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H Nowak

Publications and source records attributed to H Nowak.

At least 37 records · Page 2Linked to original sources
PubMed1999

Impaired sensory processing in male patients with schizophrenia: a magnetoencephalographic study of auditory mismatch detection.

The generation of mismatch negativity (MMN) as a component of auditory evoked event-related brain potentials has been reported previously to be severely disturbed in patients with schizophrenia. In the present study, we extended these findings to magnetoencephalography and investigated the neuromagnetic mismatch field (MMNm) in 15 male schizophrenic inpatients as compared to 16 healthy male volunteers. A standard tone of 1000 Hz and three different types of mismatch (1050-Hz tone, 5000-Hz tone, tone omission) were employed within the same paradigm, each mismatch occurring with a 10% pseudorandom probability. After correction for eye artifacts, the mean global field power of the mismatch reaction was calculated. Mismatch generation in patients with schizophrenia proved to be significantly impaired for all three conditions. This result confirms the theory of impaired auditory information processing in patients with schizophrenia at the level of the primary auditory cortex. Deficient generation of MMNm probably represents an impaired generation and/or faster decay of the sensory memory trace on the basis of disturbed sensory processing in male patients with schizophrenia.

Adult↗
PubMed1999

[Late fields in magnetcardiography and late potentials in electrocardiography after acute myocardial infarction].

UNLABELLED: A high specificity and a high positive prediction has been reached in risk stratification for a sudden cardiac death after acute myocardial infarction (AMI) by combining multiple methods. But sensitivity and negative prediction are still not satisfying. There are the same physiological processes underlying magnetocardiography (MCG) and electrocardiography (ECG). Nevertheless, the signals in each method contain different information. METHODS: We studied the cardiac magnetic fields in 50 patients after AMI and in 32 probands and calculated the magnetic late field (LF), according to Simsons late potential (LP) analysis. We defined normal values, according to the 95% confidence interval of the probands (QRS < =97 ms, RMS > = 0. 6, LAS < 25 ms). RESULTS: We compared the results of LF and LP analysis regarding pathologic-nonpathologic and found 76% of the patients with the same results in both methods. Four patients had magnetic signals with low amplitude in the ST segment in contrast to the ECG result, while 6 patients with a "LP positive" diagnosis based on RMS and LAS only, did not show LF. In addition, we have found the magnetic QRS complex to be shorter than the electrical one. DISCUSSION: In general, the results of LF measurement are similar to the ones of LP measurement. Presumably, there are intracardial currents, which are not detectable by ECG. Further studies are needed to evaluate the prognostic value in patients at high risk for cardiac arrhythmias.

Adult↗
PubMed1999

The sensitivity of instantaneous coherence for considering elementary comparison processing. Part II: Similarities and differences between EEG and MEG coherences.

The EEG (electroencephalogram) coherence depends on EEG deviation type. A high level of sensitivity of instantaneous coherence for investigating elementary cognitive tasks could be shown in the case of unipolar reference (ear lobe reference). In order to validate of this result the same investigations were performed for MEG (magnetoencephalogram) coherence, where EEG and MEG were measured simultaneously. A strong correlation between time intervals with high EEG and MEG coherence could be shown. The equivalence of the sensitivity of EEG and MEG coherence for the description of the dynamic behaviour of information processing and the distinction between different elementary cognitive tasks is proven statistically.

Adult↗
PubMed1999

Application of high-order boundary elements to the electrocardiographic inverse problem.

Eight-noded quadrilateral boundary elements are applied to the electrocardiographic inverse problem as an example for high-order boundary elements. It is shown that the choice of the shape functions used for approximation of the potentials has a remarkable influence on the solution obtained if the number of electrodes is smaller than the number of primary source points (under-determined equation system). Three different formulations are investigated considering a concentric spheres problem where an analytic solution is available: (a) the isoparametric formulation; (b) the quasi-first-order formulation; and (c) the pseudo-subparametric formulation as a new method. In a second step the pseudo-subparametric formulation (which provided the best results in the test problem) is applied to real word data. The transmembrane potential pattern of a 40 years old female suffering from severe heart failure and ventricular tachycardia after large anterior wall myocardial infarction is reconstructed for one time instant. Furthermore, an algorithm for the calculation of the transfer matrix is presented which avoids restrictions to the boundary element mesh caused by the placement of the electrodes.

Adult↗
PubMed1999

A 16-channel SQUID-device for biomagnetic investigations of small objects.

Biomagnetic investigations in basic physiological research using animals require measurement devices different from commercial biomagnetometers used in human investigations. Two major problems have to be tackled in the design of such biomagnetometers. First, the spatial sampling needs to be much higher. Second, the distance between pick-up coils and the sources needs to be much shorter in order to compensate the worse signal-to-noise ratio (SNR) due to the smaller pick-up coils. We designed and built a 16-channel biomagnetic measurement system meeting these design criteria. The pick-up coil diameter of this new biomagnetometer is 6.7 mm, thus allowing 16 channels on an area of 3.2x3.2 cm2. The pick-up coils are located 3 mm above the dewar outer bottom, hence the closest distance to the cortical surface can be a few millimetres. We provide as an example of first measurements performed with the new biomagnetometer investigations of epileptic spikes in adult rabbits by simultaneous magnetoencephalogram (MEG) and electrocorticogram (ECoG) recordings. The high SNR of the recorded MEG and the simultaneously detected electric potentials allow investigations of the spatio-temporal pattern of neuronal processes of epileptiform spikes with signal strengths of about 3.5 pT.

Animals↗
PubMed1999

Source localization in an inhomogeneous physical thorax phantom.

The influence of lung inhomogeneities on focal source localizations in electrocardiography (ECG) and magnetocardiography (MCG) is investigated. A realistically shaped physical thorax phantom with cylindrical lung inhomogeneities is used for electric and magnetic measurements. The lungs are modelled with a special ionic exchange membrane which allows different conductivity compartments without influencing the free ionic current flow. The dipolar current sources are composed of platinum wire and located at different depths and directions between the lung inhomogeneities. We localized the current dipoles with different boundary element method (BEM) models, based on electrical data and simultaneous electrical and magnetic data. Our results indicate the possibility of superadditive information gain by combining electrical and magnetic data for source reconstructions. We found a significant influence of the inhomogeneities on both the calculated source location and the calculated source strength. Mislocalizations of up to 16 mm and wrong dipole strengths of up to 52% were obtained when the lung inhomogeneities were not taken into account for source localization. Dipoles parallel to the lungs showed a larger localization error in depth than dipoles perpendicular to the lungs. We conclude that the incorporation of lung inhomogeneities will improve source localization accuracy in ECG and MCG.

Body Surface Potential Mapping↗
PubMed1999

[High-resolution magnetoencephalography--studies with a small-volume phantom].

To investigate the spatiotemporal organisation of neuronal processes in an animal model using magnetoencephalography (MEG), a high temporal resolution (ms) and an appropriate spatial resolution of about 1 mm is necessary. With the aim of determining the localization error and the resolution power of high-resolution MEG systems, we developed a phantom capable of simulating the characteristics of animal models. The phantom enables us to variably position at least two magnetic field sources to within 0.1 mm. For source localization on the basis of the magnetic field data, a spatial filtering algorithm was used. The investigation of a 16-channel micro SQUID-MEG system with a current dipole orientated tangentially to the phantom surface produced the following localization data (min ... max, x, y--horizontal plane, z--depth); systematic localization error e(x) = 1.16 ... 1.67 mm, e(y) = -1.01 ... -1.28 mm, e(z) = -5.22 ... -7.64 mm, standard deviation of the individual measurements perpendicular to the dipole axis s(perp) = 0.05 ... 0.22 mm, along this axis s(long) = 0.20 ... 1.73 mm, in the depths sz = 0.17 ... 3.17 mm. The "goodness of fit" was > 95%. Separation of two dipoles was still possible for parallel dipoles at a distance apart of d(parallel) = 0.03 mm and for those oriented perpendicularly to each other at a distance apart of d(perp) = 0.10 mm. On the basis of these results we conclude that the MEG system can achieve a resolution sufficient to permit the investigation of neuronal microstructures. The spatial errors detected were related to sensor position in the cryostatic vessel as well as to external low-frequency noise.

Magnetoencephalography↗
PubMed1999

The influence of conductivity changes in boundary element compartments on the forward and inverse problem in electroencephalography and magnetoencephalography.

Source localization based on magnetoencephalographic and electroencephalographic data requires knowledge of the conductivity values of the head. The aim of this paper is to examine the influence of compartment conductivity changes on the neuromagnetic field and the electric scalp potential for the widely used three compartment boundary element models. Both the analysis of measurement data and the simulations with dipoles distributed in the brain produced two significant results. First, we found the electric potentials to be approximately one order of magnitude more sensitive to conductivity changes than the magnetic fields. This was valid for the field and potential topology (and hence dipole localization), and for the amplitude (and hence dipole strength). Second, changes in brain compartment conductivity yield the lowest change in the electric potentials topology (and hence dipole localization), but a very strong change in the amplitude (and hence in the dipole strength). We conclude that for the magnetic fields the influence of compartment conductivity changes is not important in terms of dipole localization and strength estimation. For the electric potentials however, both dipole localization and strength estimation are significantly influenced by the compartment conductivity.

Electric Conductivity↗
PubMed1998

Hemispheric differences in frequency dependent dipole orientation of the human auditory evoked field component N100m.

Auditory evoked fields (AEF) of 19 healthy male subjects were recorded bilaterally with a Philips 31 -channel biomagnetometer, using two conditions of stimulation (1000 vs. 5000 Hz tones). The N100m latency was characterized by a single moving dipole for each condition and hemisphere using a boundary element model (BEM) as volume conductor. While the right hemispheric dipole orientations and locations did not change with respect to condition, the left hemispheric dipoles differed significantly between the 1000 and 5000 Hz tones, especially in dipole orientation. The left hemispheric dipoles were orientated on average 10.8 degrees more vertically for the 5000 Hz condition. This result points to interhemispheric differences on the level of sensory processing.

Acoustic Stimulation↗
PubMed1998

Noninvasive biomagnetic imaging in coronary artery disease based on individual current density maps of the heart.

OBJECTIVE: In this paper we present an attempt at noninvasive imaging of distributed myocardial electrical activity in patients suffering from myocardial infarction and in healthy subjects. Although advances have been made, noninvasive three-dimensional imaging of cardiac electrophysiological activity is still in its infancy and extending our knowledge of cardiac electrophysiological properties may be a valuable guide in the treatment of patients with coronary artery disease. METHODS: Magnetic field mapping data formed the input for an inverse solution that is based on a multiple dipole model. The lead field normalized minimum norm least square criterion was applied to predefined myocardial source geometry. Current density distributions were calculated for the left ventricle during ventricular depolarization. Images from two patients with previous myocardial infarction were compared to images from two healthy subjects. RESULTS: Low regional and global current density was found in the infarction patients. Regions of low current density corresponded to infarcted segments. The images of the healthy subjects displayed less marked areas of low current density. CONCLUSION: The proposed multiple dipole model may be able to distinguish viable from scarred myocardium. A prospective clinical study should be undertaken to investigate the spatial resolution and the diagnostic performance of this method.

Body Surface Potential Mapping↗
PubMed1998

Reorganization of the somatosensory cortex after amputation of the index finger.

Cortical reorganization occurs within the primary somatosensory and the primary motor cortex after amputation of the arm or forearm. Here we report on a patient showing cortical reorganization after amputation of his right index finger. Our findings indicate that the neural networks within the area of the amputated finger in the somatosensory cortex (SI) were invaded by neighbouring structures, i.e. of neural cell assemblies that subserve the thumb and middle finger of his right hand.

Amputation, Traumatic↗
PubMed1998

[Sex specific differences in hemispheric lateralization in schizophrenia? An MEG-MRI study].

In this magnetoencephalography study the issue of hemispheric lateralisation in patients with schizophrenia was addressed using acoustically evoked neuromagnetic fields. The characteristics of dipoles in the superior temporal gyrus, the primary auditory cortex, were calculated. In contrast to other studies, alterations did not concern the localisation, but rather the orientation of dipoles. Of pathophysiological interest was that the dipoles abnormalities were found left-hemispherically in male (p = 0.02) and right-hemispherically in female patients with schizophrenia (p = 0.01) when compared to controls. The findings suggest gender-specific alterations of hemispheric lateralisation in schizophrenia.

Adult↗
PubMed1998

Quantification and rejection of ocular artifacts in auditory evoked fields in schizophrenics.

RESULTS: In a magnetoencephalographic investigation of the auditory evoked field (AEF) in 17 schizophrenics and 17 controls, 37% of the schizophrenics and 12% of the controls showed eye artifacts in every second trial or even more frequently. In the uncorrected average fields, the ratio between the power of artifacts and the power of the magnetoencephalogram (MEG) exceeded the value of 0.1 for 48% of the schizophrenics and for 29% of the controls. Ocular artifacts biased the locations of equivalent current dipoles of the M100 component towards deeper positions. A regression algorithm for the correction of ocular artifacts in raw data and an identification technique of ocular artifacts based on the topography of transmission coefficients is described. CONCLUSIONS: A linear dependence of ocular artifacts in AEF on the electrooculogram (EOG) was confirmed. Possible errors introduced by the correction are discussed. Transmission coefficients should be calculated for several individual trials with the same type of artifact. Errors due to evoked potentials in the EOG were found to be comparable in amplitude to noise in the AEF. Examples of transmission coefficients from the EOG to the MEG are given.

Adult↗
PubMed1998

Gastric emptying in patients with insulin dependent diabetes mellitus and bioavailability of thioctic acid-enantiomers.

The objective of this study was to determine the impact of prolonged gastric emptying in patients with insulin dependent diabetes mellitus (IDDM) on the bioavailability of the R(+)- and S(-)-thioctic acid (TA) enantiomers. Gastric emptying time (GET) was assessed in 30 healthy volunteers and 22 patients with IDDM using sequential ultrasonography after a standardized solid-liquid test meal. Pharmacokinetics and absolute bioavailability (F) of the TA-enantiomers were studied using a randomized, open two-way crossover design with administrations of oral and intravenous single doses of 200 mg rac-TA. GET in healthy subjects was 134.7+/-21.6 min, the normal range was calculated from 88.3 to 181.1 min. The mean GET in all IDDM patients was significantly prolonged (178.2+/-28.1 min; P<0.001). Only 50% of the patients (n=11) were found to have normal GET (group A), the other half of the population (n=11) were considered to have delayed GET (group B). Mean GET values were 156.9+/-21.5 in group A (P=0.028) and 199.4+/-13.9 min in group B, respectively, suggesting that gastric motility is significantly different from non-diabetic controls even in patients with apparently normal gastric emptying. Times to peak plasma concentrations (t(max)) of both TA-enantiomers were similar in both groups and thus, unrelated to measures of gastric emptying. In contrast, maximum concentrations (C(max)) and area-under-the-curve values (AUC) of both enantiomers were reduced by about 30% in patients with delayed GET. Although these differences resulted in statistical significance for the AUC of both enantiomers (P<0.05), linear regression analysis showed only modest correlation between GET and the extent of TA-enantiomer absorption (r2=0.31 and 0.22 for R(+)-/S(-)-TA, respectively). The study suggests that prolonged gastric emptying is frequently present in IDDM. Delayed gastric emptying, however, does not substantially affect the rate and extent of absorption of both TA-enantiomers.

Administration, Oral↗
PubMed1998

MCG simulations of myocardial infarctions with a realistic heart-torso model.

Data from simulations of the anterior myocardial infarction (AMI) and inferior myocardial infarction (IMI) are presented. One infarct located in the anterior section of the left ventricle and a second one in the inferior wall of the left ventricle were modeled. A high-resolution finite element model of a heart and torso was used in this study. Differences in the normal and infarcted fields were computed. Our data suggest that the infarcted region contribution to the total magnetic field can be accounted for by an equivalent current dipole. It might also be possible to detect an infarct from these difference fields constructed for different cases of myocardial infarction. More simulations are needed to determine the relations between infarct sizes and locations and magnetic fields. These relations might then be used to detect various cases of myocardial infarction.

Biophysical Phenomena↗
PubMed1998

MCG simulations with a realistic heart-torso model.

Magnetocardiograms (MCG's) simulated with a high-resolution heart-torso model of an adult subject were compared with measured MCG's acquired from the same individual. An exact match of the measured and simulated MCG's was not found due to the uncertainties in tissue conductivities and cardiac source positions. However, general features of the measured MCG's were reasonably represented by the simulated data for most, but not all of the channels. This suggests that the model accounts for the most important mechanisms underlying the genesis of MCG's and may be useful for cardiac magnetic field modeling under normal and diseased states. MCG's were simulated with a realistic finite-element heart-torso model constructed from segmented magnetic resonance images with 19 different tissue types identified. A finite-element model was developed from the segmented images. The model consists of 2.51 million brick-shaped elements and 2.58 million nodes, and has a voxel resolution of 1.56 x 1.56 x 3 mm. Current distributions inside the torso and the magnetic fields and MCG's at the gradiometer coil locations were computed. MCG's were measured with a Philips twin Dewar first-order gradiometer SQUID-system consisting of 31 channels in one tank and 19 channels in the other.

Adult↗