Search PubMed⌕ Search

Biomedical subjects

C M Michel

Publications and source records attributed to C M Michel.

At least 55 records · Page 3Linked to original sources
PubMed1997

Prestimulus EEG microstates influence visual event-related potential microstates in field maps with 47 channels.

The influence of the immediate prestimulus EEG microstate (sub-second epoch of stable topography/map landscape) on the map landscape of visually evoked 47-channel event-related potential (ERP) microstates was examined using the frequent, non-target stimuli of a cognitive paradigm (12 volunteers). For the two frequent prestimulus microstate classes (oriented left anterior-right posterior and right anterior-left posterior), ERP map series were selectively averaged. The post-stimulus ERP grand average map series was segmented into microstates; 10 were found. The centroid locations of positive and negative map areas extracted as landscape descriptors. Significant differences (MANOVAs and t-tests) between the two prestimulus classes were found in four of the ten ERP microstates. The relative orientation of the two ERP microstate classes was the same as prestimulus in some ERP microstates, but reversed in others. Thus, brain electric microstates at stimulus arrival influence the landscapes of the post-stimulus ERP maps and therefore, information processing; prestimulus microstate effects differed for different post-stimulus ERP microstates.

Adult↗
PubMed1997

Extracranial localization of intracranial interictal epileptiform activity using LORETA (low resolution electromagnetic tomography).

Besides the standard clinical methods of EEG waveshape analysis, mathematical models for reconstruction of dipolar sources from the digitized surface EEG have been introduced in epilepsy research. Although useful for localizing focal sources, these methods are inadequate for analyzing widespread epileptiform activity. A recently introduced alternative method called LORETA (low resolution electromagnetic tomography, Pascual-Marqui et al., 1994), directly computes the current distribution throughout the full brain volume, assuming that neighboring neuronal populations are simultaneously and synchronously activated. In mathematical terms the method selects the smoothest of all possible 3-dimensional current distributions, inherently introducing a certain amount of dispersion. In 7 patients, undergoing simultaneous EEG recording from 10 intracranial (subdural) and 22 extracranial electrodes, 111 subdural discharges (61 subtemporal and 50 lateral temporal) were identified. The exact time point of maximal intracranial activity was automatically identified, and the LORETA solution at that timepoint was computed from the surface EEG. Statistical comparison revealed significantly higher LORETA current density in the area corresponding to the subdurally recorded spike compared to other areas, and a more anterior and more medial LORETA location for subtemporal compared to lateral temporal spikes. This study indicates that the LORETA technique may become a useful method to localize electrical activity in the brain.

Adolescent↗
PubMed1995

Mapping event-related brain potential microstates to sentence endings.

We analyzed topography and strength of 20 channel event-related potential maps to sentence endings differing in correctness, verbal vs. nonverbal surface form, priming, and repetition count. Seventeen healthy subjects silently read correct and incorrect versions of simple sentences with predictable color endings, and of more complex sentences with predictable composite word endings. Color endings appeared in verbal and nonverbal form. Measures of map topography (centroids of the positive and negative areas of the average referenced maps) and strength (Global Field Power) were analyzed. Adaptive segmentation distinguished a pre-N400 and a N400 microstate in the N400 time range. Topography differed between these two microstates, between verbal and nonverbal endings, and between correct color, incorrect color, and incorrect noncolor words. All verbal endings evoked left-laterlized negativity and right lateralized positivity in the pre-N400 microstates. Correct verbal endings evoked consistent posterior postivity and anterior negativity with left-lateralized gradient strength suggesting language-specific processing. New, incorrect noncolor words evoked reversed anterior-posterior N400 and pre-N400 map topographies with more anterior positivity and more posterior negativity than correct colors in each subject. Gradient strength and current source density maps also differed from those to correct colors. Strongest gradients were left-posterior in the pre-N400 but anterior in the N400 microstate, consistent with anterior activity contributing to the posterior N400 negativity. Incorrect and correct colors, which were semantically primed and repeated, showed smaller topographic differences and N400 effects with a different topography. These different maps can not arise by modulation of a single pattern of neural activity and show that the N400 time range consists of multiple distinct microstates.

Adult↗
PubMed1995

Frequency domain source localization shows state-dependent diazepam effects in 47-channel EEG.

The topic of this study was to evaluate state-dependent effects of diazepam on the frequency characteristics of 47-channel spontaneous EEG maps. A novel method, the FFT-Dipole-Approximation (Lehmann and Michel, 1990), was used to study effects on the strength and the topography of the maps in the different frequency bands. Map topography was characterized by the 3-dimensional location of the equivalent dipole source and map strength was defined as the spatial standard deviation (the Global Field Power) of the maps of each frequency point. The Global Field Power can be considered as a measure of the amount of energy produced by the system, while the source location gives an estimate of the center of gravity of all sources in the brain that were active at a certain frequency. State-dependency was studied by evaluating the drug effects before and after a continuous performance task of 25 min duration. Clear interactions between drug (diazepam vs. placebo) and time after drug intake (before and after the task) were found, especially in the inferior-superior location of the dipole sources. It supports the hypothesis that diazepam, like other drugs, has different effects on brain functions depending on the momentary functional state of the brain. In addition to the drug effects, clearly different source locations and Global Field Power were found for the different frequency bands, replicating earlier reports (Michel et al., 1992).

Acoustic Stimulation↗
PubMed1995

Electrocortical, autonomic, and subjective responses to rhythmic audio-visual stimulation.

The present study was designed to test the hypothesis that varying sensory input can affect mood, autonomic arousal, and electrocortical activity. Twenty right-handed males were exposed to three rhythmic audio-visual stimulation programs, with either a high intensity and variety of stimuli (program H), a low stimulation (program L) or with a transient from high to low (program HL). Multichannel EEG, heart rate, and skin conductance were recorded continuously, and after each trial mood was rated on a bipolar adjective list. EEG data were subjected to FFT dipole approximation procedure, and dipole locations and field strength (Global Field Power) were analyzed for the frequency bands theta, alpha, and beta 1. Mood ratings clearly differed between programs H and HL, with highest values of arousal after H. Programs L and HL decreased autonomic arousal, whereas H induced deactivating as well as activating effects. Field strength of the alpha band decreased similarly during all programs. Dipole sources were located more to the left in the alpha band and more to the right in the beta 1 band during all programs as compared with baseline. Therefore, programs affected mood and autonomic variables differently, but not electrocortical variables. The higher activation of the right hemisphere during all programs is interpreted as an indication that audio-visual stimulation does induce changes in the brain, such as are commonly found in altered states of consciousness.

Acoustic Stimulation↗
PubMed1995

Microstate segmentation of spontaneous multichannel EEG map series under diazepam and sulpiride.

Spontaneous multichannel brain electric field (EEG) map series of 20 seconds duration at 1, 15, 30, 45, and 60 minutes after the injection of a single dose of diazepam (13 ss) or sulpiride (6 ss) were segmented into microstates of quasi-constant landscape but varying durations. Post-minus-preinjection difference values were computed for the six microstate variables: specific window size, duration, orientation, distance between windows, and location of center of gravity on the anterior-posterior and left-right axis. Differences between drugs were explored with ANOVAs. Microstate duration increased after sulpiride, and the location of the microstate center of gravity on the anterior-posterior axis moved to a more anterior position after diazepam. The results are in agreement with expectations based on measurements of patients' EEG microstates and with results using estimates of EEG model source locations in the frequency domain. Microstate segmentation appears to be a useful method for physiologically meaningful reduction of multichannel brain electric field data in psychopharmacology.

Adult↗
PubMed1995

Segmentation of brain electrical activity into microstates: model estimation and validation.

A brain microstate is defined as a functional/physiological state of the brain during which specific neural computations are performed. It is characterized uniquely by a fixed spatial distribution of active neuronal generators with time varying intensity. Brain electrical activity is modeled as being composed of a time sequence of nonoverlapping microstates with variable duration. A precise mathematical formulation of the model for evoked potential recordings is presented, where the microstates are represented as normalized vectors constituted by scalp electric potentials due to the underlying generators. An algorithm is developed for estimating the microstates, based on a modified version of the classical k-means clustering method, in which cluster orientations are estimated. Consequently, each instantaneous multichannel evoked potential measurement is classified as belonging to some microstate, thus producing a natural segmentation of brain activity. Use is made of statistical image segmentation techniques for obtaining smooth continuous segments. Time varying intensities are estimated by projecting the measurements onto their corresponding microstates. A goodness of fit statistic for the model is presented. Finally, a method is introduced for estimating the number of microstates, based on nonparametric data-driven statistical resampling techniques.

Algorithms↗
PubMed1994

Low resolution electromagnetic tomography: a new method for localizing electrical activity in the brain.

This paper presents a new method for localizing the electric activity in the brain based on multichannel surface EEG recordings. In contrast to the models presented up to now the new method does not assume a limited number of dipolar point sources nor a distribution on a given known surface, but directly computes a current distribution throughout the full brain volume. In order to find a unique solution for the 3-dimensional distribution among the infinite set of different possible solutions, the method assumes that neighboring neurons are simultaneously and synchronously activated. The basic assumption rests on evidence from single cell recordings in the brain that demonstrates strong synchronization of adjacent neurons. In view of this physiological consideration the computational task is to select the smoothest of all possible 3-dimensional current distributions, a task that is a common procedure in generalized signal processing. The result is a true 3-dimensional tomography with the characteristic that localization is preserved with a certain amount of dispersion, i.e., it has a relatively low spatial resolution. The new method, which we call Low Resolution Electromagnetic Tomography (LORETA) is illustrated with two different sets of evoked potential data, the first showing the tomography of the P100 component to checkerboard stimulation of the left, right, upper and lower hemiretina, and the second showing the results for the auditory N100 component and the two cognitive components CNV and P300. A direct comparison of the tomography results with those obtained from fitting one and two dipoles illustrates that the new method provides physiologically meaningful results while dipolar solutions fail in many situations. In the case of the cognitive components, the method offers new hypotheses on the location of higher cognitive functions in the brain.

Brain↗
PubMed1994

Diazepam and sulpiride effects on frequency domain EEG source locations.

Effects of an anxiolytic (diazepam; n = 13) and an antipsychotic (sulpiride; n = 6) on the location of EEG model sources in the frequency domain were studied in normal volunteers in 19-channel eyes-closed EEG before and 1, 15, 30, 45 and 60 min after i.v. injections. Ten 2-second epochs from each recording were subjected to the fast Fourier transformation (FFT) Dipole Approximation which results in a single-source (in terms of phase angle) potential distribution map for each frequency point. Three-dimensional model dipole sources were fitted into the maps. Differences in source location before and after injection were computed, and differences between drugs were evaluated. The source location in the beta band shifted significantly more towards superior and anterior areas after diazepam than sulpiride, most prominently in the first minute after injection; during this time, conventional beta band power was significantly increased after diazepam versus sulpiride at all recording sites. The results demonstrate that the FFT dipole approximation adds direct 3-dimensional topographical information to power spectral results in pharmaco-EEG.

Adult↗
PubMed1993

EEG reactivity in high and low symptomatic schizophrenics, using source modelling in the frequency domain.

A dipole localization method in the frequency domain was used (FFT Dipole Approximation) to assess spatial differences in the spectral EEG reactivity (orienting response) between high and low symptomatic schizophrenics. Frequency bands of interest were determined empirically by comparing the two dichotomized patient groups with two matched control groups. Evidence for a correlation between EEG reactivity and severity of schizophrenic symptomatology was found, especially in the higher beta frequency range (16-25.5 Hz). Opposite effects were found in the two beta ranges of 20.5-22.5 Hz and 23.0-25.5 Hz, supporting the hypothesis that different EEG frequency bands have specific functional significances and that these bands are not necessarily those that are conventionally selected.

Adaptation, Physiological↗
PubMed1993

Global dimensional complexity of multi-channel EEG indicates change of human brain functional state after a single dose of a nootropic drug.

Viewing the multi-channel EEG as a sequence of momentary field maps corresponds to the concept of a trajectory in K-dimensional state space (K = number of channels). This approach permits a quantitative, single value measure of complexity of the brain state trajectory, the global correlation dimension that describes the ensemble characteristics of all recorded channels. In 5 normal volunteers, 4 records of 16-channel resting EEG were obtained during each of 4 randomized sessions (double blind design) after a single dose of placebo or 2.9 g or 4.8 g or 9.6 g piracetam. The global correlation dimension of a 40 sec epoch from each record was estimated, using 50 computational runs with 8192 point pairs. The results were combined for the two intermediate doses and averaged over repeated records. The dimensionality decreased from placebo (median = 5.89) to low dose (median = 5.72) to high dose (median = 5.59), significant in a Friedman ANOVA at P < 0.02, with significant differences between placebo vs. high and low vs. high dose. Thus, the subtle change of brain global functional state after a single dose of piracetam is reflected by the non-linear measure of global dimensional complexity of the multi-channel EEG.

Adult↗
PubMed1993

Adaptive segmentation of spontaneous EEG map series into spatially defined microstates.

Space-oriented segmentation can decompose multi-channel EEG map series into time segments characterized by quasi-stationary field map configurations. This assesses the dynamics of the underlying processes as activities of different neural generator ensembles. Our method of space-oriented segmentation describes the scalp field at times of maximal field strength (Global Field Power) by the locations of the centroids of positive and negative map areas. A quantitative measure of the simultaneous distance of the centroid locations evaluates the similarity between consecutive maps. A segment is defined as a sequence of maps that do not differ from each other by more than a present value. Finally, the average centroid locations for each segment are entered into an agglomerative clustering procedure to obtain a set of distinct classes of field configurations. Four records of 16 s of 42-channel resting EEG (band-pass filtered 2-16 Hz) from six subjects were analyzed. Average segment duration was 157.9 ms. Most segments belonged to a small number of classes (from 2 to 6, mean 3.7 classes for 90% of analysis time). The most frequent class showed an anterior-posterior field orientation and covered from 45 to 74% (mean 55% across subjects) of total time, with an average duration of 265 ms. The procedure was also tested using temporally and spatially unstructured data (white noise and randomly shuffled EEG) to ascertain that the methods reflect the spatio-temporal structure of the EEG processes.

Adult↗
PubMed1993

Space-oriented EEG segmentation reveals changes in brain electric field maps under the influence of a nootropic drug.

Map landscape-based segmentation of the sequences of momentary potential distribution maps (42-channel recordings) into brain microstates during spontaneous brain activity was used to study brain electric field spatial effects of single doses of piracetam (2.9, 4.8, and 9.6 g Nootropil UCB and placebo) in a double-blind study of five normal young volunteers. Four 15-second epochs were analyzed from each subject and drug condition. The most prominent class of microstates (covering 49% of the time) consisted of potential maps with a generally anterior-posterior field orientation. The map orientation of this microstate class showed an increasing clockwise deviation from the placebo condition with increasing drug doses (Fisher's probability product, p < 0.014). The results of this study suggest the use of microstate segmentation analysis for the assessment of central effects of medication in spontaneous multi-channel electroencephalographic data, as a complementary approach to frequency-domain analysis.

Adult↗
PubMed1993

Source localization of brain electric field frequency bands during conscious, spontaneous, visual imagery and abstract thought.

This paper addresses the issue of mind-brain correspondence, using a novel way to reduce brain electric field data in the frequency domain to estimates of intracerebral model source locations, and applying this method to brain electric data collected during the 2-s epochs immediately before the randomly solicited reports of spontaneous, conscious, covert experiences from 12 normal volunteers. The mentation reports were classified into visual imagery and abstract thought. The mean locations of the EEG model sources associated with abstract thoughts were generally more anterior and deeper than those of visual imagery, particularly significant for the delta/theta band; the finding was common across subjects. Thus, different brain functional states involving different geometries of activated neural populations exist during conscious, spontaneous, task-free mentations of the visual imagery type and of the abstract thought type.

Acoustic Stimulation↗
PubMed1993

Localization of sources of brain alpha/theta/delta activity and the influence of the mode of spontaneous mentation.

A method is described that accounts for multichannel brain field data (EEG) epochs, after transformation into the frequency domain, by a single oscillating dipole source in terms of phase angles. The method produces a potential distribution for each frequency point ('FFT dipole approximation'). These maps can be subjected to conventional equivalent dipole source fittings in terms of amplitudes. We studied the equivalent source locations for the different temporal EEG frequency bands (delta/theta/alpha) in 12 normal subjects during the collection of reports of spontaneous thoughts. Some of the thought reports were classed into two modes, 'visual imagery' and 'abstract', and the associated equivalent source locations during the 2 s immediately prior to these reports were computed. Different equivalent source locations were found for the different spectral components of the EEG, implying that different neural generator populations generate the different frequencies. Further, the different types of spontaneous thought, i.e. different modes of cortical functioning, were found to be associated with the activity of different neuronal generator sources that operated at the same frequency at different source locations.

Alpha Rhythm↗
PubMed1993

Single doses of piracetam affect 42-channel event-related potential microstate maps in a cognitive paradigm.

We examined whether a single administration of piracetam produces dose-dependent effects on brain functions in healthy young men. In 6 subjects, 42-channel event-related EEG potential maps (ERP) were recorded during a task requiring subjects to watch single digits presented in a pseudorandom order on a screen and to press a button after all triplets of three consecutive odd or even digits. The ERP maps to the three digits of the correctly detected triplets were analyzed in terms of their mapped ERP field configuration (landscape). Different landscapes of the maps indicate different configuration of the activated neural population and therefore reflect different functional microstates of the brain. In order to identify these microstates, adaptive segmentation of the map series based on their landscapes was done. Nineteen time segments were found. These segments were tested for direct effects on brain function of three single doses of piracetam (2.9, 4.8 or 9.6 g) and a placebo given double-blind in balanced order. Piracetam mainly affected the map landscape of the time segments following the triplet's last digit. U-shaped dose-dependent effects were found; they were strongest after 4.8 g piracetam. Since these particular ERP segments are recognized to be strongly correlated to cognitive functions, the present findings suggest that single medium doses of piracetam selectively activate differently located or oriented neurons during cognitive steps of information processing.

Adult↗