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PubMed1990

The effects of burn injury on vasoactivity in hamster peripheral microcirculation.

The effects of mild thermal shock on the vasoactivity of microvessels were studied in the hamster skin flap window preparations. Diameter fluctuations in arterioles and venules, varying in size from 10 to 50 microns and at different branching order sites, were measured prior and subsequent to a local surface skin burn. The experimental vasomotion data were characterized in terms of mean, standard deviation, skewness, and kurtosis, and by the Prony spectral line estimator (PSLE), fast Fourier transform (FFT), and auto regression (AR) methods of spectral analysis. Following a mild burn the A1 (37-50 microns), A2 (30-50 microns), and A3 (25-41 microns) arterioles relaxed to a larger diameter by an average of 58, 20, and 13%, respectively. Dispersion statistics postburn showed a skewness close to normal while the kurtosis became more negative, indicating that the transient diameter curves were flatter. Both FFT and PSLE analyses indicate less energy in the signals postburn accompanied by a shift toward lower frequencies with decreased amplitude. In addition, there was a loss of certain frequencies from the spectrum and an increase in the interval for rhythmic activity. FFT analysis gave an idea of the trend while PSLE analysis was found to be highly unstable and dependent on the window size. AR results indicate that the process is stable and definite trends exist in the data, however, the data are not purely periodic.

Animals↗
PubMed2004

Variability in /s/ production in children and adults: evidence from dynamic measures of spectral mean.

Previous research has found developmental decreases in temporal variability in speech. Relatively less work has examined spectral variability, and, in particular, variability in consonant spectra. This article examined variability in productions of the consonant /s/ by adults and by 3 groups of children, with mean ages of 3;11 (years;months), 5;04, and 8;04. Specifically, it measured the influence of age, phonetic context, and syllabic context on variability. Spectral variability was estimated by measuring dynamic spectral characteristics of multiple productions of /s/ in sV, spV, and swV sequences, where the vowel was either /a/ or /u/. Mean duration, variability in duration, and coarticulation were also measured. Children were found to produce /s/ with greater temporal and spectral variability than adults. Duration and coarticulation were comparable across the 4 age groups. Spectral variability was greater in swV contexts than in sV or spV sequences. The lack of consistent effects of phonetic context on spectral variability suggests that the developmental differences were related to subtle variability in place of articulation for /s/ in the children's productions.

Adult↗
PubMedDate not supplied

Induced periodic hemodynamics in skeletal muscle of anesthetized rabbits, studied with multiple laser Doppler flow probes.

Periodic fluctuations, regular slow-wave flowmotion, were induced in the skeletal muscle of six mature anesthetized New Zealand White rabbits by acute femoral artery pressure reduction from a median control value of 78 to 36 mm Hg. This phenomenon was monitored simultaneously with four laser Doppler flowmetry (LDF) probes placed over the gastrocnemius muscle in a linear array with a spacing of 5 mm. The median relative peak-to-trough amplitude of the oscillatory flow patterns was 47%, white a frequency of approximately 2.5 cycles per minute (cpm), which remained relatively stable over an observation period of 1 h (+/- 15-20%). Application of two frequency analysis methods, Welch's FFT method and Prony Spectral Line Estimation yielded similar results and showed a correlation coefficient of r = +0.84. Spectral coherence between pairs of regular slow-wave flowmotion records decreased with increasing probe separation (0.28 at 5 mm; 0.01 at 15 mm). However, patterns of frequency fluctuation over time were significantly correlated between concurrent record pairs regardless of probe separation. These results suggest that the regular slow-wave flowmotion signal originates in regions that are independently regulated by local vasoactive sites, which may be called pacemakers. These sites may also be influenced by an additional common control mechanism, which may be myogenic/metabolic or central in nature.

Anesthetics, Intravenous↗
PubMed2005

EEG spectral analysis and coherence in children with hemiparetic cerebral palsy.

BACKGROUND: Hemiparetic cerebral palsy (HCP) is described as having two main forms: arm-dominant, associated with large cortical/subcortical lesions and leg-dominant, associated with central white matter lesions. MATERIAL/METHODS: Twelve children with HCP underwent clinical assessment and imaging studies. For each child, 20 artifact-free EEG epochs, each of 2s duration, were selected for spectral analysis and coherence functions. The objective of this research was to estimate EEG spectral power, interhemispheric coherence (ICoh), and intrahemispheric (Hcoh) coherence in children with HCP as compared to healthy children. RESULTS: Significant differences between the HCP and control children were detected in the distribution of alpha, theta, delta and beta rhythms over the left and right hemispheres. The ICoh values in the alpha band in the temporal, parietal and occipital regions were significantly lower in the HCP patients than in the controls. There was a significant ICoh increase in the HCP in the theta and delta band, involving frontal and temporal derivations. The significantly lower ICoh values in the HCP children in the beta band involved the frontal, central, parietal and occipital derivations. A higher HCoh value in the HCP children in the alpha band was detected at the right hemisphere. CONCLUSIONS: The lower ICoh at the temporal, parietal and occipital derivations in the alpha band implies hypoconnectivity between the right and left hemispheres. HCoh asymmetry, which implies relative hypoconnectivity within the left hemisphere as compared with the right, suggests that functional hemispheric differentiation may be diminished.

Adolescent↗
PubMed2005

Functional frontoparietal connectivity during encoding and retrieval processes follows HERA model. A high-resolution study.

Recent neuroimaging studies of long-term episodic memory have suggested that left prefrontal cortex predominates in encoding condition, whereas right prefrontal cortex predominates in retrieval condition (hemispheric encoding and retrieval asymmetry, HERA model). The present electroencephalographic (EEG) study investigated the functional coupling of fronto-parietal regions during long-term memorization of visuo-spatial contents (i.e. landscapes, interiors of apartments), to test the predictions of the HERA model. Global fronto-parietal coupling was estimated by spectral coherence, whereas the "direction" of the fronto-parietal information flow was estimated by directed transfer function (DTF). The EEG rhythms of interest were theta (4-7 Hz), alpha (8-13 Hz), beta (14-30 Hz), and gamma (30-45 Hz). Statistically significant coherence in line with the HERA model was obtained at the gamma band. Namely, the fronto-parietal gamma coherence prevailed in the left hemisphere during the encoding condition and in the right hemisphere during the retrieval condition. The DTF estimates of the gamma band showed a dominant parietal-to-frontal directional flow in the right hemisphere during the encoding condition and in the left hemisphere during the retrieval condition (i.e. hemisphere-condition combination not involved by the HERA model). In contrast, a balanced bidirectional flow of the fronto-parietal coupling was observed in the left hemisphere during the encoding condition and in the right hemisphere during the retrieval condition (i.e. hemisphere-condition combination involved by the HERA model). In conclusion, the present encoding-retrieval conditions induced maximal fronto-parietal gamma coupling with bidirectional information flow in the hemisphere-condition combination predicted by the HERA model.

Alpha Rhythm↗
PubMed2005

Effects of beam steering in pulsed-wave ultrasound velocity estimation.

Experimental and computer simulation methods have been used to investigate the significance of beam steering as a potential source of error in pulsed-wave flow velocity estimation. By simulating a typical linear-array transducer system as used for spectral flow estimation, it is shown that beam steering can cause an angle offset resulting in a change in the effective beam-flow angle. This offset primarily depends on the F-number and the nominal steering angle. For example, at an F-number of 3 and a beam-flow angle of 70 degrees , the velocity error changed from -5% to + 5% when the steering angle changed from -20 degrees to + 20 degrees . Much higher errors can occur at higher beam-flow angles, with smaller F-numbers and greater steering. Our experimental study used a clinical ultrasound system, a tissue-mimicking phantom and a pulsatile waveform to determine peak flow velocity errors for various steering and beam-flow angles. These errors were found to be consistent with our simulation results.

Blood Flow Velocity↗
PubMed2003

Application of an acoustic noise removal method to aircraft-based atmospheric temperature measurements.

An acoustic noise removal method is used to reject engine acoustical disturbances from aircraft-based atmospheric temperature measurements. Removal of engine noise from atmospheric temperature measurements allows a larger wave number range to be fit while quantifying the magnitude of atmospheric temperature turbulence. The larger wave number range was found to result in a more statistically certain spectral slope estimate, with up to a 50% reduction in the standard deviation of measured spectral slopes. The noise removal technique was found to break down under conditions of weak atmospheric temperature turbulence where the engine acoustical disturbance can be several orders of magnitude larger than atmospheric temperature turbulence.

Journal Article↗
PubMed2005

Multiple solvation configurations around phthalocyanine in helium droplets.

Recent measurements of the emission spectrum of phthalocyanine solvated in superfluid helium nanodroplets exhibit a constant 10.3 cm(-1) splitting of each emission line relative to the absorption spectrum. This splitting has been attributed to two distinct helium environments near the surface of the phthalocyanine molecule. Rigid-body path-integral Monte Carlo provides a means of investigating the origin of the splitting on a detailed microscopic level. Path-integral Monte Carlo simulations of 4He(N)-phthalocyanine at 0.625 K with N ranging from 24 to 150 show two distinct helium configurations. One configuration is commensurate with the molecular substrate and the other is a triangular lattice. We investigate the energetics of these two configurations and use a method for calculating electronic spectral shifts for aromatic molecule-rare-gas clusters due to dispersive interactions to estimate the spectral splitting that would arise from the two helium configurations seen for N=150. The results are in reasonable agreement with the experimentally measured splitting, supporting the existence of two distinct local helium environments near the surface of the molecule in the nanodroplets.

Journal Article↗
PubMed1986

Color constancy: a method for recovering surface spectral reflectance.

Human and machine visual sensing is enhanced when surface properties of objects in scenes, including color, can be reliably estimated despite changes in the ambient lighting conditions. We describe a computational method for estimating surface spectral reflectance when the spectral power distribution of the ambient light is not known.

Color Perception↗
PubMed1991

Reproducibility of 24-hour serum growth hormone profiles in man.

OBJECTIVE: To study the reproducibility of 24-h serum growth hormone (GH) concentration profiles in adults. DESIGN: 24-h serum GH concentrations were constructed by drawing blood samples at 20-min intervals. Four study occasions over a period of 1 year were chosen to assess the reproducibility. SUBJECTS: Six healthy adult male volunteers of normal height and weight and aged between 20 and 22 years. MEASURES: The resulting GH data arrays were analysed by Fourier transformation. Between and within individual variations were calculated and expressed in terms of coefficients of variation and data plotted to show variations between groups and individuals. RESULTS: The frequency component of GH secretion occurred with a dominant periodicity of between 160 and 240 min. Precise estimates of spectral power (strength of oscillatory activity) and period were obtained for group data, but such estimates cannot be inferred from a single profile. There was no significant difference in 24-h mean serum GH concentration over the year of study: occasion 1 (February) mean 2.0 mU/l (SD 0.5); occasion 2 (March) mean 3.7 mU/l (SD 2.8); occasion 3 (July) mean 2.7 mU/l (SD 1.4); occasion 4 (February) 2.5 mU/l (SD 1.5) (mean within individual coefficient of variation 35%, range 9-58). The concentrations of factors known to influence GH synthesis and secretion, insulin-like growth factor I, thyroxine, testosterone and oestradiol, varied little over the year of study. CONCLUSIONS: These data demonstrate that group data are reproducible in terms of oscillatory activity and the amount of GH secreted and that 24-h GH profiles should be used predominantly for analysing group data. The variability between individual profiles limits their value in the investigation of children with growth failure and suspected GH insufficiency.

Adult↗
PubMed2004

Influence of muscle fibre shortening on estimates of conduction velocity and spectral frequencies from surface electromyographic signals.

The study of surface electromyographic (EMG) signals under dynamic contractions is becoming increasingly important. However, knowledge of the methodological issues that may affect such analysis is still limited. The aim of the study was to analyse the effect of fibre shortening on estimates of conduction velocity (CV) and mean power spectral frequency (MNF) from surface EMG signals. Single fibre action potentials were simulated, as detected by commonly used spatial filters, for different fibre lengths. No physiological modifications were included with changes in fibre length, and thus only geometrical artifacts related to fibre shortening were investigated. The simulation results showed that the dependence of CV and MNF on fibre shortening is affected by the fibre location, electrode position and the spatial filter applied. With shortening of up to 50% for a fibre of 50 mm semi-length, the variations in CV and MNF estimates with shortening in bipolar recordings were 0.5% (CV) and 0.7% (MNF) for superficial fibres, and 3.6% and 5.1% for deeper fibres. Using the longitudinal double differential filter, under the same conditions, the percent variation was 0% and 0.2%, and 24.7% and 15.8%, respectively. The main conclusions were, first, muscle fibre shortening can significantly affect estimates of CV and MNF, especially for short fibre lengths. However, for long (semi-length >50 mm) and superficial fibres, this effect is limited for shortenings of up to 50% of the initial fibre length. Secondly, CV and MNF are almost equally affected by changes in muscle length; and, thirdly, sensitivity to fibre shortening depends on the spatial filter applied for signal detection.

Electrodes↗
PubMed1996

Estimation of pulmonary artery pressure by spectral analysis of the second heart sound.

The objective of the present work was to test and validate a noninvasive method based on spectral analysis of the second heart sound (S2) to estimate the pulmonary artery (PA) systolic pressure in 89 patients with a bioprosthetic heart valve. The technique was compared with continuous-wave Doppler estimation of PA systolic pressure in these patients. The heart sounds recorded at the pulmonary area on the chest wall were digitized by computer. The spectra of S2 and those of the aortic (A2) and the pulmonary (P2) components of S2 were computed with a fast-Fourier transform. Seven features were extracted from these spectra. The statistical analysis performed with the Pearson linear correlation coefficient showed that the best estimation of PA systolic pressure obtained by spectral phonocardiography (r = 0.84, SEE +/- 5.6 mm Hg, p <0.0001) was provided by the following equation: PA systolic pressure = 47 + 0.68 Fp - 4.4 Qp - 17 Fp/Fa - 0.15 Fs, where Fs and Fp are dominant frequencies associated with the maximal amplitude of the power spectra of S2 and P2, respectively, Qp is the quality of resonance of P2, and Fp/Fa is the ratio of the dominant frequencies of P2 and A2, respectively.

Adult↗
PubMed1996

Testing the indeterminacy of linear color mechanisms from color discrimination data.

It have previously been reported that, for some choices of the fixed spatial and temporal characteristics of test stimuli, it was possible to estimate the spectral sensitivities of chromatic mechanisms from chromatic discrimination data alone. If mechanism sensitivities could be reliably estimated for any choice of test stimuli characteristics, the influence of spatial and temporal factors on chromatic discrimination performance could be directly measured. Previous studies, using test stimuli with other spatio-temporal characteristics, have found equi-discrimination contours whose ellipsoidal shapes seem to preclude estimation of mechanisms. Since there is no commonly-accepted method for testing the adequacy of ellipsoidal fits of chromatic equi-discrimination contours, it is possible that alternative psychophysical procedures combined with more powerful statistical tests could detect the pattern of deviations from ellipticality reported previously. In this paper, we described psychophysical tests and statistical analyses that, taken together, provide a more powerful test of the indeterminacy of mechanisms than previous methods. We develop a method based on analysis of residuals for detecting the pattern of deviations from ellipticality. We apply these tests under fixed experimental conditions similar to those in which other researchers have found ellipsoidal equi-discrimination contours. For these conditions, for any of the tests performed, we do not reject the hypothesis that equi-discrimination surfaces are ellipsoidal.

Color Perception↗
PubMed1985

Effects of anaesthesia for caesarean section on the computerized EEG of the neonate.

The effects of anaesthetic techniques and drugs on 18 neonates born after elective Caesarean section were studied, employing computerized EEG. Three different groups, epidural, spinal and general anaesthesia, were compared with each other and with a control group after normal vaginal delivery. A new computerized EEG method, Zero Band Power Rate (ZBPR) analysis, was devised for this study and implemented together with conventional spectral EEG estimates of five frequency bands. ZBPR analysis was found to be much more sensitive than conventional spectral analyses for detecting differences between the study groups. Significant differences occurred between the general anaesthesia group and the epidural and control groups and also between the spinal group and the epidural and control groups. No significant difference was seen between the epidural group and the control group. Significant differences were attributed to the clinical use of two drugs: thiopentone in the general anaesthesia group and ephedrine in the spinal group. The effects of these drugs on the neonatal EEG were reflected in decreased ZBPR values, particularly during sleep.

Adolescent↗
PubMed1989

The relationship between coherence and nonlinear characteristics in Renshaw cell responses to random motor axon stimulation.

Cat spinal Renshaw cells were activated by stimulating muscle nerves or ventral roots with random (pseudo-Poisson) patterns of brief electrical stimuli. This input pattern is optimal for a comparative study in both the frequency- and time-domain. The frequency-dependent variable of particular interest in this study was the coherence as a measure of the degree to which signal transmission is linear and noise-free; it was estimated via spectral analysis. Time-domain analysis consisted of calculating peri-stimulus time histograms in order to estimate the amount of nonlinearity in the cell responses to pairs of stimuli. The main result was that the amount of nonlinearity measured in this way did not profoundly depress the coherence. Two types of peri-stimulus time histogram were calculated: the "conventional" peri-stimulus time histogram (as a reference) computed with respect to all the stimuli in a train, and the "conditional" peri-stimulus time histogram computed with respect to the second in pairs of stimuli which were separated from each other by varied intervals delta. The latter type of peri-stimulus time histogram showed that Renshaw cell responses to stimuli were conditioned by preceding stimuli, which could facilitate (at small delta s) and/or more often depress (up to several hundreds of milliseconds) the subsequent responses in a nonlinear manner. The objective of this study was to test the hypothesis that nonlinear characteristics contribute significantly to depress the coherence from its optimal value (1).(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed2005

The use of stationarity and nonstationarity in the detection and analysis of neural oscillations.

Using available signal (i.e., spectral and time-frequency) analysis methods, it can be difficult to detect neural oscillations because of their continuously changing properties (i.e., nonstationarities) and the noise in which they are embedded. Here, we introduce fractally scaled envelope modulation (FSEM) estimation which is sensitive specifically to the changing properties of oscillatory activity. FSEM utilizes the fractal characteristic of wavelet transforms to produce a compact, two-dimensional representation of time series data where signal components at each frequency are made directly comparable according to the spectral distribution of their envelope modulations. This allows the straightforward identification of neural oscillations and other signal components with an envelope structure different from noise. For stable oscillations, we demonstrate how partition-referenced spectral estimation (PRSE) removes the noise slope from spectral estimates, yielding a level estimate where only peaks signifying the presence of oscillatory activity remain. The functionality of these methods is demonstrated with simulations and by analyzing MEG data from human auditory brain areas. FSEM uncovered oscillations in the 9- to 12-Hz and 15- to 18-Hz ranges whereas traditional spectral estimates were able to detect oscillations only in the former range. FSEM further showed that the oscillations exhibited envelope modulations spanning 3-7 s. Thus, FSEM effectively reveals oscillations undetectable with spectral estimates and allows the use of EEG and MEG for studying cognitive processes when the common approach of stimulus time-locked averaging of the measured signal is unfeasible.

Acoustic Stimulation↗
PubMed2001

Temporal autocorrelation in univariate linear modeling of FMRI data.

In functional magnetic resonance imaging statistical analysis there are problems with accounting for temporal autocorrelations when assessing change within voxels. Techniques to date have utilized temporal filtering strategies to either shape these autocorrelations or remove them. Shaping, or "coloring," attempts to negate the effects of not accurately knowing the intrinsic autocorrelations by imposing known autocorrelation via temporal filtering. Removing the autocorrelation, or "prewhitening," gives the best linear unbiased estimator, assuming that the autocorrelation is accurately known. For single-event designs, the efficiency of the estimator is considerably higher for prewhitening compared with coloring. However, it has been suggested that sufficiently accurate estimates of the autocorrelation are currently not available to give prewhitening acceptable bias. To overcome this, we consider different ways to estimate the autocorrelation for use in prewhitening. After high-pass filtering is performed, a Tukey taper (set to smooth the spectral density more than would normally be used in spectral density estimation) performs best. Importantly, estimation is further improved by using nonlinear spatial filtering to smooth the estimated autocorrelation, but only within tissue type. Using this approach when prewhitening reduced bias to close to zero at probability levels as low as 1 x 10(-5).

Artifacts↗
PubMed2005

[Spectral analysis of heart period variability in anorexia nervosa].

The aim of this study was to investigate the response of autonomic cardiac control to postural change using spectral analysis, in patients with anorexia nervosa. Spectral components of total variability as well as of low and high frequencies were analyzed for 17 anorexic patients with mean body mass index (14.9 +/- 1.9) kg/m2 and for 9 healthy age-matched women with body mass index (20.3 +/- 1.7) kg/m2 , in supine and standing postures. During standing posture, increased heart rate in all subjects was accompanied by the decrease in total variability and high frequency spectral powers. In supine posture, anorexic patients demonstrated the reduced low frequency spectral power. Compared to control women, during standing posture anorexic patients showed higher heart rate, reduced total variability and high frequency spectral powers. Statistically significant correlation was noticed between body mass index and spectral power of low frequency in both supine and standing posture. Alterations in autonomic cardiac control induced by anorexia nervosa could be estimated by spectral analysis of heart period variability.

Adult↗