Search PubMed⌕ Search

Biomedical subjects

H Witte

Publications and source records attributed to H Witte.

At least 55 records · Page 3Linked to original sources
PubMed1999

Identification of hemifield single trial PVEP on the basis of generalized dynamic neural network classifiers.

This paper is concerned with the application of generalized dynamic neural networks for the identification of hemifield pattern-reversal visual evoked potentials. The identification process is performed by different networks with time-varying weights using signals from different electrode positions as external inputs. Since dynamic neural networks are able to process time-varying signals, the identification of the stimulated hemiretinae is performed without feature extraction. The performance of the method presented is compared with a reference method based on the values of instantaneous frequency at the occipital electrode positions at P100 latency.

Adult↗
PubMed1999

New approaches for the detection and analysis of electroencephalographic burst-suppression patterns in patients under sedation.

An automatic EEG pattern detection unit was developed and tested for the recognition of burst-suppression periods and for the separation of burst from suppression patterns. The median, standard deviation and the 95% edge frequency were computed from single channels of the EEG within a moving window and completed by the continuous computation of frequency band power via an adapted Hilbert resonance filter. These parameters were given to the inputs of two hierarchically arranged artificial neural networks (NNs). The output signals of NNs indicate the suppression and burst phases. The burst recognition was focused on the precise recognition of the burst onset. In subsequent processing steps the time course of percentages of burst patterns within their corresponding burst-suppression-phases was calculated and the time locations of burst onsets can be used to trigger an averaging for a burst-related analysis. The data for our investigations were derived from the routine EEG derivations of 12 patients with various neurosurgical diseases. A group-related training of the NNs was realized. For the group-related trained NNs EEG data for 6 patients were used for training and the data of 6 other patients for testing the classification performance of the pattern recognition units. Additionally, the reliability of the detection algorithm was tested with data of two patients with convulsive state, resistant to treatment, and burst-suppression like pattern EEC.

Adolescent↗
PubMed1999

[Detection of quadratic phase coupling between EEG signal components by nonparamatric and parametric methods of bispectral analysis].

Recently the assumption of the independence of individual frequency components in a signal has been rejected, for example, for the EEG during defined physiological states such as sleep or sedation [9, 10]. Thus, the use of higher-order spectral analysis capable of detecting interrelations between individual signal components has proved useful. The aim of the present study was to investigate the quality of various non-parametric and parametric estimation algorithms using simulated as well as true physiological data. We employed standard algorithms available for the MATLAB. The results clearly show that parametric bispectral estimation is superior to non-parametric estimation in terms of the quality of peak localisation and the discrimination from other peaks.

Algorithms↗
PubMed1999

[Effect of common head gear on horses].

The functions of the most common head-gears for horses are analysed from a biomechanical point of view. With the exception of the stable halter are all of them designed to enlarge the tensile forces transmitted through the reins or the longe, and to concentrate the enlarged forces on sensitive parts of the horse's head: the nose, or the lips, mandible and tongue. Since the direction, duration and size of these tensile forces are the essential factors to modulate signals for controlling the horse, a device has been developed to measure, or at least roughly quantify these forces. The mechanical characteristics of bosal, caveçon, serreta, kappzaum and hackamore are demonstrated and compared with those of the two major types of bits: those with and without levers.

Animals↗
PubMed1999

Time-variant spectral analysis of LDF signals on the basis of multivariate autoregressive modelling.

Time-variant AR-modelling was used to analyse the non-stationary properties of LDF signals during provocation tests. The procedure for the estimation of time-varying AR parameters based on Kalman filtering is presented. The estimates can be used to calculate instantaneous measures, such as peak frequency, spectral band power and coherence. The resulting course of instantaneous peak frequency of a sinusoidal signal with frequency jump was compared to similar parameters derived from short-time FFT and Hilbert transformation. Univariate time-variant spectral analysis was used to investigate LDF measurements in patients with Raynaud's phenomena. The experimental protocol was splitted into phases of different room temperature. The results demonstrate time-dependent variations of spectral components (amplitude and frequency). By means of time-variant coherence analysis of LDF and diameter measurements of vessels in a hamster skin fold the existence of a main rhythm around 0.1 Hz in the LDF signals which is related to vasomotion is shown.

Algorithms↗
PubMed1999

Adaptable preprocessing units and neural classification for the segmentation of EEG signals.

In this contribution, a methodology for the simultaneous adaptation of preprocessing units (PPUs) for feature extraction and of neural classifiers that can be used for time series classification is presented. The approach is based upon an extension of the backpropagation algorithm for the correction of the preprocessing parameters. In comparison with purely neural systems, the reduced input dimensionality improves the generalization capability and reduces the numerical effort. In comparison with PPUs with fixed parameters, the success of the adaptation is less sensitive to the choice of the parameters. The efficiency of the developed method is demonstrated via the use of quadratic filters with adaptable transmission bands as preprocessing units for the segmentation of two different types of discontinuous EEG: discontinuous neonatal EEG (burst-interburst segmentation) and EEG in deep stages of sedation (burst-suppression segmentation).

Coma↗
PubMed1999

[Effect of knee joint flexion and femur rotation on retropatellar contact of the human knee joint].

The aim of the present study was to evaluate retropatellar contact characteristics at different angles of flexion of the knee joint. To this end, 6 cadaveric legs were examined using pressure sensitive film (Fuji Prescale type "super low") at angles of flexion of 45 degrees, 60 degrees, 90 degrees and 120 degrees both in neutral rotation and 10 degrees internal and external rotation of the femur in the same knee joints. A force of 140 N was applied to both the vastus medialis and lateralis, and a comparison made with a medially and a laterally dominating muscle force. The contact areas decreased with increasing angles of flexion. The medially dominating muscle traction increased the contact area. Comparison between internal and external rotation revealed a decrease in contact area on internal rotation. The pressure measurements were comparable in all loading situations. Comparison between neutral and medial traction revealed significant differences in contact area, pressure and force. The influence of femoral rotation showed no significant difference. A comparison of the different angles of flexion revealed only few significant differences. To prevent the development of retropatellar arthrosis, maximum contact areas are necessary. The study has shown an advantage for medially dominating muscle traction, and external rotation of the femur.

Aged↗
PubMed1998

[Biomechanics of injury of the cervical spine].

Whiplash injuries of the cervical spine are of special medical and socio-economic importance. Biomechanical studies of the injury have proven, that a hypertranslation of the capito-cervical region takes place first, leading to ligamentous hyperdistension and ruptures, when exceeding elastic deformation, thus possibly resulting in mechanical disturbance and rotatory malpositioning. Diagnosis of so called "functional disorders" bears difficulties concerning objective tests for structural lesions. This demands the definition of diagnostic parameters and clinical signs, which also consider vegetative phenomena. This paper aims at a systematic clinical and radiological check. Adequate therapeutic means have to be concordant to prognosis. Therefore functional parameters should already determine differential diagnosis. This is based on technical feasibilities and needs future scientific efforts.

Biomechanical Phenomena↗
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↗