Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Spectral estimation”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 883 records · Page 49Linked to original sources
PubMed2004

Spectral spatiotemporal imaging of cortical oscillations and interactions in the human brain.

This paper presents a computationally efficient source estimation algorithm that localizes cortical oscillations and their phase relationships. The proposed method employs wavelet-transformed magnetoencephalography (MEG) data and uses anatomical MRI to constrain the current locations to the cortical mantle. In addition, the locations of the sources can be further confined with the help of functional MRI (fMRI) data. As a result, we obtain spatiotemporal maps of spectral power and phase relationships. As an example, we show how the phase locking value (PLV), that is, the trial-by-trial phase relationship between the stimulus and response, can be imaged on the cortex. We apply the method to spontaneous, evoked, and driven cortical oscillations measured with MEG. We test the method of combining MEG, structural MRI, and fMRI using simulated cortical oscillations along Heschl's gyrus (HG). We also analyze sustained auditory gamma-band neuromagnetic fields from MEG and fMRI measurements. Our results show that combining the MEG recording with fMRI improves source localization for the non-noise-normalized wavelet power. In contrast, noise-normalized spectral power or PLV localization may not benefit from the fMRI constraint. We show that if the thresholds are not properly chosen, noise-normalized spectral power or PLV estimates may contain false (phantom) sources, independent of the inclusion of the fMRI prior information. The proposed algorithm can be used for evoked MEG/EEG and block-designed or event-related fMRI paradigms, or for spontaneous MEG data sets. Spectral spatiotemporal imaging of cortical oscillations and interactions in the human brain can provide further understanding of large-scale neural activity and communication between different brain regions.

Acoustic Stimulation↗
PubMed2003

VO2, VCO2, and RQ in a respiratory chamber: accurate estimation based on a new mathematical model using the Kalman-Bucy method.

A respiratory chamber is used for monitoring O(2) consumption (Vo(2)), CO(2) production (Vco(2)), and respiratory quotient (RQ) in humans, enabling long term (24-h) observation under free-living conditions. Computation of Vo(2) and Vco(2) is currently done by inversion of a mass balance equation, with no consideration of measurement errors and other uncertainties. To improve the accuracy of the results, a new mathematical model is suggested in the present study explicitly accounting for the presence of such uncertainties and error sources and enabling the use of optimal filtering methods. Experiments have been realized, injecting known gas quantities and estimating them using the proposed mathematical model and the Kalman-Bucy (KB) estimation method. The estimates obtained reproduce the known production rates much better than standard methods; in particular, the mean error when fitting the known production rates is 15.6 +/- 0.9 vs. 186 +/- 36 ml/min obtained using a conventional method. Experiments with 11 humans were carried out as well, where Vo(2) and Vco(2) were estimated. The variance of the estimation errors, produced by the KB method, appears relatively small and rapidly convergent. Spectral analysis is performed to assess the residual noise content in the estimates, revealing large improvement: 2.9 +/- 0.8 vs. 3,440 +/- 824 (ml/min)(2) and 1.8 +/- 0.5 vs. 2,057 +/- 532 (ml/min)(2), respectively, for Vo(2) and Vco(2) estimates. Consequently, the accuracy of the computed RQ is also highly improved (0.3 x 10(-4) vs. 800 x 10(-4)). The presented study demonstrates the validity of the proposed model and the improvement in the results when using a KB estimation method to resolve it.

Carbon Dioxide↗
PubMed2001

Spectral analysis of heart rate variability using the integral pulse frequency modulation model.

Spectral analysis of heart rate variability (HRV) is a widely accepted approach for assessment of cardiac autonomic function and its relationship to numerous disorders and diseases. As a rule, the non-parametric methods for HRV spectral analysis are tested using the integral pulse frequency modulation (IPFM) model. However, published results with simulated HRV signals show differences requiring further development of the existing methods. With the aim of improving estimation accuracy, an entirely IPFM-based method for HRV analysis is investigated. According to this method, the spectra are computed by finding the least squares solution of two matrix equations that are derived using the IPFM model and involve irregular samples of a signal representing the HRV. The method is validated with various synthesised signals (in all tests, the relative errors of the power estimates at the modulating frequencies are within 3%, and the relative power of the spurious terms is less than 0.8% only) and is furthermore applied to the spectral analysis of R-R interval series obtained from diabetic children. The results, with simulated and real HRV signals, show that the developed method yields very accurate estimations of the spectral region below half the mean heart rate. Moreover, it allows the detection and assessment of certain genuine modulating components beyond the traditional frequency limit of the HRV spectra.

Child↗
PubMed1989

Regression models for the estimation of circadian rhythms in the presence of effects due to masking.

The estimation of human circadian rhythms from experimental data is complicated by the presence of "masking" effects associated with the sleep-wake cycle. The observed rhythm may include a component due to masking, as well as the endogenous component linked to a circadian pacemaker. In situations where the relationship between the sleep-wake cycle and the circadian rhythm is not constant, it may be possible to obtain individual estimates of these two components, but methods commonly used for the estimation of circadian rhythms, such as the cosinor analysis, spectral analysis, average waveforms and complex demodulation, have not generally been adapted to identify the modulations that arise from masking. The estimates relate to the observed rhythms, and the amplitudes and acrophases do not necessarily refer to the endogenous rhythm. In this paper methods are discussed for the separation of circadian and masking effects using regression models that incorporate a sinusoidal circadian variation together with functions of time since sleep and time during sleep. The basic model can be extended to include a time-varying circadian rhythm and estimates are available for the amplitude and phase at a given time, together with their joint confidence intervals and tests for changes in amplitude and acrophase between any two selected times. Modifications of these procedures are discussed to allow for non-sinusoidal circadian rhythms, non-additivity of the circadian and time-since-sleep effects and the breakdown of the usual assumptions concerning the residual errors. This approach enables systematic masking effects associated with the sleep-wake cycle to be separated from the circadian rhythm, and it has applications to the analysis of data from experiments where the sleep-wake cycle is not synchronized with the circadian rhythm, for example after time-zone transitions or during irregular schedules of work and rest.

Circadian Rhythm↗
PubMed2003

Assessing Secchi and photic zone depth in the Baltic Sea from satellite data.

Long-term trends in the Secchi depth of the Baltic Sea have been interpreted in terms of eutrophication. The spectral attenuation coefficient Kd (490) can be estimated from remote sensing data. Given the empirical and theoretical relationships between diffuse attenuation and Secchi depth, it is therefore possible to estimate the trophic state from remote sensing data. This paper considers relationships among remotely sensed and in-water measured K (490), and Secchi depth data obtained during dedicated sea-truthing campaigns in the eastern Baltic Proper in 1999 (4) and in the western Baltic Proper/Himmerfjärden area during 2001 and 2002. In-water measurements are used to establish the relationship between the PAR and the spectral attenuation coefficient in the Baltic Sea via regression analysis. The analysis showed that in the area of investigation Kd(490) is about 1.48 times higher than Kd (PAR). This relationship is then used to define the link between the photic zone depth and the remote sensing optical depth, Kd (490)-1. The results show that the depth of the euphotic zone is about 6.8 times Kd (490)-1. The regression analysis between Kd (PAR) and Secchi depth confirmed previous work that Kd (PAR) is about 1.7 of the inverse Secchi depth. Furthermore, an in-water algorithm between Secchi depth and Kd (490) is used to simulate a Secchi depth map of the Baltic Sea from SeaWiFS Kd(490) data. This map is verified against sea-truthing data. Kd (490) data derived from satellite is compared to in situ Kd (490), and the sources of error are discussed.

Algorithms↗
PubMed1980

[Quantitative indicators of electrical brain activity changes during hyperbaric oxygenation (author's transl)].

In stroke patients we previously observed increases in the electrical brain activity during hyperbaric oxygenation which were quantified and documented by EEG-interval-amplitude-analysis system. The object of this study was to assess the relationship between EEG-interval-amplitude-analysis and the spectral-analysis. We approached this problem by evaluating EEG recordings obtained from stroke patients during their exposure to hyperbaric oxygen by both methods. The results showed: a) the influence of hyperbaric oxygenation over the EEG, b) a clear picture of quantitative indicators for electrical brain activity by these EEG-analysis-systems and c) a close correlation of both methods in evaluating EEG changes, apart from the more differentiated estimation of deltawave activity by the spectral-analysis.

Cerebrovascular Disorders↗
PubMed2007

Fourier transform approach for thickness estimation of reflecting interference filters. 2. Generalized theory.

A Fourier transform (FT) approach based on the evaluation of optical-density-bandwidth products in the spectral region of interest was recently proposed for the thickness estimation of reflecting thin-film dielectric filters. For simplicity, the initial discussion was limited to a particular type of immersed coating. The theory is generalized to more realistic filter configurations and confirmed by numerical examples. It is shown that good results are possible although the problem is more complex from a FT point of view.

Journal Article↗
PubMed2005

[Non-parameter estimation algorithm to determine stellar effective temperature].

The effective temperature of a star is one of the most important parameters, which determine the continuum and spectral lines in the stellar spectrum. A non-parameter estimation algorithm is proposed to estimate the stellar effective temperature in the present paper. Firstly, the spectrum data is processed by principal component analysis(PCA), then, an estimating model based on a Gaussian kernel function is set up using the PCA data and their temperatures. Experiments were carried out to verify the efficiency, and numerical robustness of the algorithm is also tested.

English Abstract↗
PubMed1986

Spectroscopic properties of bilirubin-human serum albumin complexes: a stoichiometric analysis.

Light absorption spectra, fluorescence of bound bilirubin, fluorescence of albumin as quenched by bilirubin, and circular dichroism spectra have been studied in mixtures of bilirubin and defatted human serum albumin in variable proportions at 25 degrees C and at pH 7.4, 8.2, and 9.0. Corresponding spectral data have been calculated for the stoichiometric bilirubin-albumin complexes, 1:1, 2:1, and 3:1. Light absorption spectra as well as the bound bilirubin fluorescence indicate that all three bound bilirubin dianions are internalized. These data were obtained by curve fitting to least sum of squared deviations. In addition to the best fit we obtained 30 acceptable curves, located within an F contour, thus producing a rough estimate of the variation of the resulting spectral data.

Bilirubin↗
PubMed1995

Improving the ensemble average of visual evoked potentials. II. Simulations and experiments.

Ensemble averaging is generally used for the estimation of Evoked Potentials. This paper deals with the assessment of correction procedures for the time variability of the ensemble components, this time variability reduces the improvement of the signal-to-noise ratio (SNR) by averaging. Evoked potentials were estimated by ensemble averaging, synchronized to a periodic stimulus. It is assumed that VEP-instability is partly caused by time-variability of the evoked potentials. Two time-variate models were used, from which procedures were derived to correct the single VEP-responses prior to ensemble averaging. The models are: (1) variation in response delay (jitter), (2) variable compression/expansion of the time scale of the response (wow). The Spectral Phase Difference method was applied to estimate both the delay time jitter and the wow factor of single responses with respect to a template (conventional ensemble average). The effects of the devised correction on the average VEP waveform and on the SNR of the ensemble were investigated by using data from realistic simulations and from experiments (n = 23) with a number of healthy human volunteers (n = 17). Jitter- and wow-corrections were effective on simulations with time variability due to delay time jitter and time scale distortion (wow), respectively. Both wow- and jitter correction of the single responses improved the SNR of the VEP measurements significantly and to the same amount. A combined wow-jitter approach resulted in significantly better results than the exclusive application of jitter- or wow correction.

Computer Simulation↗
PubMed2005

Doppler ultrasound spectral enhancement using the Gabor transform-based spectral subtraction.

Most of the important clinical indices of blood flow are estimated from the spectrograms of Doppler ultrasound (US) signals. Any noise may degrade the readability of the spectrogram and the precision of the clinical indiCes, so the spectral enhancement plays an important role in Doppler US signal processing. A new Doppler US spectral enhancement method is proposed in this paper and implemented in three main steps: the Gabor transform is used to compute the Gabor coefficients of a Doppler US signal, the spectral subtraction is performed on the magnitude of the Gabor coefficients, and the Gabor expansion with the spectral subtracted Gabor coefficients is used to reconstruct the denoised Doppler US signal. The different analysis and synthesis windows are examined in the Gabor transform and expansion. The signal-to-noise ratio (SNR) improvement together with the overall enhancement of spectrograms are examined on the simulated Doppler US signals from a femoral artery. The results show the denoising method based on the orthogonal-like Gabor expansion achieves the best denoising performance. The experiments on some clinical Doppler US signals from umbilical arteries confirm the superior denoising performance of the new method.

Algorithms↗
PubMed1982

Development of scotopic sensitivity and the absorption spectrum of the human ocular media.

Scotopic spectral sensitivity was measured for nine observers (aged 4.5 months to 66 years) from 400 to 650 nm (10-nm steps) by using a 42 degree naturally viewed stimulus. The dependent measure was the visually evoked cortical-potential amplitude that was phase locked to an 8-Hz flickering stimulus. Sensitivity was similar for all observers at middle and long wavelengths, but at short wavelengths there was a decrease in sensitivity with increasing age. The density of the preretinal ocular media was estimated by subtracting the log scotopic spectral-sensitivity function of each observer from the human rhodopsin-absorbance spectrum when the two sets of curves were pinned at long wavelengths. The density of the infant ocular media was lower than that for adults. To quantify the sequence of ocular-media development, scotopic sensitivity was determined for an additional 42 observers (aged 1 month to 70 years) at two spectral points: 553 nm, where the optic-media density is low, and 405-430 nm, where the density is high. From these data, optic-media density at 400 nm was calculated. Despite substantial individual differences within each age, a clear aging function emerged. Preretinal optic-media density increased monotonically from birth throughout adulthood. Thus optical density at 400 nm differs by about a factor of 22 between the average 1-month-old infant and the average 70-year-old adult.

Adolescent↗
PubMed2003

Heart rate variability and autonomic activity at rest and during exercise in various physiological conditions.

The rhythmic components of heart rate variability (HRV) can be separated and quantitatively assessed by means of power spectral analysis. The powers of high frequency (HF) and low frequency (LF) components of HRV have been shown to estimate cardiac vagal and sympathetic activities. The reliability of these spectral indices, as well as that of LF/HF ratio as a marker of autonomic interaction at rest and during exercise, is briefly reviewed. Modifications in autonomic activities induced by different physiological conditions, e.g. hypoxia exposure, training, and water immersion, have been found in HRV power spectra at rest. The changes in HF and LF powers and in LF/HF ratio observed during exercise have been shown not to reflect the decrease in vagal activity and the activation of sympathetic system occurring at increasing loads. HF peak was recognised in power spectra in the entire range of relative intensity, being responsible for the most part of HR variability at maximal load. LF power did not change during low intensity exercise and decreased to negligible values at medium-high intensity, where sympathetic activity was enhanced. There was no influence from factors such as fitness level, age, hypoxia, and blood distribution. In contrast, a dramatic effect of body position has been suggested by the observation that LF power increased at medium-high intensities when exercising in the supine position. The increased respiratory activity due to exercise would be responsible of HF modulation of HR via a direct mechanical effect. The changes in LF power observed at medium-high intensity might be the expression of the modifications in arterial pressure control mechanisms occurring with exercise. The finding of opposite trends for LF rhythm in supine and sitting exercises suggests that different readjustments might have occurred in relation to different muscular inputs in the two positions.

Altitude↗
PubMed1987

[Spectral analyses in ocular microtremor in disorders of brain stem function].

In 61 healthy and 79 patients the small involuntary movements were recorded using a piezoelecric strain gauge transducer mounted on a modified optical frame. The recording was followed by a spectralanalytic process with special programs. We used a detailed neurological examination for estimating the clinical condition of the patients. The spectral analysis of the ocular microtremor supplies results which correspond to the clinical condition in each case of the patients with increased central herniation. A irreversible loss of cerebral function (brain death) can also be documented. Patients with special clinical syndromes show tremograms which answer the probably extent of damage of (meso-)pontine parts. We investigated and discussed effects of the influence of central-nervous drugs, age-dependent relations, the significance of cardiovascular pulsations and questions and questions of the comparability of findings between both bulbi. The ocular microtremor shows burst-like components with a frequency peak between 60 and 90 oscillations per second. At present must be assessed that it could be a potential monitor of the function of the reticular (meso-)pontine formation.

Brain Death↗
PubMed2004

A new adaptive subband decomposition approach for automatic analysis of NMR data.

This paper presents a non-iterative, fast, and almost automated time-data analysis method for NMR spectroscopy, based on a new adaptive implementation of high resolution methods used in spectral subbands. It is intended to avoid the choice of the decimation factor (or the width of the spectral windows) which, in the case of a uniform decomposition, strongly conditions the estimation results, and to diminish the computational burden. It is achieved through successive decimation/estimation stages each followed by a test procedure in order to decide whether or not the process should continue. The proposed test is based on a local spectral flatness measure of the estimation residuals. This stop-criterion involves an a posteriori validation of the estimation, thus the method proposed allows one to obtain a better detection rate at a lower complexity comparatively to other stopping rules, while preserving a reasonable estimation variance. Moreover, the reliability of the fitting algorithms considered is improved, by decreasing the influence of the model order and the number of false detections. Finally, the method is more efficient than Fourier transform (FT) at low signal-to-noise ratio (SNR). The effectiveness of the method is demonstrated by analyzing a simulation signal and raw carbon-13 experimental data.

Algorithms↗
PubMed1984

Ultrasound attenuation measurements of the liver in vivo using a commercial sector scanner.

Attenuation measurements of various tissue mimicking phantoms and three different groups of patients were obtained using a modified commercial sector scanner. Estimates of attenuation were made using the spectral shift method with mean frequencies at different depths of a region of interest being obtained by both zero crossing and fast Fourier transform techniques. The accuracy and precision of both techniques was compared in phantoms and it was found that the FFT technique yielded less day-to-day variation (SD=3 percent) than the zero crossing technique (5 percent). For larger regions of interest, the range of variation in both techniques was more similar. Day-to-day variation in livers of normal patients was much larger than that seen in phantoms (10 to 15 percent) suggesting that in vivo measurements may be less precise due to actual daily changes in patients' livers. Attenuation estimates of phantoms were high by approximately 0.16 dB/MHz/cm compared to values obtained by transmission techniques. The attenuation values of livers in a group of 31 normal patients ranged from 0.214 dB/cm/MHz to 0.849 dB/cm/MHz with a mean of 0.627 +/- 0.126 dB/cm/MHz for the zero crossing technique while the mean value using the FFT technique was 0.86 +/- 0.168 dB/cm/MHz. A group of 26 Gauchers disease patients also showed wide variation with a mean attenuation value of 0.768 +/- 0.21 dB/cm/MHz using the FFT technique. This was significantly different than that of the normal group (p less than .05). Also, a group of 22 chronic B hepatitis patients was examined, having a mean attenuation value of 0.823 +/- 0.21 dB/cm/MHz, not significantly different from those of normal patients. Highly significant differences were found between the three groups when the power spectrum bandwidths of signals received were compared. These differences may be due to differences in the dependence of attenuation as a function of frequency between the groups and may represent a useful tissue characterization parameter.

Gaucher Disease↗
PubMed1998

Color vision in two observers with highly biased LWS/MWS cone ratios.

Two sisters, heterozygous carriers for congenital X-linked protanopia, were diagnosed as normal trichromats by the Rayleigh match on the anomaloscope. The heterozygous state was established by molecular analysis of their visual pigment genes. The normal color match establishes that the spectral sensitivities of their long-wavelength-sensitive (LWS) and middle-wavelength-sensitive (MWS) cone visual photopigments are within normal variability. Their FM 100-hue test error scores were low, demonstrating superior chromatic discrimination. Heterochromatic flicker photometric (HEP) spectral sensitivities were like those of protanopes. The estimated LWS/MWS cone ratios from the HFP data were 0.09/1 and 0.03/1, compared with ratios in the range of 0.6/1 to 10/1 for typical normal trichromats. Measurements of chromatic grating acuity on chromatically selective backgrounds were performed to study the cone mosaic. The data were consistent with a sparsity of LWS cones. Both protan carriers showed normal spectral sensitivities for all three cone types under cone isolating chromatic adaptation and normal three-peaked curves for increment thresholds on a white pedestal. Hue estimation, run on one carrier was normal. The equilibrium yellow locus was measured in the other carrier and was in the range of normal trichromats. The data indicate that normal color vision can occur even when the LWS/MWS cone ratio is quite abnormal.

Adaptation, Ocular↗
PubMed1977

Annoyance response to spectrally modified recorded aircraft noise during television-viewing.

Magnitude estimations of the annoyance of 27 individual noise stimuli were made by 24 Ss while viewing television; 8 different spectrum modifications of a basic aircraft noise were introduced at 3 overall intensities. The basic spectrum was that of an untreated commercial jet aircraft takeoff noise; the other 8 were created by removal of one of two amounts of energy from an octave band centerered at either .315, .8, 1.6, or 4 kc/s. An ANOVA showed significant annoyance differences for spectrum modification, overall noise intensity and their interaction. Annoyance reduction was greatest when energy was removed at the octave band centered at 1.6 kc/s, next at .8, and .315, and least at 4 kc/s. Although greater overall intensity reduction yielded progressively less annoyance with various spectrally-modified noises as well as unmodified noise, the spectrum modification was apparently most effective in reducing annoyance when the overall maximum noise intensity ranged from 88.0 to 89.1 dbA, and was least effective from 83.9 to 85.3 dbA. Annoyance reduction resulting from spectrum modification at a single octave band (centered at either .8 or 1.6 kc/s) was equivalent to that resulting from a 2.7 dbA overall intensity reduction. The results are discussed in terms of speech interference as well as intermodal effects of noise during television viewing.

Adult↗