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PubMed1996

Scaling behaviour of heartbeat intervals obtained by wavelet-based time-series analysis.

Biological time-series analysis is used to identify hidden dynamical patterns which could yield important insights into underlying physiological mechanisms. Such analysis is complicated by the fact that biological signals are typically both highly irregular and non-stationary, that is, their statistical character changes slowly or intermittently as a result of variations in background influences. Previous statistical analyses of heartbeat dynamics have identified long-range correlations and power-law scaling in the normal heartbeat, but not the phase interactions between the different frequency components of the signal. Here we introduce a new approach, based on the wavelet transform and an analytic signal approach, which can characterize non-stationary behaviour and elucidate such phase interactions. We find that, when suitably rescaled, the distributions of the variations in the beat-to-beat intervals for all healthy subjects are described by a single function stable over a wide range of timescales. However, a similar scaling function does not exist for a group with cardiopulmonary instability caused by sleep apnoea. We attribute the functional form of the scaling observed in the healthy subjects to underlying nonlinear dynamics, which seem to be essential to normal heart function. The approach introduced here should be useful in the analysis of other nonstationary biological signals.

Adult↗
PubMed2003

Analysis of frequency components of cortical potentials evoked by progressive misalignment of Kanizsa squares.

Cortical gamma oscillations (20-100 Hz) are thought to play an important role in encoding visual perception. If so they should emerge at about threshold. In the present investigation we examined the latter proposal. Visual responses were recorded in occipital, temporal and parietal areas (stimulus duration 512 ms). Oscillation strength and frequency were derived from FFT analysis and wavelet transforms. The specific goals of the present study are: 1: To examine the parallel between gamma oscillations and the psychometric threshold of perception of Kanizsa square (KS). The latter is gradually altered by a progressive misalignment of lower inducers (pacmen). Results show that the perception of the KS is altered by lateral displacements of the lower inducers as small as 0.1 to 0.2 degrees. In parallel, high frequency components of cortical responses gain in strength with misalignments. 2: Gamma oscillations emerge at or about the psychometric threshold. In addition, our data analysis demonstrates that gamma oscillations appear in short bursts (approx. 50 ms) in the time window between 200 and 500 ms after stimulus onset. Furthermore, controls indicated that these oscillations are of the induced-gamma type. Thus, our experiments suggest that gamma oscillations are associated with image structures and may be induced by local properties of the target.

Adult↗
PubMed2003

Task-dependent intermanual coupling of 8-Hz discontinuities during slow finger movements.

During slow finger movements, small discontinuities are visible at approximately 8-10 Hz. We have recorded from eight normal subjects whilst they performed index finger flexion-extension movements with the left and right hand. Movements were either performed in-phase (both fingers flexing or extending together), or anti-phase (flexion on one side coinciding with extension on the other). Coherence calculated between left and right finger velocity was significantly above zero at approximately 8 Hz for the in-phase condition, but was significantly smaller for anti-phase movements (mean coherence across all subjects at 8.7 Hz was 0.031 for in-phase, 0.010 for anti-phase). We also calculated a 'phase coherence' measure which, unlike conventional coherence analysis, was sensitive only to phase synchronization and not to amplitude co-variations. For the in-phase task, phase coherence values were smaller than coherence, but still significantly different from zero around 8 Hz; for the anti-phase task, phase coherence was not significant in this band. Measures calculated from EMG recordings yielded similar conclusions to those using finger velocity, indicating that the results were not simply due to mechanical cross-talk. Neural oscillators generating approximately 8-Hz movement discontinuities on each side of the body are therefore selectively coupled during the in-phase task. A wavelet-based analysis further suggested that intermanual coupling modulated during in-phase task performance; coupling was maximal at the start and the end of a movement. We conclude that the systems producing approximately 8-Hz movement discontinuities and those responsible for intermanual coupling are likely to share common neural elements.

Electromyography↗
PubMedDate not supplied

Concept for an intelligent anaesthesia EEG monitor.

Considering the fundamental difficulties to define the term 'depth of anaesthesia', a more feasible concept for assessment of 'adequacy of anaesthesia' will be explained. The basic requirements for a monitoring index are definite response, gradual scaling and independence from the anaesthetic technique used. Additionally the index should be predictive for appearance of clinical signs of an inadequate anaesthesia. Different signal-processing methods will be discussed to extract the relevant information from both the spontaneous and the evoked brain electrical activity. In this context well established methods like spectral analysis are investigated in combination with new and more sophisticated methods like bispectral analysis or wavelet decomposition. Since no single-parameter index has been defined for monitoring depth of anaesthesia, a set of EEG parameters may be more useful to take into account intra- and interindividual variability. In parallel to the description of the monitor concept, the investigation of neural nets and fuzzy techniques, in addition to or in substitution of conventional statistical methods, will be introduced. Examples are given for data quality assessment, parameter extraction and re-classification.

Algorithms↗
PubMed2005

Asymmetry of evoked potential latency to speech sounds predicts the ear advantage in dichotic listening.

The functional organization of the human auditory cortex is still not well understood with respect to speech perception and language lateralization. Especially, there is comparatively little data available in the brain imaging literature focusing on the timing of phonetic processing. We recorded auditory-evoked potentials (AEP) from 27 scalp and additional EOG channels in 12 healthy volunteers performing a free report dichotic listening task with simple speech sounds (CV syllables: [ba], [da], [ga], [pa], [ta], [ka]). ERP analysis employed independent components analysis (ICA) wavelet denoising for artifact reduction and improvement of the SNR. The main finding was a 15-ms shorter average latency of the N1-AEP recorded from the scalp approximately overlying the left supratemporal cortical plane compared to the N1-AEP over the homologous right side. Corresponding N1 amplitudes did not differ between these sites. The individual AEP latency differences significantly correlated with the ear advantage as an index of speech/language lateralization. The behaviorally relevant difference in N1 latency between the hemispheres indicates that an important key to understanding speech perception is to consider the functional implications of neuronal event timing.

Acoustic Stimulation↗
PubMed2004

Wavelet packet transform for R-R interval variability.

INTRODUCTION: Wavelet transform is used for time-frequency analysis. Recently, discrete wavelet transform (DWT) has been used to analyze R-R interval or heart rate variability. However, we hypothesized that wavelet packet transform (WPT) is a better way to analyze such variability. In the present study, we compared resolution of frequency band and amplitude, which are used for analysis of the variability, with DWT and WPT, followed by Hilbert transform. METHODS: A chirp signal which covers all frequency bands used for R-R interval variability was employed as a simulated signal. Levels 1-6 of DWT and level 3 of WPT were used for signal analysis. Amplitudes of the gained signal were evaluated with Hilbert transform. Differences in error of the gained amplitude from expected amplitude between CWT and DWT for low-frequency (LF) and high-frequency (HF) components were compared. To evaluate time-dependent changes in R-R interval variability, head-up tilt (HUT) was employed as an orthostatic challenge. RESULTS: Errors for both HF and LF, derived from the simulated signal with WPT, were significantly smaller than those of DWT. With HUT, time dependent changes in LF, HF, and LF/HF were observed. DISCUSSION: Although DWT is a valuable method for time-frequency analysis, WPT is a more appropriate method to utilize wavelet transform due to the equivalent resolution of the gained frequency band. WPT for time-frequency analysis improves analysis of time-dependent changes in R-R interval variability.

Adult↗
PubMed1999

Wavelet packet denoising of magnetic resonance images: importance of Rician noise at low SNR.

Wavelet packet analysis is a mathematical transformation that can be used to post-process images, for example, to remove image noise ("denoising"). At a very low signal-to-noise ratio (SNR <5), standard magnitude magnetic resonance images have skewed Rician noise statistics that degrade denoising performance. Since the quadrature images have approximately Gaussian noise, it was postulated that denoising would produce better contrast and sharper edges if performed before magnitude image formation. Signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and edge blurring effects of these two approaches were examined in synthetic, phantom, and human MR images. While magnitude and complex denoising both significantly improved SNR and CNR, complex denoising yielded sharper edges and better low-intensity feature contrast.

Artifacts↗
PubMed2005

Phase/amplitude reset and theta-gamma interaction in the human medial temporal lobe during a continuous word recognition memory task.

We analyzed intracranial electroencephalographic (EEG) recordings from the medial temporal lobes of 12 epilepsy patients during a continuous word recognition paradigm, contrasting trials of correctly recognized repeated words (hits) and correctly identified new words (correct rejections). Using a wavelet-based analysis, we investigated how power changes and phase clustering in different frequency bands contribute to the averaged event-related potentials (ERPs). In addition, we analyzed the actual mean phases of the different oscillations. Our analyses yielded the following results: (1) power changes contributed significantly only to the late components of the ERPs (>400 ms) (2) earlier ERP components were produced by a stimulus-related broad-band phase and amplitude reset of ongoing oscillatory activity about 190 ms after stimulus onset that involved not only the theta band, but also covered alpha and lower beta band frequencies (3) phase and amplitude reset occurred during an epoch of increased phase entrainment over time that lasted for about two oscillation periods for all involved frequencies and was more pronounced for correct rejections than for hits. The broad-band phase and amplitude reset was observed for both hits and correct rejections, and therefore, did not appear to support a specific cognitive function, but rather to act as a general facilitating factor for the processes involved in this memory task. Further analyses of synchronization between oscillations and power changes in different frequency bands revealed a task-dependent modulation of gamma activity by the entrained theta cycle, a mechanism potentially related to memory encoding and retrieval in the rhinal cortex and hippocampus, respectively.

Adult↗
PubMed1999

Synchronized cortical potentials and wavelet packets: a potential mechanism for perceptual binding and conveying information.

Temporal synchronization in neuronal assemblies has been linked to the functional roles of perceptual binding, sensory-motor integration, attention, and information coding. We report new evidence for a common underlying mechanism that uses specific temporal patterns of synchronized neuronal activity as a basis for conveying information. The temporal patterns of stimulus-related synchronized neuronal discharges are structured to closely resemble specific members of the Symlet wavelet packet family employed in a computational framework. Together, these results suggest that temporal patterns of synchronized activity may act as a parallel, distributed code for information through a mechanism computationally equivalent to wavelet packet analysis.

Animals↗
PubMed1999

Oscillations in the human cutaneous blood perfusion signal modified by endothelium-dependent and endothelium-independent vasodilators.

The purpose of the present study was to compare the effects of endothelium-dependent [acetylcholine (ACh)] and endothelium-independent [sodium nitroprusside (SNP)] vasodilators on the oscillatory components of the cutaneous blood perfusion signals in humans. The unstimulated basal blood perfusion and the blood perfusion during iontophoretically delivered ACh and SNP were measured using laser Doppler flowmetry (LDF). The wavelet transform was calculated before spectral analysis of the measured signals. In the frequency interval from 0.0095 to 1.6 Hz the LDF signal consists of oscillations with five different characteristic frequencies. In addition to the cardiac (1 Hz) and respiratory (0.3 Hz) rhythms, three other oscillations in the regions around 0.1, 0.04, and 0.01 Hz were detected. The oscillations with the different frequencies were observed in unstimulated blood flow and also during stimulation with ACh and SNP. Compared to the unstimulated blood flow, both ACh and SNP increased the mean amplitude of the total spectrum (P < 0. 005 for both substances). The only significant difference between the effects of ACh and SNP was observed in the amplitude of oscillations with the frequency of around 0.01 Hz. ACh increased the absolute amplitude of this frequency to a greater extent than SNP in athletes (P = 0.03), whereas only a trend was observed in controls (P = 0.2). The relative amplitude, defined as the ratio between the absolute amplitude of a particular frequency interval and the mean amplitude of the total spectrum, was also higher for ACh compared to SNP both in controls (P = 0.008) and in athletes (P = 0.004), only for oscillations with the frequency of around 0.01 Hz. We conclude that ACh selectively influences the oscillatory component of around 0.01 Hz in the cutaneous blood perfusion signal to a greater extent than SNP. This finding indicates that endothelium-mediated vasodilatation is manifested as oscillations with a repetition time of approximately 1 min. The mechanisms for the endothelial dependency of this frequency remain to be elucidated. Our data indicate that spectral analysis based on wavelet transform of the cutaneous perfusion signal can be used clinically to investigate endothelial function. The described noninvasive method might be used to evaluate endothelial function for research, for diagnostic purposes, and maybe also to assess effects of therapy in cardiovascular diseases.

Acetylcholine↗
PubMed2006

Bilateral oscillations for lateralized spikes in benign rolandic epilepsy.

PURPOSES: To elucidate the oscillatory dynamics with respect to interictal spike occurrence in benign rolandic epilepsy (BRE). METHODS: Using a whole-scalp magnetoencephalography (MEG), we recorded scalp EEG and MEG signals in 10 BRE patients (age 8-12 years) and visually identified unilateral interictal spikes that were simultaneously present on both EEG and MEG channels. We obtained the peak timing of individual spike complex based on MEG single-dipole modeling, and then applied wavelet transform to analyze the time-frequency components of corresponding MEG signals with respect to spike occurrence. RESULTS: In the hemisphere with time-domain spike waveforms, we identified a clear increase of 0.5-40 Hz activity around the spike peak, most prominent at alpha band (8-13 Hz). Notably, at the approximate timing we also observed an increase in 0.5-25 Hz oscillations over the homotopic area in the other hemisphere where no spike signals were found. CONCLUSIONS: Our results indicate bilateral increases in 0.5-25 Hz oscillations during unilateral spike formation in BRE patients. By using wavelet transform analysis, one could be able to detect some irritative feature that would in visual analysis remain undetected.

Child↗
PubMed1999

Statistical physics and physiology: monofractal and multifractal approaches.

Even under healthy, basal conditions, physiologic systems show erratic fluctuations resembling those found in dynamical systems driven away from a single equilibrium state. Do such "nonequilibrium" fluctuations simply reflect the fact that physiologic systems are being constantly perturbed by external and intrinsic noise? Or, do these fluctuations actually, contain useful, "hidden" information about the underlying nonequilibrium control mechanisms? We report some recent attempts to understand the dynamics of complex physiologic fluctuations by adapting and extending concepts and methods developed very recently in statistical physics. Specifically, we focus on interbeat interval variability as an important quantity to help elucidate possibly non-homeostatic physiologic variability because (i) the heart rate is under direct neuroautonomic control, (ii) interbeat interval variability is readily measured by noninvasive means, and (iii) analysis of these heart rate dynamics may provide important practical diagnostic and prognostic information not obtainable with current approaches. The analytic tools we discuss may be used on a wider range of physiologic signals. We first review recent progress using two analysis methods--detrended fluctuation analysis and wavelets--sufficient for quantifying monofractual structures. We then describe recent work that quantifies multifractal features of interbeat interval series, and the discovery that the multifractal structure of healthy subjects is different than that of diseased subjects.

Biophysical Phenomena↗
PubMedDate not supplied

A wavelet, fourier, and PCA data analysis pipeline: application to distinguishing mixtures of liquids.

Using a new optical engineering technique for the "fingerprinting" of beverages and other liquids, we study and evaluate a range of features. The features are based on resolution scale, invariant frequency information, entropy, and energy. They allow mixtures of beverages to be very precisely placed in principal component plots used for the data analysis. To show this we make use of data sets resulting from optical/near-infrared and ultrasound sensors. Our liquid "fingerprinting" is a relatively open analysis framework in order to cater for different practical applications, in particular, on one hand, discrimination and best fit between fingerprints, and, on the other hand, more exploratory and open-ended data mining.

Journal Article↗
PubMed2001

Travelling waves and spatial hierarchies in measles epidemics.

Spatio-temporal travelling waves are striking manifestations of predator-prey and host-parasite dynamics. However, few systems are well enough documented both to detect repeated waves and to explain their interaction with spatio-temporal variations in population structure and demography. Here, we demonstrate recurrent epidemic travelling waves in an exhaustive spatio-temporal data set for measles in England and Wales. We use wavelet phase analysis, which allows for dynamical non-stationarity--a complication in interpreting spatio-temporal patterns in these and many other ecological time series. In the pre-vaccination era, conspicuous hierarchical waves of infection moved regionally from large cities to small towns; the introduction of measles vaccination restricted but did not eliminate this hierarchical contagion. A mechanistic stochastic model suggests a dynamical explanation for the waves-spread via infective 'sparks' from large 'core' cities to smaller 'satellite' towns. Thus, the spatial hierarchy of host population structure is a prerequisite for these infection waves.

Algorithms↗
PubMed2006

Wavelet-based feature extraction applied to small-angle x-ray scattering patterns from breast tissue: a tool for differentiating between tissue types.

This paper reports on the application of wavelet decomposition to small-angle x-ray scattering (SAXS) patterns from human breast tissue produced by a synchrotron source. The pixel intensities of SAXS patterns of normal, benign and malignant tissue types were transformed into wavelet coefficients. Statistical analysis found significant differences between the wavelet coefficients describing the patterns produced by different tissue types. These differences were then correlated with position in the image and have been linked to the supra-molecular structural changes that occur in breast tissue in the presence of disease. Specifically, results indicate that there are significant differences between healthy and diseased tissues in the wavelet coefficients that describe the peaks produced by the axial d-spacing of collagen. These differences suggest that a useful classification tool could be based upon the spectral information within the axial peaks.

Algorithms↗
PubMed2002

Instantaneous frequency and amplitude identification using wavelets: application to glass structure.

This paper describes a method for extracting rapidly varying, superimposed amplitude-modulated and frequency-modulated signal components. The method is based upon the continuous wavelet transform (CWT) and uses a new wavelet that is a modification to the well-known Morlet wavelet to allow analysis at high resolution. In order to interpret the CWT of a signal correctly, an approximate analytic expression for the CWT of an oscillatory signal is examined via a stationary-phase approximation. This analysis is specialized for the new wavelet and the results are used to construct expressions for the amplitude and frequency modulations of the components in a signal from the transform of the signal. The method is tested on a representative, variable-frequency signal as an example before being applied to a function of interest in our subject area-a structural correlation function of a disordered material-which immediately reveals previously undetected features.

Journal Article↗
PubMed2004

Stochastic perturbations in vortex-tube dynamics.

A dual lattice vortex formulation of homogeneous turbulence is developed, within the Martin-Siggia-Rose field theoretical approach. It consists of a generalization of the usual dipole version of the Navier-Stokes equations, known to hold in the limit of vanishing external forcing. We investigate, as a straightforward application of our formalism, the dynamics of closed vortex tubes, randomly stirred at large length scales by Gaussian stochastic forces. We find that besides the usual self-induced propagation, the vortex tube evolution may be effectively modeled through the introduction of an additional white-noise correlated velocity field background. The resulting phenomenological picture is closely related to observations previously reported from a wavelet decomposition analysis of turbulent flow configurations.

Journal Article↗