Biomedical subjects

Asaid Khateb

Publications and source records attributed to Asaid Khateb.

14 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↗

PubMed2007

A glimpse into your vision.

Invasive recordings of local field potentials (LFPs) have been used to "read the mind" of monkeys in real time. Here we investigated whether noninvasive field potentials estimated from the scalp-recorded electroencephalogram (EEG) using the ELECTRA source localization algorithm could provide real-time decoding of mental states in healthy humans. By means of pattern recognition techniques on 500-ms EEG epochs, we were able to discriminate accurately from single trials which of four categories of visual stimuli the subjects were viewing. Our results show that it is possible to reproduce the decoding accuracy previously obtained in animals with invasive recordings. A comparison between the decoding results and the subjects' behavioral performance indicates that oscillatory activity (OA), elicited in specific brain regions) codes better for the visual stimulus category presented than the subjects' actual response, i.e., is insensitive to voluntary or involuntary errors. The identification of brain regions participating in the decoding process allowed us to construct 3D-functional images of the task-related OA. These images revealed the activation of brain regions known for their involvement in the processing of this type of visual stimuli. Electrical neuroimaging therefore appears to have the potential to establish what the brain is processing while the stimuli are being seen or categorized, i.e., concurrently with sensory-perceptual processes.

Adult↗

PubMed2005

Cognition-enhancing effects of donepezil in traumatic brain injury.

The purpose of this study was to investigate the effects of donepezil, a cholinergic agent, on chronic cognitive impairment due to traumatic brain injury (TBI). Chronic patients underwent two standardized neuropsychological evaluations--one before and the other 3 months following treatment with donepezil. Together with global inventories that appraised behaviour, fatigue, anxiety and depression, these evaluations also assessed executive functioning, memory and attention. Of the 10 patients who followed the therapy, 8 reported subjective improvement in at least one cognitive domain following therapy and most of them reported better functioning in everyday activities. This effect was supported by a slight global improvement when considering the global score of the different affectivo-behavioural scales. At the neuropsychological level, although we could observe a slight improvement in the majority of the considered tests, significant positive changes were mainly found in tests assessing speed of processing, learning and divided attention. These findings suggest that donepezil may lead to better general functioning and improve attentional skills in patients with chronic TBI.

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

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

Visual recognition of faces, objects, and words using degraded stimuli: where and when it occurs.

We studied time course and cerebral localisation of word, object, and face recognition using event-related potentials (ERPs) and source localisation techniques. To compare activation rates of these three categories, we used degraded images that easily pop out without any change in the physical features of the stimuli, once the meaning is revealed. Comparisons before and after identification show additional periods of activation beginning at 100 msec for faces and at around 200 msec for objects and words. For faces, this activation occurs predominantly in right temporal areas, whereas for objects, the specific time period gives rise to bilateral posterior but right dominant foci. Finally, words show a maximum area of activation in the left temporooccipital area at their specific time period. These results provide unequivocal evidence that when effects of low-level visual features are circumvented, faces, objects, and words are not only distinct in terms of their anatomic routes, but also in terms of their times of processing.

Adult↗

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↗

PubMed2003

Chronic cognitive impairment following laterothalamic infarcts: a study of 9 cases.

BACKGROUND: The occlusion of the lateral thalamic arteries leads to infarcts of ventrolateral thalamic nuclei, the ventroposterior nucleus, and the rostrolateral part of pulvinar, and produces hemisensory loss with or without hemiataxia. Cognitive impairment after such strokes has not been systematically studied. OBJECTIVE: To determine the nature and the extent of long-lasting cognitive deficits following lateral thalamic strokes. DESIGN: Case series. SETTING: Neurology department, Lausanne University Hospital, Lausanne, Switzerland. PATIENTS: Nine patients with hemisensory loss due to an isolated laterothalamic infarct. MAIN OUTCOME MEASURES: Three to 6 months after stroke onset, standard neuropsychologic evaluation, including testing of language, ideomotor and constructive praxis, visual gnosis, spatial attention, learning abilities, and executive functions. RESULTS: Six of 9 patients showed some degree of cognitive impairment. Executive functions tasks, particularly verbal fluency, were impaired in 5 patients (4 with right and 1 with left lesion). Learning and delayed recall in visuospatial and verbal tasks, but not in recognition, were impaired in 3 patients (2 with right and 1 with left lesion). Difficulties in visual gnosia were observed in 1 patient with right lesion while word-finding difficulties were observed in 1 patient with left lesion. CONCLUSIONS: Our observations show that while learning, naming, and gnosic difficulties fit with the classical verbal/nonverbal dichotomy (left and right hemisphere, respectively), executive dysfunctions, including verbal fluency tasks, were more dominant after right thalamic infarcts. Although the observed deficits appeared to be less severe than those generally found with dorsomedial and polar thalamic strokes, the dominance of executive dysfunction suggests that ventrolateral thalamic lesions may disrupt frontothalamic subcortical loops.

Aged↗

PubMed2003

Decoupling of autonomic and cognitive emotional reactions after cerebellar stroke.

Emotional blunting can be found after cerebellar lesions. However, the mechanism of such a modification is not clear. We present a patient with emotional flattening and increased risk taking after left cerebellar infarct who had an impaired autonomic reactivity to negative as compared with positive reinforcement. This impairment was demonstrated by the patient's undifferentiated skin conductance responses to negative and positive reinforcement, whereas controls produced larger skin conductance responses after negative feedback. The cooccurrence of emotional flattening and undifferentiated autonomic reactions to positively and negatively valenced stimuli strengthens the role of the cerebellum in the modulation of the autonomic responses.

Affect↗

PubMed2003

Processing of semantic categorical and associative relations: an ERP mapping study.

Brain imaging studies have repeatedly shown a dominant involvement of the left hemisphere (LH) in the processing of semantic information. However, some behavioural studies have suggested that the right hemisphere (RH) also processes lexico-semantic information although in a qualitatively different way. More specifically, it has been proposed that the LH shows a greater sensitivity to categorical (CR, or intraconceptual) relationships, while the RH appears more sensitive to associative (AR, or interconceptual) ones. Using semantically unrelated and related word pairs comprised of CR and AR, our aim in this study was to verify the involvement of the RH in the processing of associative relationships. Strength-independent topographical analysis of ERP map series using temporal segmentation procedures showed the same sequence of eight stable map segments in CR and AR conditions. Statistical analysis revealed no difference between conditions either in terms of duration or in terms of time of occurrence of the segmentation maps. However, field strength comparisons between the two conditions using the global field power measure at each time point revealed higher field strength in CR than in AR condition between 300 and 330 ms. Distributed source localisation showed a similar pattern of activation during this time period, involving predominantly left frontal regions. Nevertheless, the strength of the source found in the left medial frontal gyrus differed between CR than AR conditions, thus explaining the differences observed in field strength. Our data suggest that the processing of CR and AR words depend on the same neural network in the LH.

Adolescent↗

PubMed2003

[Neuropsychological test in daily practice].

Neuropsychology is a scientific discipline, born in the XIX century, and bridges the fields of neurology and psychology. Neuropsychologists apply scientific knowledge about the relationship between brain function and mental performances. The major clinical role of a neuropsychological evaluation is to help to establish medical and functional diagnosis in patients (adults or infants) with different neurological pathologies such as stroke, traumatic brain injury, dementia, epilepsy.... Such analysis necessitates accurate observation of behaviour and administration of tests of mental abilities (e.g. language, memory...). Test results can also help to clarify the nature of cognitive difficulties and to support the formulation of plans for neuropsychological therapy and functional adjustment in every day life.

Adult↗

PubMed2002

Neural processing of illusory and real contours revealed by high-density ERP mapping.

Event-related potential (ERP) studies have shown that responses to illusory (IC), as compared to no contours (NC), may differ within 200 ms following stimulus presentation. However, whether these responses reflect the modulation of similar, or the activation of distinct brain regions has never been assessed. Using ERP map series and source localisation analyses, we compared early responses (0-200 ms) to IC and NC, as well as to real contours (RC). Although IC displayed higher field strength during the N1 component, topo-graphical analyses showed a similar sequence of stable field configurations in all conditions. Source localisation applied to the N1 map revealed bilateral activation of lateral-occipital regions. These results suggest that both IC and RC processing modulate a common network associated with filling-in processes and figure-ground segregation.

Adult↗

PubMed2002

Dynamics of brain activation during an explicit word and image recognition task: an electrophysiological study.

Recent brain imaging studies suggest that semantic processing of words and images may share a common neural network, although modality-specific activation can also be observed. Other studies using event-related potentials (ERPs) report that brain responses to words and images may already differ at approximately 150 ms following stimulus presentation. The question thus remains, which differences are due to perceptual categorization processes and which differences are due to the semantic ones? Using ERP recordings and spatio-temporal source localization analysis, we investigated the dynamics of brain activation during a recognition task. The stimuli consisted of a randomized set of verbal (words vs. non-words) and pictorial items (line drawings of objects vs. scrambled drawings). After each stimulus, subjects had to decide whether it corresponds to a recognizable word or objects. ERP map series were first analyzed in terms of segments of quasi-stable map topography using a cluster analysis. This showed that verbal and pictorial stimuli elicited different field patterns in two time segments between approximately 190-400 ms. Before and after this period, map patterns were similar between verbal and pictorial conditions indicating that the same brain structures were engaged during the early and late steps of processing. Source localization analysis of map segments corresponding to the P100 and the N150 components first showed activation of posterior bilateral regions and then of left temporo-posterior areas. During the period differentiating conditions, other patterns of activation, involving mainly left anterior and posterior regions for words and bilateral posterior regions for images, were observed. These findings suggest that, while sharing an initial common network, recognition of verbal and pictorial stimuli subsequently engage different brain regions during time periods generally allocated to the semantic processing of stimuli.

Acoustic Stimulation↗

PubMed2002

Is the right amygdala involved in visuospatial memory? Evidence from MRI volumetric measures.

Quantitative MRI measurement of hippocampal sclerosis in patients suffering from temporal lobe epilepsy (TLE) have, as yet, failed to evidence any correlation between the right hippocampus and visuospatial memory. In this report, word learning and design learning tasks were carried out as well as MRI volumetric measurements of the hippocampus and amygdala in order to verify possible modality-specific correlations between function and structure. Delayed recall indices in our memory tasks provided significant results. Visuoverbal ratios differed between right and left TLE groups, as did laterality indices of hippocampal and amygdalar volumes. Furthermore, correlations were found between the left hippocampal volume and verbal memory, and between the right amygdala and visuospatial memory. We suggest that the difficulty encountered in establishing a correlation between right temporal structures and visuospatial memory could come both from the type of test employed and the structures considered.

Adult↗