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Biomedical subjects

H Witte

Publications and source records attributed to H Witte.

At least 55 records · Page 3Linked to original sources
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

Identification of the stimulated hemiretinae using a reduced number of PVEP trials.

Left and right hemifield pattern-reversal visual evoked potentials (PVEP) at P100 latency are characterised by typical field distributions of potential and the spectral parameter instantaneous frequency (IF). Both parameters can be utilised for the correct identification of the stimulated hemiretina in healthy volunteers (Hoffmann, K. et al. Electroenceph. clin. Neurophysiol., 100, 1996: 569-578). The aim of this study was the investigation of the robustness of instantaneous frequency for reduced numbers of averages. Hemifield PVEP of 15 volunteers (20 channel records) were analysed. The number of averages was reduced step-by-step (64-32-16-8-4-1). For each average, the time of P100 latency was determined by the Global field power maximum between 90 and 125 ms. The stimulated hemifield was identified using the potential or instantaneous frequency values at the occipital electrode positions O1 and O2: by maximal potential and instantaneous frequency on the stimulus-contralateral side. In summary, by reduced numbers of averages (as well as by single trials) the stimulated hemiretina was correctly identified more frequently on the basis of instantaneous frequency than of potential distribution. A number of 8 averages seems to be sufficient for the correct identification of the stimulus condition. Consequently, for the identification of left and right hemifield stimulations the recording time could be reduced immensely. Instantaneous frequency is suggested as an additional and robust parameter for the selective averaging of artefact-free trials during the recording of hemifield PVEP.

Adult↗
PubMed1998

Nonlinear dynamics of 3 Hz spike-and-wave discharges recorded during typical absence seizures in children.

One-channel routine recordings of the scalp electroencephalogram (EEG) from unmedicated children strictly classified as unprovoked typical (3 c/s) absence seizures were selected. The dynamics of spike-and-wave discharges (SWD) were then examined by means of autocorrelation, correlation dimension, averaged pointwise dimension and largest Lyapunov exponent. For one EEG signal with pronounced spike-and-wave (SW) patterns, these measures were used complementary to a surrogate data method, a nonlinear (SETAR) modeling approach, and a SW simulation procedure providing five types of SW test signals. The SETAR model exhibited stationary SW dynamics, visually very similar to the EEG target signal, and with clear nonlinear structure. According to the results, the EEG episodes investigated represent low-dimensional dynamics, possibly recorded during nonstationary periods. Arguments that justify the assumption of deterministic chaos in our EEG signals were not obtained with the current methods. From the results one may conclude that two global oscillatory modes are present for the model, and three modes are active during the EEG recording period.

Algorithms↗
PubMed1998

Adaptive AR modeling of nonstationary time series by means of Kalman filtering.

An adaptive on-line procedure is presented for autoregressive (AR) modeling of nonstationary multivariate time series by means of Kalman filtering. The parameters of the estimated time-varying model can be used to calculate instantaneous measures of linear dependence. The usefulness of the procedures in the analysis of physiological signals is discussed in two examples: First, in the analysis of respiratory movement, heart rate fluctuation, and blood pressure, and second, in the analysis of multichannel electroencephalogram (EEG) signals. It was shown for the first time that in intact animals the transition from a normoxic to a hypoxic state requires tremendous short-term readjustment of the autonomic cardiac-respiratory control. An application with experimental EEG data supported observations that the development of coherences among cell assemblies of the brain is a basic element of associative learning or conditioning.

Adaptation, Physiological↗
PubMed1998

Multimodal time-variant signal analysis of neonatal EEG burst patterns.

It can be shown that dominant rhythmic signal components of neonatal EEG burst patterns (discontinuous EEG in quiet sleep) are characterized by a quadratic phase coupling (bispectral analysis), i.e. a multiplicative interaction (connection) between the underlying electrophysiological processes can be assumed. By means of pattern recognition algorithms as well as time-variant spectral and coherence analysis, a so-called "initial wave" (narrow band rhythm within a frequency range of 3-12 Hz) can be demonstrated within the first part of the burst pattern. The detection of this signal component and of the quadratic phase coupling is more successful in the frontal region. By means of amplitude demodulation of the "initial wave" the phase coupling can be attributed to an amplitude modulation. The results were derived from 6 neonates (20 burst patterns for each neonate; 8-channel recordings). A 16-channel EEG-recording was analyzed for one neonate.

Electroencephalography↗
PubMed1997

Analysis of the interrelations between a low-frequency and a high-frequency signal component in human neonatal EEG during quiet sleep.

It can be shown that dominant rhythmic signal components of neonatal EEG burst patterns (discontinuous EEG in quiet sleep) are characterised by a quadratic phase coupling (bispectral analysis). A so-called 'initial wave' (narrow band rhythm within a frequency range of 3-12 Hz) can be demonstrated within the first part of the burst pattern. The detection of this signal component and of the phase coupling is more successful in the frontal region. By means of amplitude demodulation of the 'initial wave' and a subsequent coherence analysis the phase coupling can be attributed to an amplitude modulation, i.e. the envelope curve of the 'initial wave' shows for a distinct period of time the same qualitative course as the signal trace of a 'lower' frequency component (0.75-3 Hz). The results were derived from six neonates (20 burst patterns for each neonate; 8 channel recordings).

Brain Mapping↗
PubMed1997

Simultaneous spike detection and topographic classification in pediatric surface EEGs.

In this study, an algorithm is introduced for the automatic detection and simultaneous topographic classification of interictal regional spike activity in pediatric surface EEG records. The algorithm is based on the classification of the topographic distribution of instantaneous power by means of a 'group' trained classifier. The results of automatic spike analysis were compared with the decisions of two experienced electroencephalographers. Four routine EEG records exhibiting (multi)regional spikes were examined. The mean selectivity for the automatic spike detector was 84.6% (mean sensitivity 88.1%, mean specificity 89.3%) and for the electroencephalographers 85.3%. All spikes detected by the algorithm were simultaneously classified according to their topographic characteristics. The results of automatic spike classification (lateralization/localization) corresponded to the results of visual analysis.

Algorithms↗
PubMed1997

[Relevance of the fascia transversalis in inguinal hernia repair using total extraperitoneal plastic reconstruction].

During total extraperitoneal dissection of inguinal hernias, an artificial preperitoneal cavity is created in the supravesical space by using a balloon dissector. During the operative procedure in vivo and while examining cadavers specially fixated according to the method developed by Thiel in 1992, we investigated the position relative to the balloon the transversalis fascia could be found in and what happened to it during inflation of the balloon. In all cases, the fascia medial to the inferior epigastric arteries was adjacent to the peritoneum and lateral to the abdominal wall. We observed systematic tearing of the fascia along the lateral umbilical plica (inferior epigastric artery). These results suggested that the transversalis fascia could be used only laterally for reinforcement of the dorsal wall of the inguinal canal and/or the support of a mesh used to cover the inguinal floor.

Fascia↗
PubMed1997

Quantitative analysis of discontinuous EEG in premature and full-term newborns during quiet sleep.

To assess the spatio-temporal structure of discontinuous EEG tracing in mature and immature newborns, we analysed mean spectral power in frequency bands between 0.8 and 16.8 Hz in 6 full-term newborns and 7 premature newborns < 32 weeks of conceptional age. The most striking results showed a significantly higher mean spectral power for the first half of bursts than for the second half recorded in > 2.8-14.8 Hz frequency bands. This pattern was more pronounced in premature than in full-term newborns. No clear differences were observed in comparisons between the first and the second half of the interburst periods. In addition, as far as mid and high frequency band spectra were considered, the mean spectral power of burst was, in both groups, higher in the right as compared to the left occipital regions.

Brain↗
PubMed1997

Is elastic energy storage of quantitative relevance for the functional morphology of the human locomotor apparatus?

Elastic storage of energy in the vertebrate locomotor apparatus is supposed to be an important functional factor in cyclic and acyclic movements. In terms of physics, for humans a proof for the occurrence and quantitative relevance of this phenomenon in vivo and under physiological conditions has been missing until now. In addition to the large amount of plausible, but inconclusive information about elasticity in humans and animals, we describe a simple experiment to prove the existence of quantitatively relevant elastic energy storage in the human locomotor apparatus. Ten volunteers (5 female, 5 male) each assumed a relaxed, upright posture on a steel platform. After the release of a support, the volunteers and the platform fell for a defined distance of 33 mm. Loaded with the volunteers, the platform fell significantly (p < 0.001) faster than predicted by the laws of stiff body mechanics (50 vs. 82 ms). For a minimum time of 50 ms, the human locomotor apparatus is able to support an average external power output of more than 400 W by means of an energy transfer of more than 20 J. During the fall, no EMG activities of the ankle flexors could be recorded. We conclude that the acceleration of the platform fall is induced by elastic elements serving as energy sources. Elastic energy storage is of quantitative relevance for the functional morphology and biomechanics of the human locomotor apparatus.

Adult↗
PubMed1997

A calculation of the forces acting on the human acetabulum during walking. Based On in vivo force measurements, kinematic analysis and morphometry.

Information about the loading of the human acetabulum during walking is necessary for a functional understanding of the morphology of the pelvic girdle and the hip joint as well as for the optimization of endoprosthetic therapy in osteoarthritis. For this purpose, experimental data of the forces acting on the femur in walking taken from the literature [Bergmann et al.: J. Biomech. 1993;26: 969-990] were combined with our own kinematic and morphometric data, to transform the force vectors from the femoral into a pelvic and an acetabular frame. During the walking cycle, the resultant force vector takes a rather constant course relative to the pelvis and its orientation seems to be highly regulated to act within a small range of angles. Only small deviations occur from the angles against the vertical which the resultant peak force forms in the frontal plane (F = 11 degrees, medially orientated) and in the sagittal plane (S = 5 degrees, ventrally orientated). The experimental results form the basis for a model of the incongruous hip joint as an elastic joint, the femoral head being centered between compliant elements.

Acetabulum↗
PubMed1997

[An experimental study on the use of the Hilbert operator as a method of signal preprocessing for the determination of oxygen saturation in retinal vessels].

For the non-invasive measurement of the oxygen saturation in human retinal vessels, the light reflected by a vessel and its surroundings is evaluated. Differences in the absorption and scattering properties of the optical media provide so-called vessel profiles, but the central vessel section is often disturbed by a regular reflex. In order to eliminate this reflex, a method based on the Hilbert transform is presented, which can be used for the determination of logarithmic differences between the reflected light on and that beside the vessel. The data for our investigations were produced by simulation of the radiation transport in multi-layered tissue. A linear regression between expected and measured values based on 40 pairs was used for the evaluation of the proposed method. A linear relationship was shown to exist.

Anisotropy↗
PubMed1997

[Is indirect posturography using force plates equivalent to direct posturography by movement analysis? A physical observation].

Piezoelectric force plates are accurate and convenient aids for the monitoring of coordination in erect humans. However, the results obtained with such "indirect" posturographic measurements should not be "over-interpreted". The information on the spatial location of the centre of pressure (CoP) provided by the force plate permits conclusions to be drawn about the regulation of the "sway" of the centre of gravity (CoG) only when other information on the acceleration situation of the body is available. Otherwise the systematic error of the measurement exceeds the amount of the true "sway" even for physiological cyclic movement frequencies of 0.5 Hz or more.

Biomechanical Phenomena↗