Biomedical subjects

C M Michel

Publications and source records attributed to C M Michel.

62 records · Page 4Linked to original sources

PubMed1992

Localization of the sources of EEG delta, theta, alpha and beta frequency bands using the FFT dipole approximation.

FFT dipole approximation and 3-dimensional dipole modelling were used to determine the locations of the equivalent dipole model sources of the delta, theta, alpha, beta-1 and beta-2 frequency bands in 13 normal subjects during resting. From each subject, 2 successive data sets were analysed, each consisting of 10 epochs of 2 sec randomly collected during 30 min. ANOVAs showed that over subjects, the source locations of EEG frequency bands differed significantly in the vertical and antero-posterior dimensions. Results of data set 2 confirmed those of data set 1. The source of delta was deepest and most anterior, theta more posterior and less deep, alpha most posterior and highest on the vertical dimension, beta-1 deeper and slightly more anterior than alpha, and beta-2 again more anterior and deeper than beta-1. Thus, the depth of source location was not linearly related to temporal frequency. The sources of all 5 bands were oriented in the sagittal direction; delta mean fields had steeper gradients anteriorly, alpha and beta-1 posteriorly. The power map for any frequency was well described by a single phase angle. The results indicate that the different EEG frequency bands during a given EEG epoch are generated by neural populations in different brain locations.

Adult↗

PubMed1992

42-channel potential map series to visual contrast and stereo stimuli: perceptual and cognitive event-related segments.

Event-related potential maps to perceptual (stimulus type) and cognitive (stimulus relevance) manipulations were studied in 12 healthy volunteers using 42-channel mapping. Perceptual manipulation used three types of visual stimuli: rectangles constituted by: (1) contrast; (2) different densities of monocular Dynamic Random Dots (Flat DRD); and (3) different binocular disparities of Dynamic Random Dots (Stereo DRD). Cognitive manipulation within each stimulus type consisted of presenting the rectangles horizontally and vertically, one of the two with a probability of 33%, and requesting the subjects to count and thus attend to the 'rare' rectangles. Spatial characteristics of the maps were analyzed; this allowed conclusions about the generating sources. The map series were adaptively segmented using the minima points of the grand mean Global Field Power curve. Segment strength (Global Field Power) and segment landscape (locations of extreme potentials) were assessed. Stimulus type had effects from 78 to 310 ms, stimulus relevance was effective from 210 to 1000 ms. In the 78-174 ms segment, Stereo DRD and Flat DRD stimuli produced similar map landscapes, while contrast stimuli produced different map landscapes. Attended and ignored stimuli produced contrary effects on landscapes at 210-310 ms as compared to those at 310-546 ms, indicative of different neural populations activated by attention processes during these late event-related potential segments. Interaction between perceptual and cognitive manipulation occurred at 210-310 ms when perceiving stereo stimuli and attending to relevant monocular visible stimuli produced similar map landscapes, suggesting a common brain resource during this segment for automatic figure perception and voluntary attention. The observed functional differences of the segments contribute to the identification of global functional microstates of brain electric activity.

Adult↗

PubMed1990

Intracerebral dipole source localization for FFT power maps.

The method treats Fast Fourier Transforms (FFTs) of multichannel EEGs so that they can be used for intracerebral source localizations. For each frequency point, the FFT results are entered into a sine-cosine diagram. Into the diagram's entry constellation (FFT constellation), a straight line (single dipole model) is fitted which produces the least square deviation sum between the entry positions and their orthogonal projections onto the straight line. The map described by the voltages between the projected positions on the straight line is the single source 'FFT approximation' of the FFT constellation. The FFT approximation map is used for dipole source localization. The squared FFT approximation map and the squared map of the forward solution of the computed dipole (power maps) closely resemble the original power map.

Adult↗

PubMed1990

Pharmacokinetics of sulphadimethoxine in channel catfish (Ictalurus punctatus).

1. Plasma clearance, bioavailability, tissue disposition and elimination of 14C-sulphadimethoxine (SDM) were studied in channel catfish (Ictalurus punctatus) after intravenous (i.v.) and oral dosing (per os; p.o.) at 40 mg/kg body weight. 2. Analysis of blood SDM concentrations over time for intravascularly administered SDM showed that disposition and elimination were best described by a two-compartment pharmacokinetic model; estimated half-lives for SDM in blood were 0.09 and 12.6 h for the distribution and elimination phases, respectively. 3. SDM was found primarily in muscle tissue immediately after oral administration; however, clearance from muscle was rapid, with a half-life of 13.1 h. 4. With time, SDM-derived radioactivity became concentrated in the bile and was eliminated slowly (t 1/2 = 115.5 h). 5. Binding of SDM in channel catfish plasma was low (18%) and was non-specific and dose-independent. 6. With the exception of the initial, rapid clearance of SDM from blood, the pharmacokinetic parameters describing SDM distribution and elimination in channel catfish were similar to values reported for other vertebrate species; the rapid distribution of SDM from blood to the tissues in the catfish may be related to species differences in the plasma binding of SDM.

Administration, Oral↗

PubMed1988

Sulfadimethoxine pharmacokinetics and metabolism in the channel catfish (Ictalurus punctatus).

The pharmacokinetics of sulfadimethoxine (SDM) blood clearance, gastrointestinal absorption and tissue distribution and elimination were determined in channel catfish (Ictalurus punctatus) after intravenous and oral dosing using radiolabelled SDM. Blood clearance of SDM in catfish was rapid compared to mammals when the drug was given iv. Results from in vitro equilibrium dialysis studies suggest that this is due to a lower binding of SDM to plasma proteins in fish (18.4%) compared to mammalian species (60-88%). Results from oral dose studies indicated that SDM is readily absorbed from the gastrointestinal tract in channel catfish and distributes rapidly to body tissues, primarily the muscle. With time, concentrations of SDM in muscle decrease and the drug accumulates in the bile, primarily as the N-acetyl-metabolite.

Acetylation↗

PubMedDate not supplied

Intracerebral dipole sources of EEG FFT power maps.

The described method opens a way to compute intracerebral source localizations of ongoing EEG activity. A sine-cosine diagram of the Fourier-transformed data is constructed for each frequency point, forming a "FFT constellation" of entries. Into the FFT constellation of each diagram, a straight line is fitted which produces the least squared deviation sum between the original entry positions and their orthogonal projections onto that line. The map landscape described by the voltages between the projected positions ("FFT approximation") is the least error compromise landscape of all possible landscapes during the paradigmatic cycle of the given FFT frequency. The map thus constructed can be used in the usual dipole source localization procedures. There is one for each FFT frequency point. The squared forward solution of the fitted dipole source and the squared FFT approximation map are "power maps" which are very similar to the original power map. For an average-reference power map with two peaks, the source tends to lie between the peaks; a power map with one peak might show closely neighboring maximal and minimal potential values in the FFT approximation, indicative of a tangential source close to the surface.

Brain↗

PubMedDate not supplied

Event-related potential maps depend on prestimulus brain electric microstate map.

The brain functional microstate immediately before each of about 3000 identical tone stimuli was classified using extracted reference-free descriptors (locations of maximal and minimal potential) of the landscape of the brain's momentary electric field, in 8 volunteers. Six prestimulus microstate map classes occurred more than 30 times in each subject, and were clustered into two map class types (totals of 242 and 283 cases, respectively, on the average per subject). Event-related potential (ERP) map series were averaged for each subject and prestimulus map class. Map descriptors were extracted from the ERP maps at times of maximal Global Field Power during the component time windows N100, P200 and P330. Discriminant functions were estimated; for the maps of N100 and P330, the discriminant scores differed significantly between the maps associated with the two prestimulus map class types (paired t-tests, df = 7, p = .014 and p = .005, respectively). The dominant axis of the poststimulus class type II ERP maps deviated clockwise from that of the type I ERP maps in all components. We conclude that subtle changes in the brain's spontaneous momentary functional microstate (as classified by spatial descriptors of a single map) influence event-related information processing by the brain, following common rules over subjects.

Adult↗

PubMedDate not supplied

Event-related potential map differences depend on the prestimulus microstates.

The dependency of the landscapes of visually evoked, 47-channel, event-related potential (ERPs) on the functional microstates (momentary map landscape) just before stimulus arrival was investigated, in 12 volunteers. The prestimulus microstates were determined using the map at the last peak of Global Field Power before the stimulus. The landscapes of these maps were described by the electrode locations of the positive and negative extreme potentials, and assigned to basic classes. The two most frequently occurring map-classes were used (left anterior-right posterior, and right anterior-left posterior). ERP map series were averaged for each subject and each prestimulus microstate class. The Randomization-Monte Carlo MANOVA test was used to test the significance of the difference between the ERP map landscapes at each sample point (n = 128, 500 ms) associated with the two prestimulus microstates. At 16 samples the difference was significant at p < 0.05. The longest uninterrupted sequence (n = 9) of significant differences occurred between 164 and 195 ms, i.e. during the conventional component P200. The results demonstrate that the brain electric microstate at stimulus arrival crucially influences the active neuronal populations that contribute to the ERP. This suggests that the processing of information will differ as a function of the momentary brain microstate at information arrival.

Adult↗