field tasks. The results of experiment 1 indicated that subjects made categorical judgments in both what and where systems faster when stimuli are presented to the left hemisphere. The results of experiment 2 showed a significant interaction between visual field and difficulty of processing coordinate relationships. Indeed, a left-hemisphere advantage was observed when the task required easy processing whereas a right-hemisphere advantage was noted for difficult distinctions either in location (where system) or in lightness (what system). The left-hemisphere advantage we observed for categorization in both systems confirms the Kosslyn's hypothesis (1989) for the where system and suggests that the same left-hemisphere advantage also exists for the what system. Concerning coordinate processing, our findings highlight the influence of processing difficulty on the hemispheric lateralization and evidence a right hemispheric advantage for difficult coordinate processing and a left hemispheric advantage for easy coordinate processing. The results are discussed in terms of possible link between on the one hand difficulty and coordinate processing, and easiness and categorization on the other hand.

Adult↗

PubMed1998

ERP mapping in phonological and lexical semantic monitoring tasks: A study complementing previous PET results.

Previous PET results identified distinct neural systems involved with phonology (vicinity of the left sylvian fissure) and lexical semantics (left inferior temporal, left superior frontal, bilateral inferior parietal regions). In the time domain, the phonological task was thought to involve serial parsing of pseudo-words, whereas the lexical semantic task would correspond to probabilistic automated access to meaning. Auditory event-related potentials (ERPs) elicited by the same tasks were explored on 32 channels in 12 male volunteers. Subjects had to categorize as target a second element of a pair of pseudo-words or words if a preceding target was detected in the first element. Depending on the absence/presence of target in the first element, a RELEASE condition and a HOLD condition were distinguished. RELEASE and HOLD ERPs split earlier in the semantic task than in the phonological task (300 versus 412 ms after SOT, respectively), although words lasted longer than pseudo-words. Corresponding paired t test maps showed a predominance of differences over left perisylvian regions for the phonological task and posterior bilateral regions for the semantic task. Underlying generators were investigated using BESA (Scherg, 1990) with global task ERPs. Six dipoles-constrained according to PET clusters-brought residual variance down to 0.36%, from 364 to 565 ms after SOT, in both tasks. Relative dipole amplitudes suggested a left-sided functional asymmetry for phonology. These results support the hypothesis of left perisylvian serial processing for phonology contrasting with bihemispheric parallel access for semantics and substantiates BESA for temporally tackling cognitive processes.

Adult↗

PubMed1998

Brain correlates of memory processes in patients with dementia of Alzheimer's type: a SPECT Activation Study.

Task-induced changes in regional cerebral blood flow (rCBF) during verbal episodic memory activation were compared in 17 right-handed patients with dementia of the Alzheimer's type (DAT) and 20 healthy volunteers. Regional cerebral blood flow was assessed using single photon emission computed tomography (SPECT) and an injection of 133Xe (xenon, isotope of mass 133) in 21 regions of interest (ROI) during rest, passive listening to 36 words, and memorizing of a 12-word list repeated three times. In healthy subjects, memory-listening comparison showed activation of a distributed system involving several left-sided ROI, especially the posterior inferior frontal region. In patients with DAT, the same pattern of activation was found for listening-rest comparison, and no significant changes were found in memory-listening comparison. During listening compared with rest, significant activation was observed in left-sided hypoperfused regions. A significant correlation between memory performance and rCBF recorded in patients with DAT during the memory task was found only in the right lateral frontal region, a region that was not hypoperfused significantly in patients. The involvement of this region might relate to either retrieval effort or actual performance of patients with DAT on the memory task.

Aged↗

PubMed1997

Subcortical aphasia(s): a controversial and promising topic.

Many aspects of subcortical aphasias are not accounted for by a unique pathophysiological mechanism; rather the diversity of the observed symptoms suggests that many functional ensembles, more or less related, to language functions may be disturbed by subcortical lesions. However, such disorders are only moderate in intensity and subcortical aphasias certainly constitute an interesting model for studying the dynamics of recovery of language functions and related neural systems, combining neurolinguistic concepts with advanced functional imaging techniques.

Aphasia, Broca↗

PubMed1997

Age related cognitive decline: a clinical entity? A longitudinal study of cerebral blood flow and memory performance.

OBJECTIVES: To evaluate the changes in regional cerebral blood flow (rCBF) and memory performance in patients with age related cognitive decline (ARCD) who did and did not become demented during a follow up period. METHODS: Twenty four patients with ARCD were recruited from an outpatient memory clinic, of whom 18 were followed up over a mean period of two years. Eighteen patients with mild to moderate probable Alzheimer's disease and 18 aged normal controls were followed up over a mean period of three years. Memory performance and rCBF were evaluated quantitatively at inclusion and during follow up, using single photon emission computed tomography with xenon-133 injection and three subtests of the Wechsler memory scale (logical memory, paired associated learning, and digit span). RESULTS: Patients with ARCD showed decreased rCBF and memory performance at initial evaluation compared with controls. Five of them became demented during the follow up period, with further decline in memory and rCBF. At inclusion, the only feature that distinguished these five patients as a group from the remainder was a pronounced temporoparietal asymmetry. The 13 patients with ARCD who did not become demented still exhibited impaired memory and rCBF at follow up, but without any further decline and no increase in flow asymmetry. CONCLUSIONS: Apart from patients in the preclinical phase of Alzheimer's disease, the ARCD category includes non-demented patients who have brain dysfunction that may represent a distinct clinical entity.

Aged↗

PubMed1996

Brain functional profiles in formal and semantic fluency tasks: a SPECT study in normals.

SPECT method is used to analyze changes in regional cerebral blood flow in a group of 19 normal subjects during a baseline task (repetition of two words) and two verbal fluency tasks, a semantic fluency and a formal fluency. The semantic fluency task was associated with a relative CBF increase in the right dorso-lateral and medial frontal region when compared with that seen in the baseline condition. No specific activation was found for the formal fluency task compared to that seen in the baseline task. We suggest that the activation of the right frontal region reflects semantic categorization strategies in semantic fluency. The lack of activation of the left frontal region may be due to an activation induced by the nature of the baseline task (i.e., a self-paced repetition task).

Adult↗

PubMed1996

[Cognitive neuroanatomy of language: contribution of functional cerebral imaging].

This review summarizes the main results obtained to date, concerning the issue of brain/language relationships, from studies using the modern techniques of in vivo functional imaging of the brain, namely Positron Emission Tomography (PET) and, to a lesser extent, the still experimental method of functional Magnetic Resonance Imaging. These studies mostly rely on the so-called cognitive activation paradigm, which consists of recording blood flow variations as measured in PET in normal volunteers during various linguistic tasks compared to reference tasks. Since the late 80's, a great deal of literature has been accumulating, including studies of various aspects of language functions: perceptual (auditory and visual) processing of linguistic materials, expression, verbal memory, semantics... The first few results obtained have already led to challenge some previously widely held dogmas, derived from classical anatomo-clinical studies, such as the role of Wernicke's area in verbal comprehension and the role of Broca's area in language production. The involvement of the left supramarginal gyrus in some aspects of phonological processing has been directly demonstrated. At the same time, some studies have yielded unexpected results suggesting, for instance, a special role for the inferior lateral parts of the left frontal lobe in semantic and/or voluntary aspects of language processing and episodic verbal memory. Among other studies, those concerning the mechanisms of verbal short-term memory have provided interesting parallels with cognitive models. The demonstration, under certain experimental conditions, of an involvement of hitherto unsuspected cortical regions (such as the insular or the temporopolar cortices) is of potential considerable importance. Finally, with current technical improvements of the imaging methods, it will be shortly possible to take into account the important issue of interindividual variations as well as to perform activation studies in brain-damaged patients.

Brain↗