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At least 505 records · Page 28Linked to original sources
PubMed1996

Quantitative analysis of tremor time series.

Spectral analysis is applied to tremor time series in basic research and treatment monitoring. The estimation of the spectra from the data is usually done by averaging the squared modulus of the Fourier transform of segments of the data. We discuss drawbacks of this method and propose an alternative procedure to estimate the spectra adaptively based on the data. Thus, the method can be applied to all types of tremor. Applying the theory of spectral estimation, we propose a method to decide whether a spectrum exhibits multiple significant peaks and discuss different approaches to determine the amplitude of the tremor from the spectrum.

Electromyography↗
PubMed1996

Coherent inspiratory oscillation of cranial nerve discharges in perfused neonatal cat brainstem in vitro.

1. To understand the neural organization of respiratory movement control and its developmental transformation, we studied the temporal characteristics of inspiratory activities, especially nerve-to-nerve short-term synchronization, in an in vitro preparation of the isolated, perfused brainstem of kittens aged 0-14 days (postnatal day (P) 0-14). 2. In the inspiratory discharges of facial, vagus, glossopharyngeal and hypoglossal nerves, a stable oscillation with a period of 30-40 ms (i.e. approximately 30 Hz) was observed in all preparations examined. In addition, we demonstrated that this oscillation presents a strong short-term synchrony between distinct inspiratory nerves. This nerve-to-nerve synchronization was already apparent at approximately 12 h after birth. The degree of synchronization as evaluated by coherence spectral analysis was larger than 0.85 in all cases at any age examined. 3. This nerve-to-nerve coherence was not affected by changes in temperature (28-36 degrees C), whereas respiratory rate, oscillation frequency and oscillation amplitude as estimated by power spectral analysis were highly temperature sensitive. 4. The nerve-to-nerve synchronization, as well as the approximately 30 Hz oscillation, remained unchanged after a pontomedullary transection, indicating that the medullary network, completely isolated from other structures and afferents, is sufficient to produce both fast oscillation and nerve-to-nerve synchronization. 5. Based on these observations in vitro, we conclude that nerve-to-nerve coherent inspiratory oscillation generated in the brainstem is already functional early in life.

Age Factors↗
PubMed2002

Decision strategies of hearing-impaired listeners in spectral shape discrimination.

The ability to discriminate between sounds with different spectral shapes was evaluated for normal-hearing and hearing-impaired listeners. Listeners detected a 920-Hz tone added in phase to a single component of a standard consisting of the sum of five tones spaced equally on a logarithmic frequency scale ranging from 200 to 4200 Hz. An overall level randomization of 10 dB was either present or absent. In one subset of conditions, the no-perturbation conditions, the standard stimulus was the sum of equal-amplitude tones. In the perturbation conditions, the amplitudes of the components within a stimulus were randomly altered on every presentation. For both perturbation and no-perturbation conditions, thresholds for the detection of the 920-Hz tone were measured to compare sensitivity to changes in spectral shape between normal-hearing and hearing-impaired listeners. To assess whether hearing-impaired listeners relied on different regions of the spectrum to discriminate between sounds, spectral weights were estimated from the perturbed standards by correlating the listener's responses with the level differences per component across two intervals of a two-alternative forced-choice task. Results showed that hearing-impaired and normal-hearing listeners had similar sensitivity to changes in spectral shape. On average, across-frequency correlation functions also were similar for both groups of listeners, suggesting that as long as all components are audible and well separated in frequency, hearing-impaired listeners can use information across frequency as well as normal-hearing listeners. Analysis of the individual data revealed, however, that normal-hearing listeners may be better able to adopt optimal weighting schemes. This conclusion is only tentative, as differences in internal noise may need to be considered to interpret the results obtained from weighting studies between normal-hearing and hearing-impaired listeners.

Adult↗
PubMed2006

QT variability and HRV interactions in ECG: quantification and reliability.

In this paper, a dynamic linear approach was used over QT and RR series measured by an automatic delineator, to explore the interactions between QT interval variability (QTV) and heart rate variability (HRV). A low-order linear autoregressive model allowed to separate and quantify the QTV fractions correlated and not correlated with HRV, estimating their power spectral density measures. Simulated series and artificial ECG signals were used to assess the performance of the methods, considering a respiratory-like electrical axis rotation effect and noise contamination with a signal-to-noise ratio (SNR) from 30 to 10 dB. The errors found in the estimation of the QTV fraction related to HRV showed a nonrelevant performance decrease from automatic delineation. The joint performance of delineation plus variability analysis achieved less than 20% error in over 75% of cases for records presenting SNRs higher than 15 dB and QT standard deviation higher than 10 ms. The methods were also applied to real ECG records from healthy subjects where it was found a relevant QTV fraction not correlated with HRV (over 40% in 19 out of 23 segments analyzed), indicating that an important part of QTV is not linearly driven by HRV and may contain complementary information.

Algorithms↗
PubMed2000

Vibrational lifetime of bond-center hydrogen in crystalline silicon

The lifetime of the stretch mode of bond-center hydrogen in crystalline silicon is measured to be T1 = 7.8+/-0.2 ps with time-resolved, transient bleaching spectroscopy. The low-temperature spectral width of the absorption line due to the stretch mode converges towards its natural width for decreasing hydrogen concentration C(H), and nearly coincides with the natural width for C(H) approximately 1 ppm. The lifetimes of the Si-H stretch modes of selected hydrogen-related defects are estimated from their spectral widths and shown to range from 1.6 to more than 37 ps.

Journal Article↗
PubMed1998

Effects of known variations in photopigments on L/M cone ratios estimated from luminous efficiency functions.

The extent to which known variations in photopigment lambda max and optical density may affect cone ratios estimated from the spectral luminous efficiency function (LEF) was examined. LEFs were generated using L- and M-cone fundamentals, one of which had been shifted in lambda max (+/- 1, 2, 4 or 6 nm) or varied in peak optical density (increased or decreased by 10, 25 or 50%). A curve-fitting program was then used to estimate the L/M cone ratios for the generated LEFs assuming standard L- and M-cone fundamentals. These modeling exercises indicate that L/M cone ratios estimated from LEFs are highly correlated with long-wave sensitivity and with known variations in L-cone lambda max. Variations in M-cone lambda max and photopigment optical density for both cone types are also correlated with L/M cone ratios, but have much less impact on the estimated ratios.

Algorithms↗
PubMed1979

Determination of serum bilirubin by skin reflectance: effect of pigmentation.

A noninvasive optical technique by which serum bilrubin can be estimated from skin spectral reflectance measurements has been further investigated. The original work on 30 healthy, full-term white infants and an independent study on 14 white and 30 black infants demonstrate that the method has potential not only for clinical use, but also for the study of the transport of bilrubin to, from and within the skin. The objectives of the present study are to evaluate the method on a larger sample population with special attention to natural skin pigmentation effects and the development of a physical model of the tissue to explain the relationship between serum bilrubin concentration and skin reflectance. Reflectance spectra (380-800 nm) and concurrent serum bilirubin measurements were taken on a sample population of 58 white and 45 full-term black infants (1-3 days of age). Multiple linear regression analysis, comprised of six wavelengths gave a correlation coefficient, r = 0.831 for the white infant group. For the black infant group, a five wavelength analysis provided r = 0.877 with the standard error of estimate being +/- 1.46 mg/100 ml for both groups. The model for establishing a physical basis for the relationship shows that a transformed, normalized Kubelka-Monk function xi (460, 510, 420) is linearly related to serum bilrubin concentration. This function is determined from the spectral reflectance values at three wavelengths, 420, 460, and 510 nm. The wavelength combination is such that effects due to hemoglobin and melanin pigments are minimized. Regression analysis showed that r = 0.778 and r = 0.865 for the white and black infant groups, respectively, with standard error of estimates being +/- 1.4 mg/100 ml for both groups. Routine determinations of total serum bilrubin by laboratory methods have standard errors of estimate ranging from +/- 1 to 1.5 mg/100 ml. Thus, the method herein described shows that the relationship between skin reflectance and serum bilrubin in full-term infants is close to the acceptable limits for clinical use. Furthermore, this work shows that skin pigmentation does not obscure this relationship.

Bilirubin↗
PubMed1989

Ultradian, circadian and seasonal variations of plasma progesterone and LH concentrations during the luteal phase.

The circadian variations in plasma progesterone (P) and LH concentrations were investigated in six women, aged 23-40 years. All were studied in the mid-luteal phase (7 +/- 2 days after LH mid-cycle surge). Experiments were conducted in autumn and in spring. Blood samples were obtained every 15 min for 24 hr. Plasma P and LH concentrations were measured by RIA. Each subject's time-series was analysed using three methods; visual inspection (chronogram), spectral analysis to estimate component periods of rhythms (tau) and cosinor analysis to quantify the rhythms parameters. Marked temporal variations in plasma P concentration were observed in each subject. The maximal variations over a 24-hr period, ranged between 13-58.5 mmol/l. Differences related to sampling time were statistically validated by ANOVA (p less than 0.00001). Significant harmonic periods were detected by spectral analysis but differed among subjects. In all subjects but one, a circadian rhythm was detected. The acrophase location was similar (about 0700 hr) in the four subjects studied in autumn, but ranged from 1940 to 0320 hr in those studied in spring. An ultradian rhythm with tau = 8 hr was also validated in six time-series with similar acrophases (about 0200, 1000, and 1800 hr). Cosinor analysis of pooled data revealed that the 24-hr, 12-hr, and 8-hr rhythms were statistically significant (p = 0.001) in autumn. algebraic sum of these three cosine functions yielded a circadian waveform with peak-times occurring near 0300 and 1130 hr and a trough-time about 2200 hr. In spring, the circadian pattern appeared quite different, and peak-times were found near 0700 and 2000 hr, and trough-times near 0300 and 1500 hr. Furthermore, the 24-hr mean of P was higher in autumn (28.9 +/- 0.4 nmol/l) than in spring (17.2 +/- 0.4 nmol/l), p from ANOVA less than 0.00001. The evidence for a similar circadian LH pattern is not as strong. Seasonal, circadian and ultradian rhythms characterize the physiologic time structure of plasma P concentration in mid-luteal phase.

Activity Cycles↗
PubMed1984

Circular dichroic spectra of apolipoprotein E in model complexes and cholesterol-rich lipoproteins: lipid contribution.

Lipid-free apolipoprotein E (apo E) and canine apo E HDLc, a cholesterol-rich lipoprotein containing apo E as the only apolipoprotein, show very different circular dichroism (CD) spectra. To determine the cause of the spectral difference, we estimated the CD contribution of phospholipid, cholesterol, and cholesteryl ester in liposomes and microemulsions. We prepared microemulsions, containing nearly equal amounts of egg phosphatidylcholine (PC) and cholesteryl oleate (mean diameter 320 A), by an injection technique. Both microemulsions and cholesterol-containing liposomes exhibit intense negative CD bands in the far-ultraviolet region. Lipids contribute about 20% of the spectral difference between apo E and apo E HDLc at 222 nm, and about 60% of the spectral difference at 208 nm. The remainder of the spectral difference is attributable to lipid-protein interaction corresponding to a 15-30% increase in helicity of apo E. CD analysis indicates that the helical content of apo E in apo E HDLc resembles that in the ternary complex apo E-PC-cholesterol (or apo E-PC-cholesteryl ester) more than that in the binary complex apo E-PC, suggesting that cholesterol affects the conformation of apo E. Our data indicate that in going from a lipid-free state to a lipid environment, apo E undergoes a random to helix transition, assuming the maximal helicity predicted from its primary structure.

Animals↗
PubMed1993

[Power spectral analysis of spontaneous rhythm in peripheral blood flow in fetal lamb].

Fluctuations in fetal hemodynamic parameters are commonly observed, and we can use the variation in the fetal heart rate as an index in fetal assessment. The purpose of this study was to clarify the spontaneous rhythm in systemic circulation in fetal lambs by means of power spectral analysis. Three pregnant sheep and their fetuses at 125 and 135 days of gestation were surgically instrumented and studied. After a minimum recovery period of four days, a fetal electrocardiogram was taken and arterial blood pressure was recorded. They were also examined in the absence of heart rate variation by means of fetal cardiac pacing. We used an autoregressive model to estimate their power spectral densities. In normal fetal lambs, two spontaneous rhythms were detected in the spectra for the fetal heart rate and arterial blood pressure. Spectral analysis of arterial blood pressure variation in the absence of heart rate variation revealed that there was only one consistent major spectral component around 0.15 Hz. This suggests that fluctuations are caused not by a variation in the heart rate but by a variation in peripheral motor activity. This spontaneous rhythm in peripheral resistance may be associated with the sinusoidal heart rate pattern in the fetal cardiotochogram.

Animals↗
PubMed2006

MEG spectral profile in Alzheimer's disease and mild cognitive impairment.

OBJECTIVE: In this study, we applied a novel procedure to calculate the mean frequency from the Magnetoencephalography (MEG) signals of 22 patients with Alzheimer's Disease (AD), 22 patients with mild cognitive impairment (MCI), and 21 healthy controls. A significant mean frequency decrease was expected in pathological groups. MCI subjects are expected to show intermediate mean frequency values between AD patients and controls. METHODS: MEG signal was obtained from a whole-head 148 channels magnetometer in a resting condition. We estimated the power spectral density from the MEG signal by means of the Fourier transform of the autocorrelation function. Then, we computed the mean frequency for each subject. RESULTS: Mean frequency was higher in controls (12.46 +/- 2.00 Hz, mean +/- SD) than in MCI subjects (10.82 +/- 2.21 Hz) with significant differences (P < 0.05). Moreover, mean frequency values in MCI subjects were higher than in AD patients (9.06 +/- 2.48 Hz, P < 0.05). We also detected a decrease of 0.17 Hz per year in mean frequency from normal subjects' MEG (P < 0.05) CONCLUSIONS: Results demonstrated that the approach adopted for the calculation of a mean frequency score seems to be adequate and sensitive to detect differences between normal aging, cognitive deterioration and AD. In addition, data may contribute to the theoretical discussion on the nature of mild cognitive impairment and its similitudes with Alzheimer's disease. SIGNIFICANCE: This paper may be considered a first step to obtain a reliable measure which summarizes spectral information, and might be of a potential clinical interest.

Age Factors↗
PubMed2006

A robust method for ECG-based estimation of the respiratory frequency during stress testing.

A robust method is presented for electrocardiogram (ECG)-based estimation of the respiratory frequency during stress testing. Such ECGs contain highly nonstationary noise and exhibit changes in QRS morphology which, when combined with the dynamic nature of the respiratory frequency, make most existing methods break down. The present method exploits the oscillatory pattern of the rotation angles of the heart's electrical axis as induced by respiration. The series of rotation angles, obtained from least-squares loop alignment, is subject to power spectral analysis and estimation of the respiratory frequency. Robust techniques are introduced to handle the nonstationary properties of exercise ECGs. The method is evaluated by means of both simulated signals, and ECG/airflow signals recorded from 14 volunteers and 20 patients during stress testing. The resulting respiratory frequency estimation error is, for simulated signals, equal to 0.5% +/- 0.2%, mean +/- SD (0.002 +/- 0.001 Hz), whereas the error between respiratory frequencies of the ECG-derived method and the airflow signals is 5.9% +/- 4% (0.022 +/- 0.016Hz). The results suggest that the method is highly suitable for analysis of noisy ECG signals recorded during stress testing.

Adult↗
PubMed2004

Singular spectrum analysis applied to backscattered ultrasound signals from in vitro human cancellous bone specimens.

Mean scatterer spacing (MSS) holds particular promise for the detection of changes in quasiperiodic tissue microstructures such as may occur during development of disease in the liver, spleen, or bones. Many techniques that may be applied for MSS estimation (temporal and spectral autocorrelation, power spectrum and cepstrum, higher order statistics, and quadratic transformation) characterize signals that contain a mixture of periodic and nonperiodic contributions. In contrast, singular spectrum analysis (SSA), a method usually applied in nonlinear dynamics, first identifies components of signals corresponding to periodic structures and, second, identifies dominant periodicity. Thus, SSA may better separate periodic structures from nonperiodic structures and noise. Using an ultrasound echo simulation model, we previously demonstrated SSA's potential to identify MSS of structures in quasiperiodic scattering media. The current work aims to observe the behavior of MSS estimation by SSA using ultrasound measurements in phantom materials (two parallel, nylon-line phantoms and four foam phantoms of different densities). The SSA was able to estimate not only the nylon-line distances but also nylon-line thickness. The method also was sensitive to the average pore-size differences of the four sponges. The algorithms then were applied to characterize human cancellous bone microarchitectures. Using 1-MHz center-frequency, radio-frequency ultrasound signals, MSS was measured in 24 in vitro bone samples and ranged from 1.0 to 1.7 mm. The SSA MSS estimates correlate significantly to MSS measured independently from synchrotron microtomography, r2 = 0.68. Thus, application of SSA to backscattered ultrasound signals seems to be useful for providing information linked to tissue microarchitecture that is not evident from clinical images.

Aged↗