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C M Michel

Publications and source records attributed to C M Michel.

At least 19 recordsLinked to original sources
PubMed2006

Sex differences in semantic processing: event-related brain potentials distinguish between lower and higher order semantic analysis during word reading.

Behavioral studies suggest that women and men differ in the strategic elaboration of verbally encoded information especially in the absence of external task demand. However, measuring such covert processing requires other than behavioral data. The present study used event-related potentials to compare sexes in lower and higher order semantic processing during the passive reading of semantically related and unrelated word pairs. Women and men showed the same early context effect in the P1-N1 transition period. This finding indicates that the initial lexical-semantic access is similar in men and women. In contrast, sexes differed in higher order semantic processing. Women showed an earlier and longer lasting context effect in the N400 accompanied by larger signal strength in temporal networks similarly recruited by men and women. The results suggest that women spontaneously conduct a deeper semantic analysis. This leads to faster processing of related words in the active neural networks as reflected in a shorter stability of the N400 map in women. Taken together, the findings demonstrate that there is a selective sex difference in the controlled semantic analysis during passive word reading that is not reflected in different functional organization but in the depth of processing.

Adult↗
PubMed2006

Speech arrest with stimulation may not reliably predict language deficit after epilepsy surgery.

The authors present a patient in whom electrical cortical stimulation of the posterior temporal cortex induced speech arrest, comprehension deficits, and other language-related impairments. This area was ultimately resected because of persistence of a severe seizure disorder. No postoperative aphasia was observed despite the cortical stimulation results, and the patient is since seizure free. These findings question the well-established principle that corticography directly reflects local cortical functions in all patients.

Adult↗
PubMed2004

Patients with extratemporal lobe epilepsy do not differ from healthy subjects with respect to subcortical volumes.

BACKGROUND: Evidence from previous volumetric magnetic resonance studies has revealed that patients with chronic temporal lobe epilepsy show atrophy of distinct subcortical nuclei, predominantly ipsilateral to the focus side. We were interested to find out if there is also selective subcortical atrophy in patients suffering from long standing extratemporal lobe epilepsy. METHODS: Thirty one patients in whom pre-surgical evaluation unambiguously localised an extratemporal focus were included in this study. Using high resolution magnetic resonance imaging, the volumes of the caudate nuclei, putamen, pallidum, and thalamus were measured bilaterally in both hemispheres and compared with measurements obtained in 15 healthy volunteers. RESULTS: No significant difference in volumes was found between the two subject groups, or in any subgroup of extratemporal lobe epilepsy patients, nor was there any relation to clinical variables such as age of onset, overall seizure frequency, or disease duration. However, patients who had no or only rare generalised tonic-clonic seizures seemed to differ from the other patients and controls in that they had smaller putamen volumes bilaterally (p<0.001). CONCLUSION: We concluded that extratemporal lobe epilepsy in general is not associated with diminished volumes in the studied subcortical structures, which contrasts with findings in temporal lobe epilepsy patients. Thus, both entities differ both cortically and subcortically. However, we found that small putamen volume was bilaterally associated with absent or rare generalised tonic-clonic seizures, implicating the putamen in the control of the most disabling seizure type, independent of the site of neocortical focus.

Adult↗
PubMed2003

Pure imagery hemi-neglect of far space.

Patients with hemispatial neglect restricted to near (within reaching distance) or to far space (beyond reaching distance) have been described. This constitutes a double-dissociation considered by current neurocognitive thinking as compelling evidence for separate networks. However, a similar double-dissociation exists with respect to perceived as opposed to imagined space. If the organization of represented space was similar to that of perceived space, it should contain a far/near dissociation as well. This paper describes a patient with pure representational neglect restricted to far space.

Aged↗
PubMed2003

Mapping of the neuronal networks of human cortical brain functions.

OBJECTIVE: The principles and methodology of event-related fMRI, electromagnetic source imaging and intracranial evoked potentials will be described along with some examples of the mapping of the neuronal networks of human cortical brain functions with the use of these techniques. INTRODUCTION: Functional brain mapping using PET or fMRI has provided clues on the functioning brain and notably on the functional neuroanatomy of cognitive functions. These mapping possibilities can be used to delineate in an individual patient the brain areas subserving a cerebral function that might be compromised by a surgery in a nearby location, or to target a functional neurosurgical procedure. BACKGROUND: Brain functions and notably "higher brain functions" are served by a complex network of interrelating brain regions. Deeper insights into the functioning of a neuronal network can be gained by adding dynamic, i.e. temporal, information to the functional maps. This will demonstrate the orchestration of the activation of the different brain areas constituting the network, which gives clues to the information processing and therefore to the functioning of the different modules of the network. In order to track the flow of information and the sequential activation of the different brain regions constituting the network, brain activity has to be recorded at the speed of transfer of activation from one neuronal population to the other. The temporal resolution needed to achieve this is not in the range of traditional subtractive or comparative PET or fMRI techniques. NEW DEVELOPMENTS: Novel fMRI methods that record haemodynamic signal changes after single events (event-related fMRI) are now able to determine sequential neural processing by distinguishing the relative onset-time of activity between different areas. The temporal resolution of event-related (ER) fMRI is sufficient to detect changes of mental activity within the order of several hundreds of milliseconds. This allows the exploration of a broad range of cognitive functions. Nevertheless, this technique is currently not rapid enough to observe the transient coordinations and oscillations of neuronal activities occurring across certain cortical areas during the performance of cognitive tasks. The temporal resolution needed for that is within the order of tens or a few milliseconds and is only accessible by EEG or MEG that allow true real-time measurements of the neuronal activity elicited by a stimulus. Surface recordings of multichannel EEG or MEG combined with novel electromagnetic source localisation algorithms allow a relatively precise estimation of the activated areas. A more direct localisation of electric activity is achieved by intracranial recordings in patients having implanted electrodes for diagnostic reasons. In these cases, a high temporal and spatial resolution is achieved but with a limited sampling of brain regions. CONCLUSION: Although the temporal resolution of ER fMRI is due to improve, the temporal measures provided by EEG, MEG or intracranial event-related potentials (ERPs) are absolute, which remains a unique feature of these techniques. Therefore, ER fMRI and electromagnetic source imaging are complementary. The maps obtained with ER fMRI may be refined by electromagnetic ERPs that provide further insights into the temporal coordination or orchestration between the cortical areas already detected by ER fMRI and constituting a neuronal network, and ER fMRI can be used to precisely locate the areas coarsely situated and delineated by electromagnetic source imaging. Thus, the combination of ER fMRI and electromagnetic ERPs is essential in order to produce a mapping method with a millimetre spatial resolution and a millisecond temporal resolution. Future applications should combine these techniques to localise precisely and non-invasively relevant sensory, motor and cognitive processes in order to adequately tailor any brain surgery.

Attention↗
PubMed2003

Epileptic source localization with high density EEG: how many electrodes are needed?

OBJECTIVE: Electroencephalography (EEG) source reconstruction is becoming recognized as a useful technique to non-invasively localize the epileptic focus. Whereas, large array magnetoencephalography (MEG) systems are available since quite some time, application difficulties have previously prevented multichannel EEG recordings. Recently, however, EEG systems which allow for quick (10-20min) application of, and recording from, up to 125 electrodes have become available. The purpose of the current investigation was to systematically compare the accuracy of epileptic source localization with high electrode density to that obtained with sparser electrode setups. METHODS: Interictal epileptiform activity was recorded with 123 electrodes in 14 epileptic patients undergoing presurgical evaluation. Each single epileptiform potential was down sampled to 63 and 31 electrodes, and a distributed source model (EPIFOCUS) was used to reconstruct the sources with the 3 different electrode configurations. The localization accuracy with the 3 electrode setups was then assessed, by determining the distance from the inverse solution, maximum of each single spike to the epileptogenic lesion. RESULTS: In 9/14 patients, the distance from the EEG source to the lesion was significantly smaller with 63 than with 31 electrodes, and increasing the number of electrodes to 123 increased this number of patients from 9 to 11. Simulations confirmed the relation between the number of electrodes and localization accuracy. CONCLUSIONS: The results illustrate the necessity of multichannel EEG recordings for high source location accuracy in epileptic patients.

Adolescent↗
PubMed2003

Face versus non-face object perception and the 'other-race' effect: a spatio-temporal event-related potential study.

OBJECTIVE: To investigate a modulation of the N170 face-sensitive component related to the perception of other-race (OR) and same-race (SR) faces, as well as differences in face and non-face object processing, by combining different methods of event-related potential (ERP) signal analysis. METHODS: Sixty-two channel ERPs were recorded in 12 Caucasian subjects presented with Caucasian and Asian faces along with non-face objects. Surface data were submitted to classical waveforms and ERP map topography analysis. Underlying brain sources were estimated with two inverse solutions (BESA and LORETA). RESULTS: The N170 face component was identical for both race faces. This component and its topography revealed a face specific pattern regardless of race. However, in this time period OR faces evoked significantly stronger medial occipital activity than SR faces. Moreover, in terms of maps, at around 170 ms face-specific activity significantly preceded non-face object activity by 25 ms. These ERP maps were followed by similar activation patterns across conditions around 190-300 ms, most likely reflecting the activation of visually derived semantic information. CONCLUSIONS: The N170 was not sensitive to the race of the faces. However, a possible pre-attentive process associated to the relatively stronger unfamiliarity for OR faces was found in medial occipital area. Moreover, our data provide further information on the time-course of face and non-face object processing.

Adult↗
PubMed2002

[Variability of right hemisphere activation during semantic word processing in aphasic patients: an electrophysiologic study in three patients].

After stroke, the interhemispheric reorganisation of the neural network implicated in language is hypothesized to be a function not only of the site of lesion but also of the residual impairment. With a multiple case approach, we tested this hypothesis in three chronic aphasic patients. Two patients, GE (capsulo-lenticular stroke) and JHN (fronto-temporal stroke) showed formal residual semantic difficulties, while the third patient (EG, large sylvian lesion) did not. Brain electric activity was analysed during a categorisation task of tachistoscopically presented words in the left and the right visual field. The temporal analysis of brain activity showed that both patients with semantic residual difficulties activated the right hemisphere (RH) during some steps of word processing. In the third patient, without semantic impairment, the RH was activated only during a short time period. Further more, RH activation was shown to be dependent on the visual field of word presentation. Phonological impairment was not predictive of RH activation. These results suggest that RH activation, particularly anterior regions, can occur during semantic processing of words as a function of semantic residual impairment.

Adult↗
PubMed2001

Non-stationary distributed source approximation: an alternative to improve localization procedures.

Localization of the generators of the scalp measured electrical activity is particularly difficult when a large number of brain regions are simultaneously active. In this study, we describe an approach to automatically isolate scalp potential maps, which are simple enough to expect reasonable results after applying a distributed source localization procedure. The isolation technique is based on the time-frequency decomposition of the scalp-measured data by means of a time-frequency representation. The basic rationale behind the approach is that neural generators synchronize during short time periods over given frequency bands for the codification of information and its transmission. Consequently potential patterns specific for certain time-frequency pairs should be simpler than those appearing at single times but for all frequencies. The method generalizes the FFT approximation to the case of distributed source models with non-stationary time behavior. In summary, the non-stationary distributed source approximation aims to facilitate the localization of distributed source patterns acting at specific time and frequencies for non-stationary data such as epileptic seizures and single trial event related potentials. The merits of this approach are illustrated here in the analysis of synthetic data as well as in the localization of the epileptogenic area at seizure onset in patients. It is shown that time and frequency at seizure onset can be precisely detected in the time-frequency domain and those localization results are stable over seizures. The results suggest that the method could also be applied to localize generators in single trial evoked responses or spontaneous activity.

Adolescent↗
PubMed2001

Electric source imaging of human brain functions.

We review recent methodological advances in electromagnetic source imaging and present EEG data from our laboratory obtained by application of these methods. There are two principal steps in our analysis of multichannel electromagnetic recordings: (i) the determination of functionally relevant time periods in the ongoing electric activity and (ii) the localization of the sources in the brain that generate these activities recorded on the scalp. We propose a temporal segmentation of the time-varying activity, which is based on determination of changes in the topography of the electric fields, as an approach to the first step, and a distributed linear inverse solution based on realistic head models as an approach to the second step. Data from studies of visual motion perception, visuo-motor transfer, mental imagery, semantic decision, and cognitive interference illustrate that this analysis allows us to define the patterns of electric activity that are present at given time periods after stimulus presentation, as well as those time periods where significantly different patterns appear between different stimuli and tasks. The presented data show rapid and parallel activation of different areas within complex neuronal networks, including early activity of brain regions remote from the primary sensory areas. In addition, the data indicate information exchange between homologous areas of the two hemispheres in cases where unilateral stimulus presentation requires interhemispheric transfer.

Brain Mapping↗
PubMed2001

The time course of semantic category processing in the cerebral hemispheres: an electrophysiological study.

Using visual half-field presentations of words to the right (RVF) and to the left visual field (LVF), this study investigated the time course of the hemispheric involvement in the processing of semantic category information. Multi-channel event related brain potentials (ERPs) were recorded from 15 healthy subjects during a categorisation task of sequentially presented word pairs. Subjects had to judge mentally after the appearance of the second word whether the words of a pair were semantically related (SR) or not (SU). ERPs were computed, from 100 ms before the onset of the second word to 600 ms, for SR and SU conditions in the LVF and in the RVF separately. The temporal segmentation of ERP map series into sequences of quasi-stable map configurations revealed a total of seven segments in each visual field of which only the first five (S1-S5, appearing between 70 and 400 ms) showed different map configurations as a function of visual field but presented a similar temporal sequence in both visual fields. By contrast, of the last two segments (S6 and S7) which appeared between approximately 400 and approximately 600 ms, only S7 differentiated SR and SU conditions in terms of its duration. Source localisation analysis of the segments showed that following the initial activation of posterior brain regions as a function of the visual field of presentation, a common neural network was activated in the left hemisphere (LH) although the dynamics of activation varied as a function of visual field. Concerning the role of the right hemisphere (RH) in lexico-semantic processing, the results presented here appear to be compatible with a 'callosal relay model' and suggest that, in healthy subjects, information is transferred rapidly ( approximately 150 ms) from the RH to the language dominant-LH.

Adult↗
PubMed2001

Space-oriented segmentation and 3-dimensional source reconstruction of ictal EEG patterns.

OBJECTIVES: Characterization of the EEG pattern during the early phase of a seizure is crucial for identifying the epileptic focus. The purpose of the present investigation was to evaluate a method that divides ictal EEG activity into segments of relatively constant surface voltage distribution, and to provide a 3-dimensional localization of the activity during the different segments. METHODS: For each timepoint the electrical voltage distribution on the scalp (the voltage map) was determined from the digitized EEG recording. Through a spatial cluster analysis time sequences where the maps did not change much (segments) were identified, and a 3-dimensional source reconstruction of the activity corresponding to the different mean maps was performed using a distributed linear inverse solution algorithm. RESULTS: Segments dominating early in seizure development were identified, and source reconstruction of the EEG activity corresponding to the maps of these segments yielded results which were consistent with the results from invasive recordings. In some cases a sequence of consecutive segments was obtained, which might reflect ictal propagation. CONCLUSIONS: Segmentation of ictal EEG with subsequent 3-dimensional source reconstruction is a useful method to non-invasively determine the initiation and perhaps also the spread of epileptiform activity in patients with epileptic seizures.

Adolescent↗
PubMed2001

Noninvasive localization of electromagnetic epileptic activity. I. Method descriptions and simulations.

This paper considers the solution of the bioelectromagnetic inverse problem with particular emphasis on focal compact sources that are likely to arise in epileptic data. Two linear inverse methods are proposed and evaluated in simulations. The first method belongs to the class of distributed inverse solutions, capable of dealing with multiple simultaneously active sources. This solution is based on a Local Auto Regressive Average (LAURA) model. Since no assumption is made about the number of activated sources, this approach can be applied to data with multiple sources. The second method, EPIFOCUS, assumes that there is only a single focal source. However, in contrast to the single dipole model, it allows the source to have a spatial extent beyond a single point and avoids the non-linear optimization process required by dipole fitting. The performance of both methods is evaluated with synthetic data in noisy and noise free conditions. The simulation results demonstrate that LAURA and EPIFOCUS increase the number of sources retrieved with zero dipole localization error and produce lower maximum error and lower average error compared to Minimum Norm, Weighted Minimum Norm and Minimum Laplacian (LORETA). The results show that EPIFOCUS is a robust and powerful tool to localize focal sources. Alternatives to localize data generated by multiple sources are discussed. A companion paper (Lantz et al. 2001, this issue) illustrates the application of LAURA and EPIFOCUS to the analysis of interictal data in epileptic patients.

Artifacts↗
PubMed2001

Noninvasive localization of electromagnetic epileptic activity. II. Demonstration of sublobar accuracy in patients with simultaneous surface and depth recordings.

Seven patients with complex partial epileptic seizures undergoing invasive video/EEG-monitoring were investigated with a combination of 10 subdural strip electrode contacts (subtemporal + lateral temporal), and 22 extracranial recording sites. In each patient spikes with different intracranial distributions were identified, and for those with similar distributions the extracranial activity was averaged. A new inverse solution method called EPIFOCUS (Grave et al. 2001, this issue) was used to reconstruct the sources of both single and averaged spikes in a standard 3D-MRI, and a statistical analysis was performed in order to demonstrate location differences between spikes with different intracranial distributions. The results revealed significantly more anterior and ventral source locations for subtemporal compared to lateral temporal spikes. Within the subtemporal group, medial spikes had more mesial and dorsal locations compared to lateral ones. In the lateral temporal group, more anterior and ventral locations were obtained for anterior compared to posterior spikes. The results demonstrate the applicability of EPIFOCUS in the localization of sources in the temporal lobe with sublobar accuracy. This possibility may become important in the future, for instance in identifying cases where amygdalo-hippocampectomy or other limited temporal lobe resections may replace the standard en bloc resections.

Adult↗
PubMed2001

EEG mapping and functional MRI in presurgical epilepsy evaluation.

The major goal of presurgical epilepsy evaluation is the correct and precise identification of the epileptic focus. Functional magnetic resonance imaging (fMRI) and EEG source analysis (EEG mapping) are two recent developments that are increasingly used in this particular clinical context. fMRI yields anatomically precise information but low temporal resolution, whereas the inverse situation is true for EEG mapping. Important complementary information is therefore obtained by integrating the results from each technique. Recent advances in these two fields are discussed, with particular reference to epileptic focus localization. Such a combined approach is shown to be a powerful tool, in particular in patients with non-lesional and/or extratemporal epilepsy still considered to be difficult surgical candidates.

Brain Mapping↗
PubMed2001

Localization of distributed sources and comparison with functional MRI.

Functional mapping of the human brain has made tremendous progress in the past years thanks to new technical developments. Imaging methods are now available; they allow to study brain functions with high spatial and temporal resolution. Single photon emission computer tomography (SPECT), positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and high resolution electro- and magnetoencephalography (EEG and MEG) are currently intensively applied techniques to functional studies, each one having specific properties concerning spatial and temporal resolution. The success of these methods in basic neuroscience research has led to the demand for applying them to clinical questions. Diseases of the central nervous system that lead to brain dysfunction can be ideally explored using these techniques. Of particular importance are those diseases in which a focal neuronal dysfunction is the primary cause and where surgical resection of this focus might be the cure. This is often the case for epilepsy, where a discrete primary focus might exist from which pathological rhythms evolve and propagate throughout the brain, leading to seizures that severely handicap the patient. Surgical resection of the primary focus is only possible if the focus can be exactly localized and adequately separated from functionally important areas. This is where these new functional imaging tools become important. The use of SPECT and PET for focus localization has been most extensively studied and their specificity and sensitivity are intensively discussed. In the last few years functional MRI has evolved as a new interesting tool in epileptic focus localization. The most important limitation of these techniques, however, is the temporal resolution. Since epileptic activity can propagate very fast, several hyper- or hypoactive regions are seen in the images and primary areas cannot be distinguished from regions of propagation. The only methods that have sufficient temporal resolution to follow neuronal activity in real time are the electrophysiological measures, i.e. the EEG and the MEG. Localization of the sources in the brain that produced a given surface electromagnetic field has become possible through algorithms that solve the so-called "inverse problem". Several different algorithms exist and many groups begun to apply them to epileptic data with the aim to localize the focus of the pathological electrical discharges. This review article discusses the use of distributed EEG source localization procedures in the presurgical evaluation of patients with intractable focal epilepsy. In contrast to equivalent dipole models, distributed localization methods do not localize one active point in the brain but rather assume extended active areas, which is generally the case in epileptic activity. The methods shown here are based on linear numerical methods and are therefore less prone to errors when working with scattered solution spaces such as the one defined by anatomical constraints. Solutions constraint to the gray matter determined in the individual MRI are shown here. We illustrate three methods to increase the spatial resolution of the source localization procedures: One is to increase the number of recording channels to more than 100, the second to use linear methods of high precision to detect focal sources (EPIFOCUS), and the third to combine EEG source localization with EEG-triggered functional magnetic resonance imaging. The importance of EEG source localization for the interpretation of fMRI data will be particularly discussed in view of the important difference of the temporal resolution by the two methods. The localization methods can be applied to interictal as well as to ictal activity. In case of analysis of ictal EEG we propose to use full scalp frequency analysis to determine the time period of seizure onset and to localize the sources of the initial dominant frequency.

Journal Article↗
PubMed2000

Visually induced activity in human frontal motor areas during simple visuomotor performance.

Visuomotor tasks elicit neuronal activity in primate motor areas at relatively short latencies. Although this early activity embodies features of visual responses (short latency, stimulus-dependency), its sensory nature has been questioned. We investigated neural correlates of visuomotor performance in human motor areas using scalp and intracranial event-related potential measures. A simple visuomanual reaction-time task evoked early potentials at 133-145 ms post-stimulus which occurred much earlier than the motor potentials of the same region. The amplitude of the early potentials covaried with stimulus location and was independent of parameters of the motor response. Because of their timing, stimulus-dependency and characteristics of our behavioral task, the early potentials are suggested to reflect neuronal responses of sensory nature rather than processing related to pure motor aspects of the task.

Adult↗