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At least 1,387 records · Page 77Linked to original sources
PubMed2006

Spectral F-test power evaluation in the EEG during intermittent photic stimulation.

Intermittent photic stimulation (IPS) is an important functional test, which can induce the photic driving in the electroencephalogram (EEG). It is capable of enhancing latent oscillations manifestations not present in the resting EEG. However, for adequate quantitative evaluation of the photic driving, these changes should be assessed on a statistical basis. With this aim, the sampling distribution of spectral F test was investigated. On this basis, confidence limits of the SFT-estimate could be obtained for different practical situations, in which the signal-to-noise ratio and the number of epochs used in the estimation may vary. The technique was applied to the EEG of 10 normal subjects during IPS, and allowed detecting responses not only at the fundamental IPS frequency but also at higher harmonics. It also permitted to assess the strength of the photic driving responses and to compare them in different derivations and in different subjects.

Adolescent↗
PubMed2005

Causal influence: advances in neurosignal analysis.

The analysis of multichannel recordings such as electroencephalography (EEG) and magnetoencephalography (MEG) is important both for basic brain research and for medical diagnosis and treatment. Multivariate linear regressive analysis such as the AutoRegressive (MAR) modeling is an effective means to characterize, with high spatial, temporal, and frequency resolution, functional relations within multichannel neuronal data. Recent advances in MAR modeling show promise for the analysis and visualization of large-scale network interactions, especially in the ability to assess their causal relations. This article provides a detailed review of the advances in the development and application of causal influence measures for analyzing neurosignal within the framework of the MAR spectral analysis. First, we outline mathematical formulations of the MAR model and its related estimation procedures, with emphasis on the development of causal influence measures for analyzing brain circuits. Second, we address the technical issues on the practical applications of the causal measures to the neurobiological data. Of particular interest is the recent development of adapting the MAR to analyze neural spike train data. Third, we present a variety of applications ranging from basic neuroscience research to clinical applications as well as functional neuroimaging. We finally conclude with a brief summary and discuss future research development in this field.

Action Potentials↗
PubMed1977

[Analysis of gamma-ray spectra by using fast Fourier transform (author's transl)].

In order to simplify the mass data processing in a response matrix method for gamma-ray spectral analysis, a method using a Fast Fourier Transform devised. The validity of the method was confirmed by a computer simulation for spectra of a NaI detector. The method uses the fact that spectral data can be represented by Fourier series with reduced number of terms. The estimation of intensities of gamma-ray components is performed by a matrix operation using the compressed data of an observation spectrum and standard spectra in Fourier coefficients. The identification of gamma-ray energies is also easy. Several features in the method and a general problem to be solved in a response matrix method are described.

Fourier Analysis↗
PubMed2005

Transmissibility of helicopter vibration in the spines of pilots in flight.

INTRODUCTION: The high incidence of back pain in helicopter pilots (HP) has been attributed to vibration and the in-flight pilot's posture. Helicopter vibration has a peak power at frequencies around 5 Hz, which is within the range that the human upper body presents resonance frequency. This study investigates the transmissibility (TR) of helicopter vibration from the seat to the spine of HP, and the seat resultant vibration (RSV). METHODS: We monitored 12 male HP during flights lasting 2 h on average. Uniaxial accelerometers measured the vibration at the L3 and T1 spinae processes of the pilots, and the helicopter backrest. The vibration at the pilot's seat, measured by triaxial accelerometers, was taken as the reference for the estimation of transmissibility values (TR) to L3, T1, and backrest using the cross-spectral method. TR was considered only for the frequency presenting the maximum power along the seat Z acceleration, and RSV was calculated according to ISO 2631-1. RESULTS: The TR values found for T1 suggest the presence of resonance in the pilot's spine during flight. Most of the HP presented TR within the range reported in the literature, but two of them had higher values at T1. Five flights showed RSV within the "caution zone" for 4-8 h of daily exposure. CONCLUSION: Our findings indicate that cyclic compressive force due to helicopter vibration can potentially increase the load imposed on the spines of pilots during flight. This might explain the incidence of back pain and other injuries to the spines of these professionals.

Acceleration↗
PubMed2005

[Estimation of age-related changes in the regulation of peripheral blood flow in humans].

The age-dependent features in the state of skin microvascular bed has been studied with laser Doppler flowmetry in healthy volunteers of different age groups. To reveal the reaction of skin blood flow in response to short-term ischemia, the occlusive test has been carried out. To estimate the contribution of rhythmic components to blood flow signal, continuous wavelet-transform spectral analysis was used. Age-dependent increase of pulse-wave amplitude and decrease of respiratory wave amplitude reflecting age-dependent changes in functioning of arteriolar and venular links of microvascular bed have been observed at rest. In response to short-term ischemia the age-dependent reduction of reserve resources has been revealed in functioning of arteriolar link of microvascular bed. The reduction of activity of myogenic, neurogenic and endothelial regulation systems have been shown at rest in ageing.

Adult↗
PubMed2006

[Spectral control of metal admixtures in tetracycline].

Analysis of circular dichroism spectra made it possible to offer a method for estimation of tetracycline solutions contamination with metal ions. By its sensitivity the method is much superior to the spectrophotometric one used at present for determination of the antibiotic purity. In the latter method formation of complexes with metals is traced by batochromic displacement of the absorption spectra. The new method is rapid, relatively selective and requires comparatively small quantities of the substance for the analysis, which provides its use under both laboratory and manufacture conditions. The method is based on identification of the circular dichroism spectra of tetracycline complexes with metals in the long wavelength region. The presence of the circular dichroism concervative bands with strictly defined extremums in the spectra of tetracycline low acid solutions contaminated by multiply charged metal ions allowed vs. the circular dichroism spectra of pure tetracycline sample to conclude that the solution contained admixtures and to suggest their nature. It was shown that the charge, ion radius and tetracycline:metal relation were the factors defining the mark and location of the dichroism band extremums. At lambda(extr)-410-415 nm the tetracycline complexes with light metal ions such as Mg2+, Al3+ and Ca2+ were detected by the circular dichroism negative band in the spectra, while the complexes with heavy metal ions such as Sc3+, Sr3+, Cu3+, Cd3+, Ba2+, Y3+ and the cerium subgroup lanthanides were detected by the circular dichroism positive band. The tetracycline complexes with the lanthanides of the second half of the yttrium subgroup (Ho(3+)-Lu3+) were characterized by the presence of the circular dichroism minimum at lambda(min)-425 nm. When the tetracycline concentration was above 1.5 x 10(-3) M, multiligand complexes with circular dichroism negative extremum at lambda(min)-400 nm formed.

Circular Dichroism↗
PubMed1991

[The periodic components of the integral tonic activity of the sympathetic nerves].

Sympathetic spontaneous activity in peripheral nerves of limbs and activity of C-fibres of the cervical sympathetic nerve trunk (CSNT) were recorded in situ from anaesthetized rabbits using multichannel coherent recording technique. Spectral characteristics of the activity and correlation between nerve activity and blood pressure were estimated. The data obtained have suggested that C-fibres in the CSNT are mainly vasoconstrictor fibres and their activity is more baroreceptor-dependent than that of sympathetic fibres in peripheral nerves. It is found that latencies of changes in the arterial blood pressure in response to changes in sympathetic activity are between 4 and 10 s.

Animals↗
PubMedDate not supplied

[Correlation of electric potentials of the hippocampus, amygdala and neocortex during instrumental conditioned reflexes in the dog].

Coherence and spectral functions of biopotentials of the hippocampus, amygdala, frontal and sensorimotor cortical areas were estimated in dogs during instrumental alimentary behaviour and during self-stimulation of emotionally positive brain areas. Formation of functional systems: frontal cortex--amygdala, frontal cortex--hippocampus, and sensorimotor cortex--amygdala, synchronously acting in delta- and theta bands of electrical activity, was observed at the stage of a stabilized motor habit. THe hippocampus and amaygdala poorly co-operated, probably reflecting their different roles in the organization of behaviour.

Amygdala↗
PubMed1981

Analysis of the time series of the EEG frequency spectra and of EEG spectral power densities.

Some examples of the use of the principal component model for the economic description of the structure of the multiple time series and for the data reduction in the quantitative EEG studies are presented. The broad-band EEG frequency spectra were measured with the use of an electronic system designed by J. Dvorák. The EEG spectral power densities were computed via the discrete Fourier Transform (namely FFT) algorithm. The estimated two or three first principal components account for the major part of the total variance of individual EEG variables: The results hold for the used elementary epoch of measurement, i.e. 5 sec. - With the use of the algorithms and FORTRAN IV programs developed by J. Andĕl, T. Cipra and L. Tomásek a data reduction by a factor of 1:2000 can be achieved without any substantial loss of biological information. - The described methods help to obtain a better insight into the structure of the data and represent a powerful tool for data reduction at least in a certain class of experimental EEG studies (experimental toxicology, pharmacology, experimental neurology).

Animals↗
PubMed1995

Reappraisal of quantitative evaluation of pulmonary regurgitation and estimation of pulmonary artery pressure by continuous wave Doppler echocardiography.

This study assessed the usefulness of continuous wave Doppler echocardiography and color flow mapping in evaluating pulmonary regurgitation (PR) and estimating pulmonary artery (PA) pressure. Forty-three patients were examined, and high quality Doppler spectral recordings of PR were obtained in 32. All patients underwent cardiac catheterization, and simultaneous PA and right ventricular (RV) pressures were recorded in 17. Four Doppler regurgitant flow velocity patterns were observed: pandiastolic plateau, biphasic, peak and plateau, and early diastolic triangular types. The peak diastolic and end-diastolic PA-to-RV pressure gradients derived from the Doppler flow profiles correlated well with the catheter measurements (r = 0.95 and r = 0.95, respectively). As PA pressure increased, the PR flow velocity became higher; a linear relationship between either systolic or mean PA pressure and Doppler-derived peak diastolic pressure gradient was noted (r = 0.90 and 0.94, respectively). Based on peak diastolic gradients of < 15, 15-30 or > 30 mm Hg, patients could be separated as those with mild, moderate or severe pulmonary hypertension, respectively (p < 0.05). A correlation was also observed between PA diastolic pressure and Doppler-derived end-diastolic pressure gradient (r = 0.91). Moreover, the Doppler velocity decay slope of PR closely correlated with that derived from the catheter method (r = 0.98). The decay slope tended to be steeper with the increment in regurgitant jet area and length obtained from color flow mapping. In conclusion, continuous wave Doppler evaluation of PR is a useful means for noninvasive estimation of PA pressure, and the Doppler velocity decay slope seems to reflect the severity of PR.

Adolescent↗
PubMed1995

Effect of head-down tilt on cardiovascular control in healthy subjects: a spectral analytic approach.

1. The purpose of this investigation was to determine non-invasively the changes in autonomic cardiovascular control observed in normal subjects submitted to acute cardiopulmonary blood volume expansion by 100 degrees head-down tilt. The effect of head-down tilt on finger blood pressure and heart rate fluctuations was studied by means of power spectral analysis in 12 healthy men. 2. Amplitude spectra of heart rate and blood pressure rhythmicity were estimated at the low-frequency (60-140 mHz, 10-s rhythm) and high-frequency (area under the curve at mean respiration rate +/- 50 mHz) component. Transfer gain and phase were calculated between systolic blood pressure and heart rate. Forearm vascular resistance was estimated to validate the head-down procedure. 3. Forearm vascular resistance decreased significantly from 19.82 (16.34-26.46) mmHg ml-1 min 100 ml to 18.05 (13.69-22.88) mmHg ml-1 min 100 ml (P < 0.01) during head-down tilt (values are medians and 25 and 75 percentiles). The overall variability (total area under the curve of the spectrum from 20 to 500 mHz) of blood pressure and heart rate time series was consistently reduced with head-down tilt. 4. The spectral pattern of systolic blood pressure showed a diminution of the absolute and relative low-frequency component during head-down tilt: absolute log-transformed values, 2.86 (2.80-2.94) mmHg/Hz1/2 versus 2.77 (2.72-2.82) mmHg/Hz1/2 (P < 0.05); relative values, 35% (32-37%) versus 32% (29-32%) (P < 0.05). In heart rate spectra only the absolute low-frequency component decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1998

Noninvasive assessment of spontaneous baroreflex sensitivity in patients with liver cirrhosis.

AIMS/BACKGROUND: An impairment of baroreceptor sensitivity has been found in liver cirrhosis. Noninvasive and spontaneous estimates of baroreflex sensitivity are obtained from beat-to-beat blood pressure and heart rate recordings by means of cross-spectrum analysis and calculation of alpha-index (as a measure of baroreflex gain). The aim of the present study was to investigate the function of the spontaneous baroreflex sensitivity related to clinical Child score in liver cirrhosis. METHODS: The alpha-index was evaluated in 40 cirrhotic patients (18 with and 22 without ascites) and 17 healthy subjects by analysing finger arterial pressure recorded noninvasively with the Portapres device. RESULTS: Baroreflex sensitivity was significantly lower in cirrhotic patients with and without ascites compared with healthy subjects (p<0.01). Furthermore, in patients with ascites the baroreflex gain was significantly related to plasma sodium (p<0.01). A significant inverse relationship was present between baroreflex gain and grade of Child score and the severity of ascites (p<0.01). There were no significant relationships between hormonal parameters (catecholamines, renin, aldosterone, arginine-vasopressin, atrial natriuretic peptide and nitric oxide) and baroreflex gain. No significant differences were found between healthy subjects and cirrhotic patients with respect to systolic and diastolic blood pressure total variability in a supine position, whilst it was lower in cirrhotic patients with ascites in a tilted position (p<0.05). CONCLUSION: Our findings showed that baroreflex sensitivity was significantly impaired in cirrhotic patients when compared with healthy subjects. In addition, there was a significant trend toward lower baroreflex sensitivity values with the grade score of Child class (p<0.01). Spectral analysis of the alpha-index provides viable alternatives to the pharmacological approach for estimation of baroreflex sensitivity and may represent a prognostic tool to identify cirrhotic patients at increased risk of adverse events.

Aldosterone↗
PubMed1996

Prolongation of QTc interval and autonomic nervous dysfunction in diabetic patients.

We have attempted to clarify the relationships of the QTc interval to alpha and beta sympathetic, as well as parasympathetic function tests including spectral analyses of the R-R interval and systolic blood pressure. The QTc intervals were estimated in 76 diabetic patients and 76 age-matched healthy controls whose R-R intervals were comparable to those of the diabetic patients. We also investigated the relationships of the QTc interval to various clinical features of diabetes mellitus and to autonomic function tests. The QTc interval in diabetic patients was significantly greater than that in healthy controls. The QTc intervals were unrelated to diabetic control, retinopathy and nephropathy, but were prolonged in patients with a long duration of diabetes as compared to those with a short disease duration. There were significant correlations between the QTc interval and orthostatic hypotension, R-R variation and the Valsalva maneuver. Significant correlations were observed between the QTc interval and both the low and the high frequency component of spectral analysis of the R-R interval, whereas no correlations were found with spectral analysis of systolic blood pressure. An abnormal QTc interval is an indicator of cardiac sympathetic and parasympathetic nervous dysfunction, but not vasomotor dysfunction.

Blood Glucose↗
PubMed1978

Recent advances in EEG data processing.

It is argued that the most interesting advances in EEG signal processing are with methods based on descriptive mathematical models of the process. Formulation of auto-regressive (AR) and mixed autoregressive and moving average (ARMA) models is reviewed for the scalar and the multidimensional cases and extensions to allow time-varying coefficients are pointed out. Data processing with parametric models, DPPM, involves parameter estimation and a large number of algorithms are available. Emphasis is put on those that are simple to apply and require a modest amount of computation. A recursive algorithm by Levinson, Robinson and Durbin is well suited for estimation of the coefficients in the AR model and for tests of model order. It is applicable to both the scalar and multidimensional cases. The ARMA model can be handled by approximation of an AR model or by nonlinear optimization. Recursive estimation with AR and ARMA models is reviewed and the connection with the Kalman filter pointed out. In this way processes with time-varying properties may be handled and a stationarity index is defined. The recursive algorithms can deal with AR or ARMA models in the same way. A reformulation of the algorithm to include sparsely updated parameter estimates significantly speeds up the calculations. It will allow several EEG channels to be handled simultaneously in real time on a modern minicomputer installation. DPPM has been particularly successful in the areas of spectral analysis and detection of short transients such as spikes and sharp waves. Recently some interesting attempts have been made to apply classification algorithms to estimated parameters. A brief review is made of the main results in these areas.

Electroencephalography↗
PubMed2004

Sequential F0 comparisons between resolved and unresolved harmonics: no evidence for translation noise between two pitch mechanisms.

Carlyon and Shackleton [J. Acoust. Soc. Am. 95, 3541-3554 (1994)] suggested that fundamental-frequency (F0) discrimination performance between resolved and unresolved harmonics is limited by an internal "translation" noise between the outputs of two distinct F0 encoding mechanisms, in addition to the encoding noise associated with each mechanism. To test this hypothesis further, F0 difference limens (DLF0s) were measured in six normal-hearing listeners using sequentially presented groups of harmonics. The two groups of harmonics presented on each trial were bandpass filtered into the same or different spectral regions, in such a way that both groups contained mainly resolved harmonics, both groups contained only unresolved harmonics, or one group contained mainly resolved and the other only unresolved harmonics. Three spectral regions (low: 600-1150 Hz, mid: 1400-2500 Hz, or high: 3000-5250 Hz) and two nominal F0s (100 and 200 Hz) were used. The DLF0s measured in across-region conditions were well accounted for by a model assuming only two sources of internal noise: the encoding noise estimated on the basis of the within-region results plus a constant noise associated with F0 comparisons across different spectral regions, independent of resolvability. No evidence for an across-pitch-mechanism translation noise was found. A reexamination of previous evidence for the existence of such noise suggests that the present negative outcome is unlikely to be explained by insufficient measurement sensitivity or an unusually large across-region comparison noise in the present study. While the results do not rule out the possibility of two separate pitch mechanisms, they indicate that the F0s of sequentially presented resolved and unresolved harmonics can be compared internally at no or negligible extra cost.

Acoustic Stimulation↗
PubMed1991

Assessment of the neural control of the circulation during psychological stress.

In this study, we used spectral analysis of short-term R-R and systolic arterial pressure (SAP) variabilities to estimate the changes in neural control of the circulation produced by psychological stress. The 0.1 Hz low-frequency (LF) component of R-R and SAP variabilities provided a quantitative index of the sympathetic activity controlling heart rate and vasomotion. Conversely the high-frequency (HF) respiratory component of R-R variability provided an index of vagal tone. In conscious dogs we used the seemingly stressful situation of being accompanied for the first time to the experimental laboratory as a stimulus. In human subjects we used mental arithmetic. In both cases LF of R-R and SAP variabilities increased significantly suggesting enhanced sympathetic activity both to the SA node and the vasculature. In man, the index alpha, a measure of the overall gain of baroreceptor mechanisms, was found to be reduced during mental arithmetic. Spectral analysis of cardiovascular variabilities thus suggests that in man and in conscious dogs psychological challenges induce a profound re-arrangement of neural control of the circulation, which appears to be characterised by sympathetic predominance and which can be monitored by this technique.

Adult↗
PubMed1995

Approximate solutions for the maximum-likelihood estimates in models of univariate human twin data.

We present numerical results concerning the accuracy of approximate maximum-likelihood estimators of variance components for several models of univariate human twin data. The approximations are obtained via a spectral decomposition of the twin model covariance matrix. The results apply to likelihood functions for univariate twin data based on either the Wishart distribution or the bivariate normal distribution. For sample sizes of 100 twin pairs for each zygosity group, if the difference of the traces of the sample covariance matrices is 10% or less of the sum of the traces, the approximate solutions can be used as the maximum-likelihood estimators for some models.

Analysis of Variance↗
PubMedDate not supplied

Radiographic information theory: correction for x-ray spectral distribution.

A more complete computational method is developed to account for the effect of the spectral distribution of the incident x-ray fluence on the minimum exposure required to record a specified information set in a diagnostic radiograph. It is shown that an earlier, less rigorous, but simpler computational technique does not introduce serious errors provided that both a good estimate of the mean energy per photon can be made and the detector does not contain an absorption edge in the spectral range. Also shown is that to a first approximation, it is immaterial whether the detecting surface counts the number of photons incident from each pixel or measures the energy incident on each pixel. A previous result is confirmed that, for mammography, the present methods of processing data from the detector utilize only a few percent of the incident information, suggesting that techniques can be developed for obtaining mammograms at substantially lower doses than those presently used. When used with film-screen combinations, x-ray tubes with tungsten anodes should require substantially lower exposures than devices using molybdenum anodes, when both are operated at their optimal voltage.

Humans↗