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Biomedical subjects

B Schack

Publications and source records attributed to B Schack.

27 records · Page 2Linked to original sources
PubMed1995

Dynamic power and coherence analysis of ultra short-term cognitive processes--a methodical study.

This EEG mapping study was designed to explore ultra short-term cognitive processes in human thinking. The EEG was recorded while the subjects performed two tasks randomly mixed in time: category concept activation and pattern comparison. A novel approach is introduced to examine these tasks by means of band powers and coherence with high time and frequency resolution. This approach is based on general adaptive principles and the adaptive fit of a bivariate linear model (ARMA model) with time varying parameters, and allows the estimation of band powers and coherences continuously in time. The application of mapping to these spectral parameter functions results in map sequences of band powers, and local band coherences. It was proved that these map sequences of the frequency band 13-20Hz reflect the dynamic behaviour of information processing in a characteristic topographical manner. Based on these dynamic topographical examinations, conceptual and imaginal representations are distinguishable.

Brain Mapping↗
PubMed1995

Methods of dynamic spectral analysis by self-exciting autoregressive moving average models and their application to analysing biosignals.

Dynamic methods in the spectral domain are necessary to analyse biological signals because of the frequently nonstationary character of the signals. The paper presents an adaptive procedure of fitting time-dependent ARMA models to nonstationary signals, which is suitable for on-line calculations. The properties of the model parameter estimations are examined, and in the stationary case are compared with the results of convergent estimation methods. On this basis time-varying spectral parameters with high temporal and spectral resolution are calculated, and the possibility of their application is shown in EEG analysis and laser-Doppler-flowmetry.

Electroencephalography↗
PubMed1995

A new approach for detecting the position of retinal vessels and calculating their diameter on the basis of dynamic spectral analysis.

Fast adaptive algorithms were used for the detection of retinal vessels and for pre-processing the signals. By using different methods of dynamic spectral analysis, the local frequency of the vessels was determined. On this basis a method of calculating the diameters of the vessels was constructed; it is robust in relation to disturbances in the domain of the edges.

Algorithms↗
PubMed1995

[Adaptive procedures for measuring arterial blood flow velocity in retinal vessels using indicator technique].

There are highly significant differences in the measuring results of arterial blood velocity between the indicator and laser-Doppler techniques (up to 800%). A new measuring procedure for the analysis of indicator dilution curves was developed based on indicator model and experimental results. The use of this new measuring procedure results in reduced mean systematic error between the indicator and laser-Doppler techniques to values around 10%. With the introduction of adaptive measuring arrays for the creation of indicator dilution curves and the application of adaptive algorithms for centering and spectral normalizing of the dilution curves, improved reproducibility can be expected.

Blood Flow Velocity↗
PubMed1994

Dynamic description of stochastic signal by adaptive momentary power and momentary frequency estimation and its application in analysis of biological signals.

The importance of dynamic spectral analysis of time-varying signals in medicine, biology and technology is increasing rapidly. The basic spectral parameters are momentary power and momentary frequency. The paper presents adaptive recursive estimation methods for these spectral parameters. Their specific properties are investigated, and the possibilities of applications in computer-assisted analysis of biological and technical signals are demonstrated, even satisfying real-time requirements.

Algorithms↗
PubMed1990

[Adaptative procedures for pre-processing of evoked potentials].

The investigation of evoked potentials requires suitable consideration of physiological and pathophysiological characteristics of spontaneous and evoked electrical activity of the brain. For this purpose a preprocessing strategy based on adaptive recursive estimation of statistical parameters was developed. In this way, artifact handling, classification, filtering and further preprocessing of spontaneous EEG and evoked potentials can be improved.

Algorithms↗
PubMedDate not supplied

Quantification of transient quadratic phase couplings within EEG burst patterns in sedated patients during electroencephalic burst-suppression period.

The time dynamics of the quadratic phase coupling within burst patterns during electroencephalic burst-suppression has been quantified. It can be shown that a transient quadratic phase coupling (QPC) exists between the frequency ranges 0 to 2.5 and 3 to 7.5 Hz and between the frequency ranges 0 to 2.5 and 8 to 12 Hz. The QPC can be explained by an amplitude modulation, where the slow rhythm modulates the rhythmic activities with a higher frequency. By means of time-variant bicoherence analysis, a strong phase-locking between the modulating and the modulated component can be identified. The phase-locking is demonstrable within the first 250 ms after the burst onset and comes up to the maximum between 750 and 1250 ms. The effect is maintained over the whole first part of the burst (2 s) with a decreasing tendency after 1250 ms. All these effects cannot be found in the EEG before entering the burst suppression period (BSP). The transient coupling phenomena in the EEG bursts during BSP can be regarded as indicators for short-term interrelations between the underlying electrophysiologic processes.

Brain↗
PubMedDate not supplied

Filtering properties of an adaptive trend operator and the application for determining the arterial blood velocity in retinal vessels.

This study deals with methods focused on estimating blood velocity. The estimation of the linear trend function of a non-stationary signal based on the adaptive recursive estimation of the mean value function is used for the determination of the time delay of two indicator dilution curves. The filter property of this trend operator depends on the choice of a constant parameter c, the so-called adaptation factor. The functional connection between the filter property and the adaptation factor is considered in such a way that an objective calculation of arterial blood velocity in retinal vessels is possible.

Algorithms↗