Spectral Coherence Analysis Research Articles

Intercentral relationships of cortical bioelectrical activity of the human brain following operative interventions in brainstem structures.

The dynamics of the total bioelectrical activity and the parameters of its intercentral interaction was investigated in 52 neurosurgical patients with a focal lesion at the level of the brainstem with the aim of analysis of the compensatory neurodynamic reorganizations of the human brain in the early postoperative period. The study was based on the precise verification of the topography of the pathological focus within the limits of the brainstem; on the dynamic clinical-neurological assessment of different variants of the postoperative course; and on a study of the structure of cortical bioelectrical activity based on visual and spectral coherence analysis. The dependance is demonstrated of the compensatory neurodynamic reorganizations of the brain on the character and degree of the stem lesion (transitory, stable, irreversible).

A spatial length scale analysis of turbulent temperature and velocity fluctuations within and above an orchard canopy

Turbulent flow within and above an almond orchard was measured with three-dimensional wind sensors and fine-wire thermocouple sensors arranged in a horizontal array. The data showed organized turbulent structures as indicated by coherent asymmetric ramp patterns in the time series traces across the sensor array. Space-time correlation analysis indicated that velocity and temperature fluctuations were significantly correlated over a transverse distance more than 4m. Integral length scales of velocity and temperature fluctuations were substantially greater in unstable conditions than those in stable conditions. The coherence spectral analysis indicated that Davenport's geometric similarity hypothesis was satisfied in the lower frequency region. From the geometric similarity hypothesis, the spatial extents of large ramp structures were also estimated with the coherence functions.

Low coherence reflectometry and spectral analysis for detection of gain anomalies in semiconductor lasers

Commercial semiconductor lasers have been analysed by low coherence reflectrometry and by spectral analysis to reveal the location and relative strength of internal gain anomalies. The authors report the first observation of a correlation between the longitudinal co-ordinates of gain anomalies observed by low coherence reflectrometry and by independent mathematical analysis of the laser spectra.

Coherence analysis of the electrical activity of the rabbit brain in the process of the formation of the polarization dominant

It has been demonstrated by means of the method of spectral coherence analysis in rabbits, under conditions of a chronic experiment, that when a motor "dominant" reaction is still absent in the early stages of the dominant, interhemispheric asymmetry appears in the Coh spectra of the electrical activity of the sensorimotor cortex and the VPL nucleus of the thalamus. On the other hand, interhemispheric asymmetry appears in the Coh spectra of the electrical activity of the sensorimotor cortex and the CA3 field of the dorsal hippocampus only at the stage at which the motor "dominant" reaction is recorded. The asymmetry in the alpha and beta ranges of the frequencies in the Coh spectra of the biopotentials of the investigated regions, which coincides with the attainment of the motor "dominant" reaction, is associated with the processes of the organization of movement.

Performance of the human vestibuloocular reflex during locomotion.

1. The stability of gaze was measured in nine normal subjects during 30-s epochs of standing, walking in place, and running in place. The angle of gaze and head rotations in horizontal and vertical planes were measured using the magnetic search coil technique. Subjects visually fixed on a stationary object located at a distance of 100 m; thus measurements of gaze indicated the stability of images on the retina. 2. During standing, walking, or running in place, the standard deviation of the angle of gaze was less than 0.4 degrees, both horizontally and vertically. During standing and walking in place, peak gaze velocity (Gp) was less than 3.0 degrees/s. During running in place, Gp was less than 3.0 degrees/s horizontally but ranged up to 9.3 degrees/s vertically. 3. Visual acuity was measured during standing, walking, and running in place. During walking in place, five of nine subjects showed a small but significant (P = 0.03) decline in visual acuity compared with standing. During running in place, all nine subjects showed a small but significant (P = 0.002) decline in visual acuity compared with standing. 4. Stability of gaze was also measured during vigorous, voluntary head rotations in the horizontal (yaw) or vertical (pitch) planes, for 15-s epochs. Gp ranged as high as 70 degrees/s horizontally and 41 degrees/s vertically. All subjects reported illusory movement of the seen environment during these head rotations. 5. The suitability of linear systems techniques for analysis of the horizontal and vertical vestibuloocular reflex (VOR) during walking and running in place was assessed using coherence spectral analysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of spectral coherence analysis to describe the relationships between ambient ozone exposure and crop growth

Growth season-based time series spectral coherence analysis was performed between weekly changes in hourly ambient O 3 concentrations and weekly changes in alfalfa height growth. Weekly median hourly O 3 concentration and the corresponding weekly cumulative integral (sum of all hourly concentrations within the week) were used as indicators of weekly O 3 spectral density and coherence with the change in weekly alfalfa height growth. In general, the weekly cumulative integral performed much better than the weekly median O 3 concentration. A conceptual analysis of the results is presented, along with a recommendation that crop growth stage-based cumulative integrals merit further evaluation towards a better understanding of cause-effect relationships.

Moving Spectral Variance and Coherence Analysis and Some Applications on Long Air Temperature Series

Climatic records of a long time series (e.g., centuries) may be nonstationary. Thus, the stability of the variance (power) spectrum over a sequence of time periods is examined. Moreover, it is important to use different algorithms and tests in cases where there is an unknown or problematical physical background. Variance spectra of Hohenpeissenberg (FRG) annual mean air temperatures are compared using two methods, autocorrelation spectral analysis (ASA) and maximum entropy spectral analysis (MESA). These spectra are then compared with corresponding spectra based on Northern Hemisphere mean air temperature reconstructions where the ASA and MESA results are very similar. The application of a moving (running, “dynamic”) variance spectrum analysis shows that, in general, the signals found in the customary “integrated” spectrum vary as time varies, namely in their occurrence, significance and “bandwidth.” These findings are presented in terms of either contour lines of the relative variance (MESA) or contour lines of the confidence levels exceeded (ASA), where 50-yr subsamples are “moved” in 10-yr steps. Similarly, coherence spectra can be computed in moving terms. As an example the Northern Hemisphere data are spectrally correlated with the corresponding central England and Philadelphia air temperature series. It is shown that the coherencies are not stable in time, and that the spectral characteristics throw considerable doubt on the reliability of the reconstructed Northern Hemisphere temperature series prior to 1881. In general, moving spectral analysis of climatic time series improves the interpretation of climatic change.

Evidence for slow mode MHD turbulence in the solar wind: Post‐slow shock observations at 0.31 AU

First observational evidence is presented for low‐frequency slow mode MHD turbulence in the solar wind (ƒ ≤ 5 × 10−3 Hz, δB‐δN coherence greater than 0.6) by employing power and coherence spectral analysis of the Helios high time resolution solar wind plasma and magnetic field observations. This turbulence occurs directly behind an interplanetary slow forward shock wave at 0.31 AU. As the foreshock plasma conditions are such that steepening of slow waves is favored in comparison to Landau damping, the observations presented here do in addition provide unique support to the theoretical results of Hada and Kennel (1985). It is also shown that the turbulence upstream is predominantly Alfvénic (ƒ≲ 10−2 Hz, 10−2 Hz, δB‐δV coherence greater than 0.94) and propagating away from the Sun (shock). Thus we provide an example of an interplanetary slow shock where the turbulence upstream and downstream is completely different in nature. Moreover, the “evolutionary conditions” for slow shocks indicate that the downstream (upstream) turbulence could not have been generated by an interaction and wave mode conversion of the upstream (downstream) turbulence with and by the slow shock, respectively.

EEG spectral power and coherence analysis in healthy full-term neonates.

Continuous, bipolar EEG recording was performed in nineteen healthy full-terms during quiet sleep (Q), REM sleep (R) and wakefulness (W). Spectral power was calculated for the bilateral fronto-central (FC), centro-parietal (CP), parieto-occipital (PO) and temporal (TT) derivations. Coherence was determined for the interhemispheric biFC, biCP, biPO and biTT locations, as well as for the left and right intrahemispheric FC-PO and TT-PO combinations. Power was highest in Q in the CP derivations at 0.4-12/s. Generally, power was higher in Q than in R. In most derivations, Q and R power ran parallel. W power behaviour was different. Up to 8/s, power in W was intermediate between Q and R power. From 10/s, it became highest, particularly in the TT and FC derivations. PO power was low. Highest coherence values occurred in the lowest frequencies. Up to 8/s, biFC, biCP and biPO interhemispheric coherences were highest in R and usually lowest during Q. In the alpha-beta range, highest coherences occurred during Q and lowest during R or W, without much variance between stages. BiPO coherences were remarkably high during all stages. BiTT and intrahemispheric coherences were low and less consistent. These data suggest high synchronization of theta-delta activity between biPO regions, particularly during R. We hypothesize active common generators of neonatal theta-delta activity in the posterior regions, indicating less autonomy and more interdependent behaviour of these areas especially during R.

Signal and noise estimation from seismic reflection data using spectral coherence methods

Spectral coherence analysis is unrivaled as a quantitative tool over a range of practical problems in seismic interpretation, data processing, quality assessment for data acquisition, and research. Its great virtue is its ability to supply the detailed error information necessary for a thorough interpretation of results. Ordinary coherence analysis is employed in line intersection analysis and the design of filters to cross-equalize differently acquired seismic sections in a given area; both ordinary and partial coherence methods are indispensable in matching synthetic seismograms and seismic data; and multiple coherences are used to estimate the coherent signal and incoherent noise content of seismic sections and gathers. The precise meaning of the signal and noise estimates output by coherent analysis has to be related to the particular technique employed and the type of data input to it. The principles and procedures for analyzing seismic data with these methods are reviewed and illustrated with practical examples.

Spatial resolution and repeatability of MAGSAT crustal anomaly data over the Indian Ocean

Preliminary results from analysis of individual MAGSAT tracks in the eastern Indian Ocean demonstrate the resolution capability and repeatability of these data. Spectrum analysis indicates that 250 km is the absolute lower limit for resolving crustal magnetic anomalies in Magsat along‐track data. Spectral coherence analysis of closely‐spaced tracks shows that magnetic anomaly features with wavelengths greater than 700 km are highly repeatable from track to track.

EEG power and coherence during non-REM and REM phases in humans in all-night sleep analyses.

All-night spectral power and coherence analysis of six channels of EEG data from 6 healthy volunteers was performed. Integrated power and integrated coherence for the frequency bands of 0.1-7 and 7-12 Hz in 20 s epochs was plotted over the entire nights. Power and coherence increased with deepening slow wave sleep. With the onset of REM periods, power expectedly decreased, whereas coherence showed a further increase or maintained levels. With post-REM phase awakenings, power showed further reductions, and coherence decreased. The REM coherence results were most pronounced in interhemispheric right to left parietal comparisons (recorded vs. a Cz reference) in the 0.1-7 Hz band. It is hypothesized that the high interhemispheric coherence facilitates or reflects right-left transfer of information.

The use of partial and multiple coherence spectral analysis of accelerometer recordings in source and path identification of structure-borne noise on surface ships

Partial and multiple coherence spectral analysis was performed on accelerometer data, recorded on a U.S. Navy surface ship. A total of 48 accelerometers were mounted at predetermined locations on the ship, and vibration data were recorded on a 14-channel instrumentation tape recorder. The data were recorded in four groups, each consisting of twelve pickups. Taking groups of four pickups at a time with high pairwise coherences, a four-channel analog-to-digital converter and minicomputer were used to digitize and process the data. Partial and multiple coherence spectral analysis was performed on the digitized data. The analysis was found to be very useful in identifying major sources of structure-borne noise at various frequencies. Also the analysis was helpful in identifying major noise paths. The analysis was performed at the David W. Taylor Naval Ship Research and Development Center at Annapolis, MD, under the Surface Ship Silencing Program.

Separation of Hydrodynamic, Entropy, and Combustion Noise in a Gas Turbine Combustor

This paper deals with noise sources which are centra! to the problem of core engine noise in turbopropulsion systems. The sources dealt with are entropy noise and direct combustion noise, as well as a nonpropagating psuedosound which is hydrodynaniic noise. It is shown analytically and experimentally that a transition can occur from a combustion noise-dominant situation to an entropy noise-dominant case if the contraction of a terminating nozzle to the combustor is high enough. In the conibustor tested, entropy noise is the dominant source for propagationai noise if the combustor is choked at the exit. It is also speculated that there might be another unexplored noise source interior of the combustor. Analysis techniques include spectral, cross spectral, correlation, and ordinary and partial coherence analysis. Measurements include exterior and interior fluctuating and mean pressures and temperatures. T has been known for some time that there are at least two probable causes for core noise—entropy or indirect noise and direct combustion noise.! Direct combustion noise is caused by a fluid dilatation caused by a fluctuating heat release. Entropy noise is caused by hot (or cold) spots passing through the pressure gradients of. the turbine assembly. Both noise sources have the same fundamental cause—heat release fluctuations—but they are formed in a different manner. Entropy noise depends upon the heat release history following a fluid element through the combustor, whereas combustion noise depends on the instantaneous aggregate heat release rate fluctuation. The purpose of this program was to isolate the two suspected causes of core noise and determine their relative importance to the core noise problem. It is clear that core noise presents a noise floor in current turbopropulsion systems, but there is controversy concerning the strength of core noise relative to other sources.2'4 It is sufficient to remark here that core noise exists and is measurable. Entropy and combustion noise are not the only possibilities for core noise. Another possibility is vorticity- nozzle interaction nose,5 which is essentially a resistance of a nozzle to pass an axial velocity fluctuation. While sources other than entropy and combustion noise are not directly investigated here, the analysis techniques do reveal whether or not entropy and combustion noise are dominant over other sources. The analysis techniques presented essentially try to relate processes taking place inside a combustor to the noise radiated to the surroundings. One troublesome problem encountered in a prior program6 was the contamination of interior pressure fluctuation measurements by non- propagating psuedosound (hydrodynamic noise). Another purpose of this paper was to eliminate this contaminant as much as possible in order to concentrate on propagationai sound and its causes.

Power and cross-power spectral studies of electric and meteorological parameters under fair weather conditions in the atmospheric surface layer

The influence of micrometeorological parameters (wind speed, temperature stratification and atmospheric stability) upon electric polar conductivity, potential gradient, vertical total current density and vertical convection current density is studied by means of power spectral and coherence spectral analysis. The analyses show great variations in the atmospheric electric parameters for frequencies corresponding to the diurnal wave (0.042 cph) and in many cases great variances are noted for the frequencies 0.083, 0.125 and 0.17 cph. The influence of the micrometeorological parameters upon atmospheric electricity seems to be significant for these frequencies. Annual changes of the power and coherence spectra are also observed.

Reduction of epileptic seizures through EEG biofeedback training

Biofeedback training of the sensorimotor rhythm (SMR) was carried out in three male and three female adolescent epileptics and in two normal controls. The patients represented a cross-section of epilepsies including grand mal, myoclonic, afocal and psychomotor types. Three of the cases were mentally retarded. 12–14 Hz (SMR) activity was detected by a combination of sharp analog filtering and digital processing. The patients were provided with feed-back whenever they produced 0.5 sec of 12–14 Hz activity of a specified amplitude. Additional feedback was provided for epileptiform activity slow waves or movement. Furthermore, feedback for SMR production was inhibited by digital logic circuitry when movement, slow waves or spikes were present. Seizure reduction was obtained in five of the six epileptics. Several patients showed increased percentage of SMR when feedback was provided and varying degrees of normalization in their EEG as demonstrated by fast Fourier, crossed power spectral density and coherence analyses.

Intensity and coherence contours during self-regulated high alpha activity

Spectral intensity and coherence analysis of EEG activity during rest and during a period of self-regulated high alpha in two subjects indicate that: (1) Slow frontal activity, which is due primarily to eye movements, is less intense during successful alpha regulation than during baselines. (2) There is little evidence for increased intensity in frequency ranges other than the alpha range during alpha regulation. (3) Occipital alpha tends to be more coherent with frontal than with temporal alpha during occipital alpha regulation.