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At least 487 records · Page 27Linked to original sources
PubMed2005

Surface EMG of jaw elevator muscles: effect of electrode location and inter-electrode distance.

This study addresses methodological issues on surface electromyographic (EMG) signal recording from jaw elevator muscles. The aims were (i) to investigate the sensitivity to electrode displacements of amplitude and spectral surface EMG variables, (ii) to analyse if this sensitivity is affected by the inter-electrode distance of the bipolar recording, and (iii) to investigate the effect of inter-electrode distance on the estimated amplitude and spectral EMG variables. The superficial masseter and anterior temporalis muscles of 13 subjects were investigated by means of a linear electrode array. The percentage difference in EMG variable estimates from signals detected at different locations over the muscle was larger than 100% of the estimated value. Increasing the inter-electrode distance resulted in a significant reduction of the estimation variability because of electrode displacement. A criterion for electrode placement selection is suggested, with which the sensitivity of EMG variables to small electrode displacements was of the order of 2% for spectral and 6% for amplitude variables. Finally, spectral and, in particular, amplitude EMG variables were very sensitive to inter-electrode distance, which thus should be fixed when subjects or muscles are compared in the same or different experimental conditions.

Adult↗
PubMed2002

Optimization of Ga-67 imaging for detection and estimation tasks: dependence of imaging performance on spectral acquisition parameters.

UNLABELLED: We have compared the use of two (93 and 185 keV) and three (93, 185, and 300 keV) photopeaks for Ga-67 tumor imaging and optimized the placement of each energy window. METHODS: The bases for optimization and evaluation were ideal and Bayesian signal-to-noise ratios (SNR) for the detection of spheres embedded in a realistic anthropomorphic digital torso phantom and ideal SNR for the estimation of their size and activity concentration. Seven spheres of radii ranging from 1 to 3 cm, located at several sites in the torso, were simulated using a realistic Monte Carlo program. We also calculated the ideal SNR for the detection from simple phantom acquisitions. RESULTS: For detection and estimation tasks, the optimum windows were identical for all sphere sizes and locations. For the 93 keV photopeak, the optimal window was 84-102 keV for the detection and 87-102 keV for estimation; these windows are narrower than the 20% window often used in the clinic (83-101 keV). For the 185 keV photopeak, the optimal window was 170-220 keV for the detection and 170-215 keV for estimation; these are substantially different than the 15% window used in our clinic (171-199 keV). For the 300 keV photopeak, the optimal window for detection was 270-320 keV, and for estimation, 280-320 keV. Using the three optimized, rather than only the two lower-energy, windows yielded a 9% increase in the SNR for the detection of the 3 cm diam sphere (a 12% increase for a 2 cm diam sphere) and a 7% increase in the SNR for estimation of its size. For the acquired phantom data, detection also increased by 9%-12% when using three, rather than two, energy windows.

Abdomen↗
PubMedDate not supplied

Relationships between circulating catecholamines and low frequency heart period variability as indices of cardiac sympathetic activity during mental stress.

Heart rate increases during psychological stress are the product of cardiac sympathetic activation and parasympathetic withdrawal. Levels of plasma epinephrine (E) and norepinephrine (NE) have a long history as indicators of cardiac adrenergic activity and, accordingly, generally increase in response to psychological challenge. Recently, several investigators have suggested that indices derived from power spectral analysis of heart period variability (HPV) also may provide estimates of cardiac sympathetic nervous system activity. These indices include power in the low frequency band (0.04-0.15 Hz, LF), and the ratio of low to high frequency (0.15-0.50 Hz, HF) power (LF/HF). The relationship between spectral and neurohumoral indices during psychological stress has not been investigated. This issue was addressed by studying spectrally defined measures of HPV and levels of plasma E and NE in 34 normal subjects who participated in a study of responsiveness to a psychologically challenging arithmetic task. Heart rate (HR), LF and HF power, the LF/HF ratio, and blood pressure were measured during the 5-minute baseline and 5-minute task periods. Integrated samples of forearm venous blood were collected for both periods. E and NE were analyzed by high performance liquid chromatography. The task produced significant increases in HR, systolic and diastolic pressures, and NE. Of the 12 Pearson correlation coefficients used to examine the relationships between power spectral measures and catecholamines for the baseline, task, and delta values, none achieved statistical significance, suggesting little relationship between neurohumoral and spectral estimates of cardiac sympathetic activity. We conclude that under conditions of psychological stress, LF power provides no useful information about cardiac sympathetic activity, both because power in this frequency band falls whereas HR rises and because there is no relationship between LF power and plasma NE.

Adult↗
PubMed1995

Dipole modelling of MEG rhythms in time and frequency domains.

Local generators of spontaneous brain rhythms can be identified from the measured magnetic field pattern both in the time and frequency domains when the sources are dipolar. The dipole assumption is most efficient when only a few sources contribute to the signal at one time or frequency. We have quantified the effects of different filters and spectral transformation sizes in the analysis of spontaneous magnetic activity measured simultaneously over the whole head to determine the optimal values for maximum identification probability of localized (dipolar) activity. The criteria employed were (i) the percentage of dipolar sources, (ii) the goodness-of-fit of the dipole model, (iii) the confidence volumes and (iv) spatial distributions of the source sites, and (v) the effective spectral resolving power of the various Fast Fourier transform (FFT) sizes. The systematic changes of (i-v) suggested the use of 3-5 Hz passbands around the major spectral peaks and FFT lengths of 2-3 s for extraction of the maximum amount of information from this data set. The most successful choices of filter passband and spectral transformation length for source localization simultaneously provide estimates for the inherent spectral fluctuation of cortical rhythms (0.5 Hz) and for the activation lifetime of individual sources (0.3 s).

Adult↗
PubMed2004

[Effect of chronic lead poisoning on the parameters of heart rate variability].

BACKGROUND: The dysfunction of the autonomic nervous system is one of the effects of chronic exposure to lead. Heart rate variability (HRV) is a non-invasive method used to estimate the autonomic system. The aim of the study was to estimate HRV in people occupationally exposed to lead. MATERIALS AND METHODS: The study group included 43 copper-smelters and 43 healthy subjects matched individually. A 24-hour ECG Holter monitoring was performed in all men. RESULTS: The HRV time analysis in a 24-hour observation, as well as for daily activities (6:00-22:00) and nocturnal sleep (22:00-6:00) showed that pNN50 parameter, an estimator of the vagus nerve function, was significantly lower in copper-smelters than in healthy subjects (p < 0.05). The majority of time and spectral HRV parameters, estimated at 15-min intervals during day- and night-time in the group of lead exposed men, were lower than in the control group (p < 0.05). CONCLUSIONS: In subjects occupationally exposed to lead, HRV is lower than in healthy men, which results rather from the decreased parasympathetic than from the increased sympathetic activity.

Adult↗
PubMed2002

Visible spectral dependence of the scattering and absorption coefficients of pigmented coatings from inversion of diffuse reflectance spectra.

A spectral-projected gradient method and an extension of the Kubelka-Munk theory are applied to obtain the relevant parameters of the theory from measured diffuse reflectance spectra of pigmented samples illuminated with visible diffuse radiation. The initial estimate of the spectral dependence of the parameters, required by a recursive spectral-projected gradient method, was obtained by use of direct measurements and up-to-date theoretical estimates. We then tested the consistency of the Kubelka-Munk theory by repeating the procedure with samples of different thicknesses.

Journal Article↗
PubMed1996

Estimation of arterial blood pressure variability by spectral analysis: comparison between Finapres and invasive measurements.

In order to assess the accuracy of spectral analysis of beat-by-beat arterial blood pressure fluctuations recorded by the finger technique we analysed simultaneous ipsilateral resting sequences (180 s) of systolic blood pressure (SP) and diastolic blood pressure (DP) obtained by brachial artery cannulation and by the Finapres device in 26 post-myocardial infarction (post-MI) and 24 chronic heart failure (CHF) patients. We estimated: (i) the total variability and the power in the bands 0.01-0.04 Hz (band 1), 0.04-0.07 Hz (band I), 0.07-0.15 Hz (band III) and 0.15-0.45 Hz (band IV); and (ii) the group-average transfer function between the invasive and non-invasive channel. The total variability of intra-arterial SP was greatly enhanced by the Finapres device in both populations (p < 0.01) mainly due to the increase of power in bands I and II (mean percentage errors 157 and 111% in post-MI and 368 and 245% in CHF patients respectively). The power in band IV was significantly reduced in post-MI (p < 0.001) but not in CHF patients. The total DP variability increased in post-MI patients (p < 0.001) due to the increase in bands I and II (mean percentage errors 62 and 43% respectively). No significant difference was found in bands III and IV of the same group and in the overall variability of CHF. Linear system analysis showed that intra-arterial SP oscillations in both groups are amplified by the Finapres device in bands I and II, with a higher gain in CHF patients, whereas they are attenuated in band IV. Band III acts as a transition band. DP oscillations recorded non-invasively are slightly amplified in bands I and II of post-MI patients only and are attenuated in band IV of both groups. A substantial fraction (approximately 30%) of the non-invasive SP variability in band I cannot be explained as a pure linear transformation (amplification) of the invasive signal. A phase delay in the range 7 degrees-10 degrees relates the non-invasive signal to the invasive one in the region 0.025-0.17 Hz and approaches zero elsewhere. Several mechanisms were examined as potential causes of the invasive-non-invasive differences. We argue that these differences are mainly due to the complex rhythmic behaviour of peripheral resistance vessels.

Adult↗
PubMed2003

Comparison of various techniques used to estimate spontaneous baroreflex sensitivity (the EuroBaVar study).

This study compared spontaneous baroreflex sensitivity (BRS) estimates obtained from an identical set of data by 11 European centers using different methods and procedures. Noninvasive blood pressure (BP) and ECG recordings were obtained in 21 subjects, including 2 subjects with established baroreflex failure. Twenty-one estimates of BRS were obtained by methods including the two main techniques of BRS estimates, i.e., the spectral analysis (11 procedures) and the sequence method (7 procedures) but also one trigonometric regressive spectral analysis method (TRS), one exogenous model with autoregressive input method (X-AR), and one Z method. With subjects in a supine position, BRS estimates obtained with calculations of alpha-coefficient or gain of the transfer function in both the low-frequency band or high-frequency band, TRS, and sequence methods gave strongly related results. Conversely, weighted gain, X-AR, and Z exhibited lower agreement with all the other techniques. In addition, the use of mean BP instead of systolic BP in the sequence method decreased the relationships with the other estimates. Some procedures were unable to provide results when BRS estimates were expected to be very low in data sets (in patients with established baroreflex failure). The failure to provide BRS values was due to setting of algorithmic parameters too strictly. The discrepancies between procedures show that the choice of parameters and data handling should be considered before BRS estimation. These data are available on the web site (http://www.cbi.polimi.it/glossary/eurobavar.html) to allow the comparison of new techniques with this set of results.

Adult↗
PubMed2004

The subthalamic nucleus in Parkinson's disease: power spectral density analysis of neural intraoperative signals.

To test a new tool for the neurophysiological identification of the human subthalamic nucleus (STN) during stereotactic surgery for the implantation of deep-brain-stimulation (DBS) electrodes, we analysed off-line the intraoperative signals recorded from patients with Parkinson's disease. We estimated the power spectral density (PSD) along each penetration track (8 patients, 13 sides) and determined the spatial correlation of the PSD with the target location estimated from neuroimaging procedures ("anatomical target"), and with the final target location derived from standard intraoperative neurophysiological procedures for STN localization ("clinical target"). At each step we recorded the 'on-line' signal for 120 seconds; because the PSD was estimated by calculating the periodogram for 6-second epochs of neural signal, we had 20 epochs at each step. When the electrode track crossed the STN, the PSD in the 0.25-2.5 kHz band increased, peaking on average <0.5 mm cranial to the clinical target and 1.00+/-1.51 mm caudal to the anatomical target. When the track was outside the nucleus, the PSD remained unchanged. Even on recordings with low signal-to-noise ratio, off-line PSD analysis of neural signals showed a good correspondence with the target indicated by the surgical team. On-line intraoperative estimation of the PSD may be a simple, reliable, rapid and complementary approach to electrophysiological monitoring during STN surgery for Parkinson's disease.

Aged↗
PubMedDate not supplied

Quantitative 31P MRS of the normal adult human brain. Assessment of interindividual differences and ageing effects.

The characteristics of the 31P MR spectra from a large central volume in the brain of 47 healthy adults (aged 25-85 years) were assessed. Spectral parameters were estimated by means of a time-domain fitting technique. Statistical uncertainties of the estimates were determined by means of the Cramer-Rao theory and minimized by introducing a priori knowledge into the fitting procedure. Age-dependency of the spectral parameters was assessed by means of linear regression. Significant differences between individuals were established for some parameters. A significant age-dependency (p < or = 0.001) of ca 20% over the age range considered was found for the intensity of the phosphocreatine resonance line.

Adenosine Triphosphate↗
PubMed1989

An automated iterative algorithm for the quantitative analysis of in vivo spectra based on the simplex optimization method.

The success in utilizing in vivo NMR to identify and/or monitor metabolic abnormalities will be determined in large part on the reliability with which the spectral parameters of the metabolites present can be measured. For these reasons it is clear that there is a need for the development of algorithms with which to obtain quantitatively reliable estimates of the spectral parameters of the peaks present. In this report we describe an adaptation of the simplex algorithm which we have found useful in fitting in vivo spectral data in the frequency domain. This simplex algorithm was implemented on an IBM-PC AT compatible computer. We evaluated the simplex algorithm on three representative kinds of spectral data: a simulated spectrum, 31P spectrum of normal calf muscle, and the 31P spectrum of a pediatric patient with a brain tumor. In each case we generated a set of spectra by adding varying amounts of noise. On the basis of our simulations and the two examples discussed, we conclude that the simplex method generates parameters which are reliable estimates of the areas of the peaks present when the signal-to-noise is above 8:1 for phosphocreatine. We found that the speed of convergence of the algorithm was improved by overestimating the linewidths of the peaks present. We also found that the method converged more rapidly in the presence of a moderate amount of noise. We conclude that the algorithm described here can provide a robust method with which to analyze in vivo spectra in a quantitative manner. Because the method requires little user intervention, it lends itself to implementation in a semi-, or fully, automated fashion.

Algorithms↗
PubMed2004

Spectral pattern complexity analysis and the quantification of voice normality in healthy and radiotherapy patient groups.

Vocal fold functionality may alter in response to direct radiotherapy or indirectly by perturbation of the hypothalamic-pituitary axis. Perceptual assessment of voice quality is difficult to summarise in a single, reliable figure of normality and normality itself is undefined. In this study spectral analysis of vocal fold vibration, based on impedance variations measured across the larynx using an electro-glottogram, is used to build a single parameter description of standard vowel phonation in the normal male population. Patient data and perceptual assessment are then compared to this standard. The spectral pattern of the vowel/i/ electro-glottogram time series is analysed using approximate entropy after dynamic fundamental-harmonic frequency normalisation. The approximate entropy provides a single estimate of the spectral pattern complexity. A cohort of 89 normal males formed two statistically distinct groups, G1, with strong spectral pattern and high complexity 0.338 (+/-0.036), and G2 with a weak spectral pattern and low complexity 0.175 (+/-0.049). Membership ratio G1:G2 was 2:1. A cohort of 30 male larynx cancer cases were analysed approximately 3-6 months after irradiation, and three male prophylactic cranial irradiation cases some years after treatment. Two-thirds of patients had G2 or lower levels of complexity. The lower G2 complexity level appears to be the subjective, as well as the objective, threshold for voice normality.

Algorithms↗
PubMed1992

Evaluation of a method of simulating reduced frequency selectivity.

The accuracy of a method of simulating reduced frequency selectivity by the spectral smearing of complex stimuli has been evaluated. First an excitation pattern that would be evoked by a given nonsmeared stimulus in an impaired ear with broad auditory filters was estimated. Then the spectral smearing of the stimulus that would be necessary to create the same excitation pattern in a normal ear was calculated. The smearing was based on the shapes of simulated broad auditory filters; both symmetric and asymmetric broad filters were simulated. The method was used to process notched noise, and tones in notched noise, and the processed stimuli were used in a series of experiments with normally hearing subjects measuring the threshold for the tone in notched noise. The resulting data were used to derive auditory filter shapes. The derived filter shapes were generally similar to the expected shapes (based on the type of spectral smearing used), but there were some systematic discrepancies and some individual differences. The discrepancies do not seem to be due to the use of information derived from phase locking, since they were observed both at 1 kHz (where phase locking occurs) and at 6 kHz (where phase locking probably does not occurs). The discrepancies also do not seem to be due to the transmission characteristics of the outer/middle ear, since they occurred both when these characteristics were taken into account in the fitting procedure, and when the stimuli were preshaped to compensate for these characteristics. The influence of the subjects' own auditory filters probably can explain some of the discrepancies; the excitation pattern evoked by the spectrally smeared stimuli can be significantly influenced by the subjects' own filters when those filters are not much sharper than the simulated filters used to produce the smeared stimuli. Finally, some of the discrepancies can probably be explained by subjects combining information across auditory filters, rather than just using the single 'best' filter in each condition; this represents a limitation of the fitting procedure rather than of the simulation itself. Overall, the simulation worked reasonably well, especially when the smearing was based on symmetric filters.

Attention↗
PubMed1998

pH-dependent fluorescence of a heterologously expressed Aequorea green fluorescent protein mutant: in situ spectral characteristics and applicability to intracellular pH estimation.

The green fluorescent protein of Aequorea victoria (GFP) is a natural peptide chromophore without substrate or cofactor requirements for fluorescence. In vitro, a recombinant F64L/S65T GFP mutant (GFPmut1) exhibited pH sensitive fluorescence within the physiologic range. When heterologously expressed in BS-C-1 cells or rabbit proximal tubule cells, uniform cytosolic and nuclear fluorescence was observed. Cytosolic fluorescence constituted over 80% of the total. Excitation scanning of transfected cells revealed two GFPmut1-specific regions that were pH-sensitive over the physiologic range, and each region exhibited a unique pH "bias" in fluorescence emission. Excitation at or near the expected maximum of 488 nm (region II) uniformly resulted in fluorescence that was preferentially altered at acidic pH. In contrast, a novel "wild-type" excitation peak at 400 nm (region I) resulted in alkaline-biased fluorescence similar to that described for the wild-type chromophore in vitro, suggesting that wild-type spectral features disrupted in vitro by mutagenesis may be recovered in intact cells. Calibration of intracellular pH (pHi) with in situ fluorescence following excitation in either region revealed a semilogarithmic relationship between fluorescence intensity and pH within the physiologic range. We therefore measured pHi changes attributable to altered Na/HCO3 cotransport (NBC) activity both in GFPmut1-expressing cells and in paired untransfected cells loaded with BCECF. Basal NBC activity was the same in each group, as was the stimulation of activity by 10% CO2, thus validating the utility of GFPmut1 as a fluorescent probe for pHi and establishing a novel, useful, and practical application for GFPmut1 in monitoring pHi in real time.

Animals↗
PubMed1979

Optimum probabilistic processing in colour perception. II. Colour vision as template matching.

A statistical approach to account for psychophysical phenomena in human colour vision is presented. The central visual processor is viewed as an optimum recognizer of stochastic patterns supplied by the periphery. The processor makes an optimum estimate of the spectral parameters of the stimulus, given the wavelength filter characteristics of the periphery, the stochastic nature of the information and an internal template to which the external stimulus is matched. The estimate is constrained in ways inferred from empirical phenomena. Subjective brightness of monochromatic stimuli and related constant brightness manifolds in the colour space constitute the constraint for brightness estimation. Results analogous and in accord with those of earlier line element theories are obtained. The Bezold-Brücke hue shift constitutes the basic constraint for hue estimation. The hue estimate involves interrelation between the fields in the experiment. Similarities and differences both in basic conceptions and results introduced by the template matching notions are discussed.

Color Perception↗
PubMed1986

Robust spectral analysis of the EEG.

Robust methods for the spectral analysis of time series are briefly reviewed and seen to have applications in the field of EEG. After presenting two simple schemes for outliers (artifacts) generation and discussing their implications for estimation of the spectral density, the robust filtering algorithm of Kleiner et al. [J.R. Statist. Soc. Ser. B, 41: 313-351, 1979] is introduced and shown to work well for simulated data and for true EEG data containing artifacts. A new use of the robust methods for the detection of artifacts and possibly other transients in long-time EEG recordings is suggested and a preliminary implementation illustrated.

Electroencephalography↗
PubMed1987

Component analysis of spatial and spectral patterns in multispectral images. I. Basis.

A new (to our knowledge) theory of component pattern analysis in multispectral images is developed by using the methods of principal component analysis and nonlinear optimization with a nonnegativity constraint. Given images of a scene in different color bands, we estimate both the spectral curves of components included in the image and the spatial pattern corresponding to each spectral curve. In this method, neither spatial nor spectral features of the components are necessary, but the physical rule of nonnegative absorptivity and density nonnegativity is used for any material of any optical frequency at any position in the image. Experimental results of component analysis with real microscopic image data are shown to demonstrate the effectiveness of the proposed method.

Form Perception↗