Biomedical subjects

Mohamed L Seghier

Publications and source records attributed to Mohamed L Seghier.

At least 19 recordsLinked to original sources

PubMed2007

Rhyme processing in the brain: an ERP mapping study.

The event-related potential (ERP) N450 component has been described in rhyme detection tasks as a negative response elicited by non-rhyming words in comparison to rhyming ones. This response, which peaked around 450 ms over the midline and right hemisphere recording sites, has been subsequently suggested to start already at approximately 300 ms. Moreover, although, the phonological N450 has first been linked to the semantic N400 component, its cognitive nature and cerebral origin remained debated. In this study, we re-investigated the time course of the electrophysiological responses to rhyming and non-rhyming words and estimated their cerebral generators using source localization methods. Waveform analysis showed that, prior to the N450 response to non-rhyming, a slightly earlier negativity characterized the rhyming condition over left fronto-temporal electrodes and peaked at approximately 350 ms. The analysis of the ERP map series in terms of functional microstates revealed a specific map segment in the rhyming condition and another one in the non-rhyming condition. Source localization indicated that the rhyming-elicited microstate engaged predominantly left frontal and temporal areas while the non rhyming-specific response recruited temporal and parietal regions bilaterally. Our results suggest that, similar to the N400 component that is also induced by mismatch contexts, the N450 might rely on temporal generators.

Adult↗

PubMed2006

Functional MRI of the newborn.

In order to provide accurate prognosis and developmental intervention to newborns, new methods of assessing cerebral functions are needed. The non-invasive technique of functional magnetic resonance imaging (fMRI) can be considered as the leading technique for functional exploration of the infant's brain. Several studies have previously applied fMRI in both healthy and diseased newborns with different sensory and cognitive tasks. In this chapter, the methodological issues that are proper to the use of fMRI in the newborn are detailed. In addition, an overview of the major findings of previous fMRI studies is provided, with a focus on notable differences from those in adult subjects. More specifically, the functional responses and the localization of cortical activations in healthy and diseased newborns are discussed. We expect a rapid expansion of this field and the establishment of fMRI as a valid clinical diagnostic tool in the newborn.

Brain↗

PubMed2006

Neural systems for orienting attention to the location of threat signals: an event-related fMRI study.

Attention may reflexively shift towards the location of perceived threats, but it is still unclear how these spatial biases recruit the distributed fronto-parietal cortical networks involved in other aspects of selective attention. We used event-related fMRI to determine how brain responses to a neutral visual target are influenced by the emotional expression of faces appearing at the same location during a covert orienting task. On each trial, two faces were briefly presented, one in each upper visual field (one neutral and one emotional, fearful or happy), followed by a unilateral target (a small horizontal or vertical bar) replacing one of the faces. Participants had to discriminate the target orientation, shown on the same (valid) or opposite (invalid) side as the emotional face. Trials with faces but no subsequent target (cue-only trials) were included to disentangle activation due to emotional cues from their effects on target detection. We found increased responses in bilateral temporo-parietal areas and right occipito-parietal cortex for fearful faces relative to happy faces, unrelated to the subsequent target and cueing validity. More critically, we found a selective modulation of intraparietal and orbitofrontal cortex for targets following an invalid fearful face, as well as an increased visual response in right lateral occipital cortex for targets following a valid fearful face. No such effects were observed with happy faces. These results demonstrate that fearful faces can act as exogenous cues by increasing sensory processing in extrastriate cortex for a subsequent target presented at the same location, but also produce a cost in disengaging towards another location by altering the response of IPS to invalidly cued targets. Neural mechanisms responsible for orienting attention towards emotional vs. non-emotional stimuli are thus partly shared in parietal and visual areas, but also partly distinct.

Adult↗

PubMed2006

The fusiform face area is tuned for curvilinear patterns with more high-contrasted elements in the upper part.

The ability to identify conspecifics from the face is of primary interest for human social behavior. Newborns' visual preference for schematic face-like stimuli has been recently related to a sensitivity for visual patterns with a greater number of elements in the upper compared to the lower part. At the adult level, neuroimaging studies have identified a network of cortical areas devoted to the detection and identification of faces. However, whether and how low-level structural properties of face stimuli contribute to the preferential response to faces in these areas remain to be clarified. Using functional magnetic resonance imaging (fMRI), here we investigated whether the adults' face-sensitive cortical areas show a preference for top-heavy patterns, similarly to newborns' preference. Twelve participants were presented with head-shaped and square patterns with either more elements in the upper or the lower vertical part. In the right fusiform gyrus ('fusiform face area', FFA), an area showing a preference for faces over other visual object categories, there was a larger activation for curvilinear patterns with more high-contrast elements in the upper part, even though these patterns were not perceived as face stimuli. These findings provide direct evidence that the FFA is tuned for geometrical properties fitting best with the structure of faces, a computational mechanism that might drive the automatic detection of faces in the visual world.

Adult↗

PubMed2005

Visual recovery after perinatal stroke evidenced by functional and diffusion MRI: case report.

BACKGROUND: After perinatal brain injury, clinico-anatomic correlations of functional deficits and brain plasticity remain difficult to evaluate clinically in the young infant. Thus, new non-invasive methods capable of early functional diagnosis are needed in young infants. CASE PRESENTATION: The visual system recovery in an infant with perinatal stroke is assessed by combining diffusion tensor imaging (DTI) and event-related functional MRI (ER-fMRI). All experiments were done at 1.5T. A first DTI experiment was performed at 12 months of age. At 20 months of age, a second DTI experiment was performed and combined with an ER-fMRI experiment with visual stimuli (2 Hz visual flash). At 20 months of age, ER-fMRI showed significant negative activation in the visual cortex of the injured left hemisphere that was not previously observed in the same infant. DTI maps suggest recovery of the optic radiation in the vicinity of the lesion. Optic radiations in the injured hemisphere are more prominent in DTI at 20 months of age than in DTI at 12 months of age. CONCLUSION: Our data indicate that functional cortical recovery is supported by structural modifications that concern major pathways of the visual system. These neuroimaging findings might contribute to elaborate a pertinent strategy in terms of diagnosis and rehabilitation.

Asphyxia Neonatorum↗

PubMed2005

Emotion and attention interactions in social cognition: brain regions involved in processing anger prosody.

Multiple levels of processing are thought to be involved in the appraisal of emotionally relevant events, with some processes being engaged relatively independently of attention, whereas other processes may depend on attention and current task goals or context. We conducted an event-related fMRI experiment to examine how processing angry voice prosody, an affectively and socially salient signal, is modulated by voluntary attention. To manipulate attention orthogonally to emotional prosody, we used a dichotic listening paradigm in which meaningless utterances, pronounced with either angry or neutral prosody, were presented simultaneously to both ears on each trial. In two successive blocks, participants selectively attended to either the left or right ear and performed a gender-decision on the voice heard on the target side. Our results revealed a functional dissociation between different brain areas. Whereas the right amygdala and bilateral superior temporal sulcus responded to anger prosody irrespective of whether it was heard from a to-be-attended or to-be-ignored voice, the orbitofrontal cortex and the cuneus in medial occipital cortex showed greater activation to the same emotional stimuli when the angry voice was to-be-attended rather than to-be-ignored. Furthermore, regression analyses revealed a strong correlation between orbitofrontal regions and sensitivity on a behavioral inhibition scale measuring proneness to anxiety reactions. Our results underscore the importance of emotion and attention interactions in social cognition by demonstrating that multiple levels of processing are involved in the appraisal of emotionally relevant cues in voices, and by showing a modulation of some emotional responses by both the current task-demands and individual differences.

Acoustic Stimulation↗

PubMed2005

The voices of wrath: brain responses to angry prosody in meaningless speech.

We report two functional magnetic resonance imaging experiments showing enhanced responses in human middle superior temporal sulcus for angry relative to neutral prosody. This emotional enhancement was voice specific, unrelated to isolated acoustic amplitude or frequency cues in angry prosody, and distinct from any concomitant task-related attentional modulation. Attention and emotion seem to have separate effects on stimulus processing, reflecting a fundamental principle of human brain organization shared by voice and face perception.

Acoustic Stimulation↗

PubMed2005

Anatomical variability of the lateral frontal lobe surface: implication for intersubject variability in language neuroimaging.

The lateral surface of the frontal lobe shows functional activation in a large number of language related tasks. Group analyses, however, demonstrate remarkable intersubject variability of activation. There are different sources for functional variability, anatomical variability being considered as one of them. The aim of the present study therefore was to qualitatively and quantitatively investigate the anatomical variability of the lateral frontal lobe surface and to search for reliable and stable landmarks connected to language functions. MRIs of 23 healthy right-handed subjects were investigated using the publicly available software "Anatomist/BrainVISA". After standardization of the brains (SPM) and sulci identification, the most frequent pattern was determined and the variance of selected landmarks calculated. The variability of the lateral frontal lobe surface is remarkable, particularly in the prefrontal region. Relatively stable landmarks were selected as follows: (1) connection between the superior frontal sulcus (SFS) and the superior precentral sulcus (SPCS); (2) connection between the inferior frontal sulcus (IFS) and the inferior precentral sulcus (IPCS); (3) inferior end of the precentral sulcus (PCS); and (4) origin of the ascending ramus (AscR) of the Sylvian fissure (SYF). The variability (standard deviation) of the spatial coordinates along the 3 axis of these landmarks after normalization ranged from 2.5 to 5.7 mm. The present study demonstrates that intersubject variability of selected landmarks of the frontal lobe surface remains notable even after spatial normalization of the brains. These results support the concept that anatomical variability is a relevant source of functional variability. We therefore suggest to express functional activation in relation to landmarks obtained from individual anatomy. This approach may contribute to a better analysis of the differences between individuals.

Adult↗

PubMed2005

Functional magnetic resonance imaging and diffusion tensor imaging in a case of central poststroke pain.

The role of the lesion location within functional pain systems is not fully understood for central poststroke pain (CPSP) pathogenesis. In a patient with CPSP we used data from both functional magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) for anatomo-functional correlations. Structural MRI showed a small residual cavity confined to the right thalamic ventral posterolateral nucleus and the adjacent posterior arm of the internal capsule. DTI maps showed selective reduction of right sensory thalamocortical fibers. Functional MRI, performed with different thermonociceptive stimuli, showed pain-specific signal changes in the anterior cingulate gyrus (BA 24/32) and in the associative parietal regions (BA 5/7). These findings underline, for CPSP pathogenesis, the role of damage of lateral nociceptive thalamoparietal fibers together with the release of activity of anterior cingulate and posterior parietal regions. In a patient with CPSP, we combined noninvasive neuroimaging techniques (functional and diffusion MRI) to assess the anatomo-functional relationship in CPSP. Our investigations show, for CPSP pathogenesis, the role of damage of lateral nociceptive thalamoparietal fibers together with the release of activity of anterior cingulate and posterior parietal regions.

Aged↗

PubMed2005

Portraits or people? Distinct representations of face identity in the human visual cortex.

Humans can identify individual faces under different viewpoints, even after a single encounter. We determined brain regions responsible for processing face identity across view changes after variable delays with several intervening stimuli, using event-related functional magnetic resonance imaging during a long-term repetition priming paradigm. Unfamiliar faces were presented sequentially either in a frontal or three-quarter view. Each face identity was repeated once after an unpredictable lag, with either the same or another viewpoint. Behavioral data showed significant priming in response time, irrespective of view changes. Brain imaging results revealed a reduced response in the lateral occipital and fusiform cortex with face repetition. Bilateral face-selective fusiform areas showed view-sensitive repetition effects, generalizing only from three-quarter to front-views. More medial regions in the left (but not in the right) fusiform showed repetition effects across all types of viewpoint changes. These results reveal that distinct regions within the fusiform cortex hold view-sensitive or view-invariant traces of novel faces, and that face identity is represented in a view-sensitive manner in the functionally defined face-selective areas of both hemispheres. In addition, our finding of a better generalization after exposure to a 3/4-view than to a front-view demonstrates for the first time a neural substrate in the fusiform cortex for the common recognition advantage of three-quarter faces. This pattern provides new insights into the nature of face representation in the human visual system.

Adult↗

PubMed2004

View-independent coding of face identity in frontal and temporal cortices is modulated by familiarity: an event-related fMRI study.

Face recognition is a unique visual skill enabling us to recognize a large number of person identities, despite many differences in the visual image from one exposure to another due to changes in viewpoint, illumination, or simply passage of time. Previous familiarity with a face may facilitate recognition when visual changes are important. Using event-related fMRI in 13 healthy observers, we studied the brain systems involved in extracting face identity independent of modifications in visual appearance during a repetition priming paradigm in which two different photographs of the same face (either famous or unfamiliar) were repeated at varying delays. We found that functionally defined face-selective areas in the lateral fusiform cortex showed no repetition effects for faces across changes in image views, irrespective of pre-existing familiarity, suggesting that face representations formed in this region do not generalize across different visual images, even for well-known faces. Repetition of different but easily recognizable views of an unfamiliar face produced selective repetition decreases in a medial portion of the right fusiform gyrus, whereas distinct views of a famous face produced repetition decreases in left middle temporal and left inferior frontal cortex selectively, but no decreases in fusiform cortex. These findings reveal that different views of the same familiar face may not be integrated within a single representation at initial perceptual stages subserved by the fusiform face areas, but rather involve later processing stages where more abstract identity information is accessed.

Adult↗

PubMed2004

Discriminating emotional faces without primary visual cortices involves the right amygdala.

Destruction of the brain's primary visual areas leads to blindness of cortical origin. Here we report on a subject who, after bilateral destruction of his visual cortices and ensuing cortical blindness, could nevertheless correctly guess the type of emotional facial expression being displayed, but could not guess other types of emotional or non-emotional stimuli. Functional magnetic resonance imaging showed activation of the right amygdala during the unconscious processing of emotionally expressive faces.

Amygdala↗

PubMed2004

Illusory persistence of touch after right parietal damage: neural correlates of tactile awareness.

We studied a patient who experienced 'palinaesthesia', an illusion of persistent touch following tactile stimulation on the left hand, subsequent to a right parietal meningioma affecting primary somatosensory regions in the postcentral gyrus (SI) and superior parietal gyrus (Brodmann area 7), but preserving the secondary somatosensory cortex (SII) in the upper lateral sulcus. This subjective sensation was accompanied by transient increases in objective measures of tactile threshold. The patient had mild deficits in superficial tactile perception, but showed severe left-sided extinction for offsets of tactile stimuli during bilateral stimulation, but not for onsets of stimuli. Functional MRI revealed increased neural activity during palinaesthesia selectively arising within the ipsilesional-right SI cortex, but no abnormality within left SI and bilateral SII. Right SI responded to the onset of new tactile stimuli on the left hand but not to their offset. By contrast, any tactile events on either hand modulated activity in contralateral SII regions, even undetected left-sided offsets. These data demonstrate that illusory persistence of touch following stimulation on the hand may result from sustained neural activity in a restricted region of the SI cortex outlasting the offset of the actual tactile stimuli. These findings also provide direct evidence for a critical role of SI in mediating conscious somatosensory experience on contralateral parts of the body.

Aged↗

PubMed2004

FMRI evidence for activation of multiple cortical regions in the primary auditory cortex of deaf subjects users of multichannel cochlear implants.

To investigate the activation of the auditory cortex by fMRI, three deaf subjects users of the Ineraid cochlear implant participated in our study. Possible interference between fMRI acquisition and the implanted electrodes was controlled and safe experimental conditions were obtained. For each subject, electrical stimuli were applied on different intracochlear electrodes, in monopolar mode. Stimulation of each electrode was actually producing auditory sensations of different pitches, as demonstrated by psychophysical pitch-ranking measurements in the same subjects. Because deaf subjects did not hear scanner noise, the data were collected in 'silent background' conditions, i.e. as a result of pure auditory sensations. Functional maps showed activation of the primary auditory cortex, predominantly in the left hemisphere. Stimulation of each different intracochlear electrode revealed different clusters of activation. After cluster grouping, at least three regions have been identified in the auditory cortex of each subject, and comparisons with previous architectonic and functional studies are proposed. However, a tonotopic organization could not be clearly identified within each region. These arguments, obtained without interference with unwanted scanner noise, plead in favor of a functional subdivision of the primary auditory cortex into multiple cortical regions in cochlear implant users.

Adult↗

PubMed2004

Transient crossed aphasia evidenced by functional brain imagery.

Crossed aphasia refers to language deficits induced by unilateral right hemisphere injuries in right-handed people who had no previous history of brain damage. One of the intriguing questions concerning crossed aphasia is the atypical language representation in the brain. In this respect, fMRI is a valuable tool for understanding the neural basis of crossed aphasia. Here, we used neuropsychological and fMRI language tasks in a right-handed subject who presented a crossed aphasia due to a right frontal meningioma. fMRI maps from two language tasks showed bilateral patterns of activation. In the light of previous studies reporting much frequent bilateral than exclusive right hemisphere representations, we hypothesise that some crossed aphasia cases could occur in subjects with bilateral language representation.

Aphasia↗

PubMed2004

Tactile awareness and limb position in neglect: functional magnetic resonance imaging.

We studied a patient with right parietal damage for whom tactile stimuli on the right/ipsilesional hand (projecting to the intact left hemisphere) were extinguished from awareness during double simultaneous stimulation, when his right hand was positioned in the left/contralesional space. This demonstrates the role of an egocentric spatial reference frame in attention that can determine awareness of stimuli despite intact sensory pathways. Using functional magnetic resonance imaging to elucidate the neural correlates of such perceptual extinction, we found that limb position modulated neural responses to tactile stimuli at early cortical stages (SI) in the intact hemisphere. Activity in bilateral middle frontal gyri also was modulated by limb position and may contribute to integrate sensory inputs into a supramodal, egocentric representation of space.

Aged↗

PubMed2004

Variability of fMRI activation during a phonological and semantic language task in healthy subjects.

Assessing inter-individual variability of functional activations is of practical importance in the use of functional magnetic resonance imaging (fMRI) in a clinical context. In this fMRI study we addressed this issue in 30 right-handed, healthy subjects using rhyme detection (phonologic) and semantic categorization tasks. Significant activations, found mainly in the left hemisphere, concerned the inferior frontal gyrus, the superior/middle temporal gyri, the prefrontal cortex, the inferior parietal lobe, the superior parietal lobule/superior occipital gyrus, the pre-central gyrus, and the supplementary motor area. Intensity/spatial analysis comparing activations in both tasks revealed an increased involvement of frontal regions in the semantic task and of temporo-parietal regions in the phonologic task. The frequency of activation analyzed in nine regional subdivisions revealed a high inter-subject variability but showed that the most frequently activated regions were the inferior frontal gyrus and the prefrontal cortex. Laterality indices, strongly lateralizing in both tasks, were slightly higher in the semantic (0.76 +/- 0.19) than the phonologic task (0.66 +/- 0.27). Frontal dominance indices (a measure of frontal vs. posterior left hemisphere dominance) indicated more robust frontal activations in the semantic than the phonologic task. Our study allowed the characterization of the most frequently involved foci in two language tasks and showed that the combination of these tasks constitutes a suitable tool for determining language lateralization and for mapping major language areas.

Adult↗

PubMed2004

Combination of event-related fMRI and diffusion tensor imaging in an infant with perinatal stroke.

Focal ischemic brain injury, or stroke, is an important cause of later handicap in children. Early assessment of structure-function relationships after such injury will provide insight into clinico-anatomic correlation and potentially guide early intervention strategies. We used combined functional MRI (fMRI) with diffusion tensor imaging (DTI) in a 3-month-old infant to explore the structure-function relationship after unilateral perinatal stroke that involved the visual pathways. With visual stimuli, fMRI showed a negative BOLD activation in the visual cortex of the intact right hemisphere, principally in the anterior part, and no activation in the injured hemisphere. The functional activation in the intact hemisphere correlated clearly with the fiber tract of the optic radiation visualized with DTI. DTI confirmed the absence of the optic radiation in the damaged left hemisphere. In addition, event-related fMRI (ER-fMRI) experiments were performed to define the characteristics of the BOLD response. The shape is that of an inverted gamma function (similar to a negative mirror image of the known positive adult BOLD response). The maximum decrease was reached at 5-7 s with signal changes of -1.7 +/- 0.4%.Thus, this report describes for the first time the combined use of DTI and event-related fMRI in an infant and provides insight into the localization of the fMRI visual response in the young infant and the characteristics of the BOLD response.

Asphyxia Neonatorum↗