Biomedical subjects

Guillaume Thierry

Publications and source records attributed to Guillaume Thierry.

14 recordsLinked to original sources

PubMed2006

Perceptual and lexical effects in letter identification: an event-related potential study of the word superiority effect.

Most classical models of visual word recognition are based on sequentially organized levels of representation and involve feedback mechanisms to various extents. In this study, we aim at clarifying which of the early processing stages of visual word recognition are modulated by top-down lexical effects. We studied the identification of letters embedded in briefly presented words (e.g., TABLE) and illegal nonwords (e.g., GTFRS) using event-related potentials (ERPs). Participants were involved in the Reicher-Wheeler paradigm: they were asked to indicate which of two letters displayed above and below a string of hashes was flashed immediately before at fixation within a letter string, which was either a word or a nonword. Event-related potentials were significantly modulated by the lexical status of stimuli around 200 ms after stimulus onset, i.e., in the peaking window of the N1 component. In light of our results, we propose that visual word form representations can constrain letter identification at a prelexical stage i.e., during the extraction of letter-shape information. In addition, we show that this facilitatory top-down effect is sensitive to stimulus exposure duration.

Adult↗

PubMed2006

An event-related potential component sensitive to images of the human body.

One of the critical functions of vision is to provide information about other individuals. Neuroimaging experiments examining the cortical regions that analyze the appearance of other people have found partially overlapping networks that respond selectively to human faces and bodies. In event-related potential (ERP) studies, faces systematically elicit a negative component peaking 170 ms after presentation - the N170. To characterize the electrophysiological response to human bodies, we compared the ERPs elicited by faces, bodies and various control stimuli. In Experiment 1, a comparison of ERPs elicited by faces, bodies, objects and places showed that pictures of the human body (without the head) elicit a negative component peaking at 190 ms (an N190). While broadly similar to the N170, the N190 differs in both spatial distribution and amplitude from the N1 components elicited by faces, objects and scenes and peaks significantly later than the N170. The difference between N190 and N170 was further supported using topographic analyses of ERPs and source localization techniques. A unique, stable map topography was found to characterize human bodies between 130 and 230 ms. In Experiment 2, we tested the four conditions from Experiment 1, as well as intact and scrambled silhouettes and stick figures of the human body. We found that intact silhouettes and stick figures elicited significantly greater N190 amplitudes than their scrambled counterparts. Thus, the N190 generalizes to some degree to schematic depictions of the human form. Overall, our findings are consistent with intertwined, but functionally distinct, neural representations of the human face and body.

Adult↗

PubMed2006

Dissociating verbal and nonverbal conceptual processing in the human brain.

Functional neuroimaging has highlighted a left-hemisphere conceptual system shared by verbal and nonverbal processing despite neuropsychological evidence that the ability to recognize verbal and nonverbal stimuli can doubly dissociate in patients with left- and right-hemisphere lesions, respectively. Previous attempts to control for perceptual differences between verbal and nonverbal stimuli in functional neuroimaging studies may have hidden differences arising at the conceptual level. Here we used a different approach and controlled for perceptual confounds by looking for amodal verbal and nonverbal conceptual activations that are common to both the visual and auditory modalities. In addition to the left-hemisphere conceptual system activated by all meaningful stimuli, we observed the left/right double dissociation in verbal and nonverbal conceptual processing, predicted by neuropsychological studies. Left middle and superior temporal regions were selectively more involved in comprehending words--heard or read--and the right midfusiform and right posterior middle temporal cortex were selectively more involved in making sense of environmental sounds and images. Thus, the neuroanatomical basis of a verbal/nonverbal conceptual processing dissociation is established.

Acoustic Stimulation↗

PubMed2005

Phonological oddballs in the focus of attention elicit a normal P3b in dyslexic adults.

Difficulties in phonological processing have been proposed to be the core symptom of developmental dyslexia. Phoneme awareness tasks have been shown to both index and predict individual reading ability. In a previous experiment, we observed that dyslexic adults fail to display a P3a modulation for phonological deviants within an alliterated word stream when concentrating primarily on a lexical decision task [Fosker and Thierry, 2004, Neurosci. Lett. 357, 171-174]. Here we recorded the P3b oddball response elicited by initial phonemes within streams of alliterated words and pseudo-words when participants focussed directly on detecting the oddball phonemes. Despite significant verbal screening test differences between dyslexic adults and controls, the error rates, reactions times, and main components (P2, N2, P3a, and P3b) were indistinguishable across groups. The only difference between groups was found in the N1 range, where dyslexic participants failed to show the modulations induced by phonological pairings (/b/-/p/ versus /r/-/g/) in controls. In light of previous P3a differences, these results suggest an important role for attention allocation in the manifestation of phonological deficits in developmental dyslexia.

Adult↗

PubMed2005

Age of acquisition modulates the amplitude of the P300 component in spoken word recognition.

Words acquired earlier in life are easier to process in adulthood than words acquired later; this is known as the age of acquisition (AoA) effect. The goal of this study was to establish whether the P300 component of event-related potentials (ERPs) is sensitive to AoA. Early-acquired words (12.5%), late-acquired words (12.5%) and pseudo-words (75%) were presented in an auditory lexical decision task. The two sets of words were matched for length, word type, concreteness, imageability and, crucially, word frequency. Early-acquired words were recognised faster and more accurately than late-acquired words. In addition, AoA modulated ERP activity in centroparietal electrode sites, with early-acquired words eliciting a larger positivity (P300) than late-acquired words. This is the first study to demonstrate an ERP correlate of AoA effects. An important implication of our findings is that AoA may need to be controlled in ERP studies of lexical processing, especially in designs in which it is likely to be a confound (e.g., studies of lexical category effects).

Adolescent↗

PubMed2005

Speech-specific auditory processing: where is it?

Are speech-specific processes localized in dedicated cortical regions or do they emerge from developmental plasticity in the connections among non-dedicated regions? Here we claim that all the brain regions activated by the processing of auditory speech can be re-classified according to whether they respond to non-verbal environmental sounds, pitch changes, unfamiliar melodies, or conceptual processes. We therefore argue that speech-specific processing emerges from differential demands on auditory and conceptual processes that are shared by speech and non-speech stimuli. This has implications for domain- vs. process-specific cognitive models, and for the relative importance of segregation and integration in functional anatomy.

Auditory Perception↗

PubMed2005

Renewal of the neurophysiology of language: functional neuroimaging.

Functional neuroimaging methods have reached maturity. It is now possible to start to build the foundations of a physiology of language. The remarkable number of neuroimaging studies performed so far illustrates the potential of this approach, which complements the classical knowledge accumulated on aphasia. Here we attempt to characterize the impact of the functional neuroimaging revolution on our understanding of language. Although today considered as neuroimaging techniques, we refer less to electroencephalography and magnetoencephalography studies than to positron emission tomography and functional magnetic resonance imaging studies, which deal more directly with the question of localization and functional neuroanatomy. This review is structured in three parts. 1) Because of their rapid evolution, we address technical and methodological issues to provide an overview of current procedures and sketch out future perspectives. 2) We review a set of significant results acquired in normal adults (the core of functional imaging studies) to provide an overview of language mechanisms in the "standard" brain. Single-word processing is considered in relation to input modalities (visual and auditory input), output modalities (speech and written output), and the involvement of "central" semantic processes before sentence processing and nonstandard language (illiteracy, multilingualism, and sensory deficits) are addressed. 3) We address the influence of plasticity on physiological functions in relation to its main contexts of appearance, i.e., development and brain lesions, to show how functional imaging can allow fine-grained approaches to adaptation, the fundamental property of the brain. In closing, we consider future developments for language research using functional imaging.

Aphasia↗

PubMed2004

Electrophysiological evidence for language interference in late bilinguals.

The mechanism by which late bilinguals access the meaning of words presented in their second language (conceptual mediation or word association) is highly debated. Here we asked late Chinese-English bilinguals undergoing event-related potential (ERP) recordings to indicate whether pairs of English words were related or unrelated in meaning. Participants were unaware that half of the pairs concealed a repeated Chinese character when translated into Chinese. As expected, native English controls showed no sensitivity to this hidden factor. However, pairs of words unrelated in meaning and concealing form repetition through Chinese translation elicited longer reaction times, greater error rates, and larger N400 ERP shifts in Chinese participants. These findings demonstrate that Chinese participants unconsciously translate words into Chinese while reading English.

Adult↗

PubMed2004

P300 investigation of phoneme change detection in dyslexic adults.

A specific impairment in phoneme awareness has been hypothesized as one of the current explanations for dyslexia. We examined attentional shifts towards phonological information as indexed by event-related potentials (ERPs) in normal readers and dyslexic adults. Participants performed a lexical decision task on spoken stimuli of which 80% started with a standard phoneme and 20% with a deviant phoneme. A P300 modulation was expected for deviants in control adults, indicating that the phonological change had been detected. A mild and right-lateralized P300 was observed for deviant stimuli in controls, but was absent in dyslexic adults. This result suggests that dyslexic adults fail to make shifts of attention to phonological cues in the same way that normal adult readers do.

Acoustic Stimulation↗

PubMed2003

Familiar words capture the attention of 11-month-olds in less than 250 ms.

The capacity of human infants to discriminate contrasting speech sounds specializes to the native language by the end of the first year of life, when the first signs of word recognition have also been found, using behavioural measures. The extent of voluntary attentional involvement in such word recognition has not been explored, however, nor do we know what its neural time-course may be. Here we demonstrate that 11-month-old children shift their attention automatically to familiar words within 250 ms of presentation onset by measuring event-related potentials elicited by familiar and unfamiliar words. A significant modulation of the first negative peak (N200), known to index implicit change detection in adults, was induced by word familiarity in the infants.

Acoustic Stimulation↗

PubMed2003

Hemispheric dissociation in access to the human semantic system.

Patient studies suggest that speech and environmental sounds are differentially processed by the left and right hemispheres. Here, using functional imaging in normal subjects, we compared semantic processing of spoken words to equivalent processing of environmental sounds, after controlling for low-level perceptual differences. Words enhanced activation in left anterior and posterior superior temporal regions, while environmental sounds enhanced activation in a right posterior superior temporal region. This left/right dissociation was unchanged by different attentional/working memory contexts, but it was specific to tasks requiring semantic analysis. While semantic processing involves widely distributed networks in both hemispheres, our results support the hypothesis of a dual access route specific for verbal and nonverbal material, respectively.

Acoustic Stimulation↗

PubMed2003

Demand on verbal working memory delays haemodynamic response in the inferior prefrontal cortex.

Event-related functional magnetic resonance imaging was used to test the involvement of the inferior prefrontal cortex in verbal working memory. Pairs of French nouns were presented to ten native French speakers who had to make semantic or grammatical gender decisions. Verbal working memory involvement was manipulated by making the categorization of the second noun optional. Decisions could be made after processing the first noun only (RELEASE condition) or after processing the two nouns (HOLD condition). Reaction times suggested faster processing for gender than for semantic category in RELEASE. Despite the absence of anatomical difference across tasks and conditions in the wide activated network, the haemodynamic response peak latencies of the inferior prefrontal cortex were significantly delayed in HOLD versus RELEASE while no such peak delay was observed in the superior temporal gyrus. Interestingly, this pattern did not interact with language tasks. This study shows that cognitive manipulation can influence haemodynamic time-course and suggests that the main cognitive process determining inferior prefrontal activation is verbal working memory rather than specific linguistic processes such as grammatical or semantic analysis.

Acoustic Stimulation↗

PubMed2003

Electrophysiological comparison of grammatical processing and semantic processing of single spoken nouns.

Access to meaning from speech input is a high-speed process. However, models of word comprehension stipulate that words must be analyzed at phonological, grammatical and semantic levels. Here we used event-related potentials (ERPs) to compare on-line semantic categorization (natural/manufactured) and grammatical gender categorization (masculine/feminine) of spoken monosyllabic French noun pairs. Twenty four native French speakers were instructed to spot pairs of nouns in which both stimuli pertained to a category specified prior to each block. They could make a decision either after processing the first noun (i.e., ignore the second, Release condition), or after processing both (Hold condition). Virtually identical N4 components affecting Hold and Release ERPs in relation to category expectations were elicited by the first noun in both the tasks. Their topography was intermediary between that of the N400 and that of the Left Anterior Negativity. Despite shorter Release reaction times (RTs) for gender decisions than semantic ones, Release and Hold ERPs diverged 84 ms earlier in the semantic context than in the gender context, indicating faster onset for the processing of meaning. Conversely, the offset of Hold/Release differences was observed 42 ms earlier in the gender task than in the semantic task, suggesting an earlier completion for gender categorization. In sum, electrophysiological differences induced by semantic operations commenced earlier but recovered later than those relating to grammatical gender analysis. In convergence with previous Lateralized Readiness Potential and N200 studies, our results suggest that conscious semantic access can precede access to other types of lexical information during word comprehension in highly controlled conditions.

Adult↗

PubMed2002

Relationship between ventilatory settings and barotrauma in the acute respiratory distress syndrome.

OBJECTIVE: High pressures or volumes may increase the risk of barotrauma in the acute respiratory distress syndrome (ARDS). METHODS: The first part of the study analyzed data from a prospective trial of two ventilation strategies in 116 patients with ARDS retrospectively, and ventilatory pressures and volumes were compared in patients with or without pneumothorax. The second part consisted of a literature analysis of prospective trials (14 clinical studies, 2270 patients) describing incidence and risk factors for barotrauma in ARDS patients, and mean values of ventilatory parameters were plotted against incidence of barotrauma. RESULTS: In our clinical trial comparing two tidal volumes, 15 patients (12.3%) developed pneumothorax. There was no significant difference in any pressure or volume between these patients and the rest of the population, including end-inspiratory plateau pressure (P(plat)), driving pressure (P(plat)-PEEP), respiratory rate and compliance. Multiple trauma was more frequent among patients with pneumothorax (27%) than in those without (7%). Duration of mechanical ventilation tended to be longer with pneumothorax. In the literature review, the incidence of barotrauma varied between 0% and 49%, and correlated strongly with P(plat), with a high incidence above 35 cmH(2)O, and with compliance, with a high incidence below 30 ml/cmH(2)O. CONCLUSION: Clinical studies maintaining P(plat) lower than 35 cmH(2)O found no apparent relationship between ventilatory parameters and pneumothorax. Analysis of the literature suggests a correlation when patients receive mechanical ventilation with P(plat) levels exceeding 35 cmH(2)O.

Barotrauma↗