Low Coherence Values Research Articles

Bilateral effect of a unilateral voluntary modulation of physiological tremor

Verify whether a unilateral physiological tremor modulation attempt has a bilateral effect on central oscillators responsible for physiological tremor. Bilateral finger physiological tremor was recorded in 35 right-handed participants with laser displacement sensors in: (A) control condition, (B) modulation attempt of physiological tremor of the right index finger, (C) modulation attempt of physiological tremor of the right index finger by using co-contraction as a means to stabilize the finger. Physiological tremor amplitude was significantly reduced between the control and modulation conditions for the right index finger. Physiological tremor amplitude was also significantly reduced for the left index finger. Regression analysis of oscillation amplitudes showed little relationship between both fingers, even during modulation attempts. The lack of relationship between fingers is also evident by low coherence values obtained in the control and modulation conditions. The coherence in the co-activation condition was significantly higher, albeit still low, in each frequency band. Our results confirm that physiological tremor can be voluntarily reduced, and this reduction is bilateral. The modulation attempt did not however increase the frequency relationship between both sides. A central command aiming at modulating tremor amplitude will not increase the synchronization between oscillators responsible for the central components of physiological tremor.

Repeat-pass multi-temporal interferometric SAR coherence variations with Amazon floodplain and lake habitats

We have analysed interferometric coherence variations in Japanese Earth Resources Satellite (JERS-1) L-band synthetic aperture radar (SAR) data at three central Amazon sites: Lake Balbina, Cabaliana and Solimões-Purús. Because radar pulse interactions with inundated vegetation typically follow a double-bounce travel path that returns energy to the antenna, coherence will vary with vegetation type as well as with physical and temporal baselines. Lake Balbina consists mostly of upland forests and inundated trunks of dead, leafless trees whereas Cabaliana and Solimões-Purús are dominated by flooded forests. Balbina has higher coherence values than either Cabaliana or Solimões-Purús probably because the dead, leafless trees support strong double-bounce returns. The mean coherences of flooded woodland are 0.28 in Balbina and 0.11 in both Cabaliana and Solimões-Purús. With increasing temporal baselines, flooded and nonflooded wetland habitats show a steadily decreasing trend in coherence values whereas terra-firme and especially open-water habitats have little variation and remain lower in value. Flooded and nonflooded wetland coherence varies with the season whereas terra-firme and open water do not have similarly evident seasonal variations. For example, flooded habitats in all three study regions show annual peaks in coherence values that are typically 0.02 greater than coherence values from temporal baselines 180 days later, yet open water shows no variation with time. Our findings suggest that, despite overall low coherence values, repeat-pass interferometric coherence of flooded habitats is capable of showing the annual periodicity of the Amazon flood wave.

Very low frequency blood pressure fluctuations: not only myogenic responsiveness

Very low frequency blood pressure fluctuations: not only myogenic responsiveness

Modulation of human corticomuscular beta‐range coherence with low‐level static forces

Although corticomuscular synchronization in the beta range (15-30 Hz) was shown to occur during weak steady-state contractions, an examination of low-level forces around 10% of the maximum voluntary contraction (MVC) is still missing. We addressed this question by investigating coherence between electroencephalogram (EEG) and electromyogram (EMG) as well as cortical spectral power during a visuomotor task. Eight healthy right-handed subjects compensated isometrically static forces at a level of 4% and 16% of MVC with their right index finger. While 4% MVC was accompanied by low coherence values in the middle to high beta frequency range (25-30 Hz), a significant increase of coherence mainly confined to low beta frequencies (19-20 Hz) was observed with force of 16% MVC. Furthermore, this increase was associated with better performance, as reflected in decreased relative error in force during 16% MVC. We additionally show that periods of good motor performance within each condition were associated with higher values of EEG-EMG coherence and spectral power. In conclusion, our results suggest a role for beta-range corticomuscular coherence in effective sensorimotor integration, thus stabilizing corticospinal communication.

Interferometric SAR coherence magnitude estimation using second kind statistics

Coherence magnitude is a fundamental parameter for the analysis of applications using interferometric synthetic aperture radar (InSAR). The coherence magnitude estimators are biased and need bias removal. The sample coherence magnitude estimation, computed on a window basis, depends on the number of independent samples and theoretical coherence. It has been shown that the sample coherence magnitude estimator is the maximum-likelihood one. It is a biased estimator, especially for low coherence values. In this paper, we present a novel coherence magnitude estimator obtained from the method of moments using "second kind statistics". Classical methods (with regular statistics) for coherence computation are based on a probability density function (pdf) model for estimating regular moments (first kind statistics) defined with the Fourier transform. The proposed approach is based on the same pdf model to compute the second kind statistics defined with the Mellin transform (log-moment). Thus, it is shown that the estimated coherence given by the first log-moment is less biased. Moreover, it is shown that the coherence magnitude estimation from complex coherence maps (interferometric data) using second kind statistics is the optimal estimation procedure of interferometric coherence. It gives the smallest bias near zero comparing with existing estimators. The developed estimation approaches have been applied to obtain coherence measurements from tandem European Remote Sensing 1 and 2 satellite interferometric data, collected over varying terrain with a variety of ground cover types (agriculture field, forest, lake, urban area, sea) in Tunisia, France, and Nepal

EEG Coherence Analysis in Never-Medicated Patients with Panic Disorder

To investigate the functional abnormalities in the central nervous system (CNS) of patients with panic disorder (PD), we compared the electroencephalography (EEG) coherence values in 18 never-medicated PD patients with those in age-matched normal control subjects, and examined the relationships between EEG coherence values and both the duration of disease and the severity of panic attacks. EEG data were recorded in the resting state. The PD patients had lower coherence values with significant differences in F3-F4, C3-C4, P3-P4, F7-T5, and F8-T6. There were positive correlations for the higher alpha band between coherence values and both the duration of disease and the severity of panic attacks. These findings provide further evidence that PD patients have a lower degree of inter-hemispheric functional connectivity in the frontal region and intra-hemispheric functional connectivity in the bilateral temporal region, and that chronic condition or frequent panic attacks in PD patients may be related to the pathophysiological CNS changes.

Time-Series Analysis of Supersonic Base-Pressure Fluctuations

An investigation into the dynamic nature of supersonic base-pressure fluctuations was conducted. Two axisymmetric models, a blunt and a boattailed afterbody, were studied at a freestream Mach number of 2.46 and zero angle of attack. High-frequency pressure measurements were recorded with both radially and circumferentially positioned transducers. Blunt afterbody measurements showed rms pressure fluctuations on the order of 5% of the average base pressure. Power spectral density (PSD) estimates at the two outermost radial locations for the blunt afterbody revealed that a significant portion of the pressure fluctuation energy is contained in a peak centered near 850 Hz (Sr = 0.094). RMS pressure fluctuations on the boattailed afterbody were on the order of 4.3% of the average base pressure. The PSD estimates at the two outermost locations contained a more defined and larger peak centered near 800 Hz (Sr = 0.089) as compared to the blunt base. The two outermost radial locations on both afterbodies are subjected to similar pressure histories and, hence, display large values of coherence at low frequencies. The pressure history at the center position is not well correlated to those at the two outermost locations, resulting in low values of coherence. Possible mechanisms for the substantial pressure fluctuation energy contained at low frequencies are pulsing or flapping of the recirculation region, whereas higher-frequency energy could be attributed to large-scale turbulent structures in the shear layer.

Brain Electrical Activity Associated With Cognitive Processing During a Looking Version of the A‐Not‐B Task

This work was designed to investigate individual differences in brain electrical activity during a looking version of the A-not-B task. It was proposed that this spatial task required the cognitive skills of working memory and inhibitory control, each associated with frontal lobe function. Electroencephalograms (EEGs) were recorded from 54 8-month-old infants during baseline and task. Only high performers on the looking task exhibited increases in 6- to 9-Hz EEG power from baseline to task. These task-related changes were evident at frontal and posterior scalp locations. High performers on the looking task exhibited lower EEG coherence values at right hemisphere frontal locations relative to the low performers. These lower coherence values were evident during baseline and task. All infants showed increased frontal-parietal coherence during the spatial working memory task relative to baseline values. These data confirm previous cognitive neuroscience work associating frontal lobe function with cognitive performance levels during infancy.

Spectral properties of EEG fast activity ictal discharges associated with infantile spasms

Objective: The aim of this study was to evaluate the characteristics of the ictal EEG event accompanying infantile spasms.Methods: Quantitative analysis was used, based on the application of a bivariate autoregressive (AR) parametric model; autospectra, coherence, phase functions and inter-hemispheric time differences were estimated on homologous EEG channels in 18 infants presenting with either cryptogenic or symptomatic West syndrome.Results: The AR analysis of the 500 ms EEG epochs preceding spasm onset revealed the presence of a short discharge of fast activity restricted to a narrow frequency band in 13 of the 18 cases included in the study. The fast discharge peaked at 17.5±2.1 Hz, with rather low inter-hemispheric coherence values (0.52±0.17) and asymmetric amplitude on homologous EEG derivations. It persisted briefly after spasm onset, reaching a higher coherence value (0.71±0.16). The inter-hemispheric time difference, estimated in those cases with the coherence values significantly different from zero, ranged from 9.1 to 14.3 ms (11.4±1.9) in the epoch preceding spasm onset.Conclusion: The data obtained from the analysis of the ictal EEG events, compared with clinical and interictal EEG features, indicate that an asymmetric EEG pattern (mainly consisting of a rhythmic burst of fast activity) consistently preceded both symmetric and asymmetric spasms, thus suggesting a localized cortical origin of the ictal discharge giving rise to the spasms.

Adaptive noise cancellation with directional microphones

The use of adaptive noise cancellation in communication devices has now been under investigation for more than two decades. Early studies predicted large SNR gains that were rarely obtained when actual devices were tested in typical acoustic noise environments. One of the problems encountered was the lack of coherence between the ‘‘noise’’ sensor and the actual corrupting noise signal. The measured low coherence values were most probably due to time-varying multipath, multiple uncorrelated noise sources, and nonlinearities in the signal and noise transmission paths. This talk will address the expected noise cancellation that can be obtained by using directional microphones to spatially filter the acoustic field into signal-plus-noise and noise channels. Analytical results for spherically and cylindrically isotropic noise fields will be shown and discussed.

Transduction and adaptation in spider slit sense organ mechanoreceptors.

1. Mechanoreceptor neurons in spider (Cupiennlus salei) slit sense organ were examined by intracellular current- and voltageclarry recordings. Steps and pseudorandomly modulated displacement stimuli were delivered to the mechanosensitive cuticular slits. The resulting responses were used to determine the response dynamics and signal-to-noise ratio (SNR) of mechanoelectrical transduction. 2. Neurons were separated into two groups that, in terms of their afferent discharges, displayed different adaptations to displacement stimuli. Both responded at the onset of the step but then adapted fully, either immediately or within 10-200 ms. Voltage-clamp recordings showed only small differences in the receptor currents of the two groups. 3. Displacement of the slit caused a large inward current that decayed in seconds to a steady level of approximately 10-25% of the initial transient. When adapted to a steady displacement, the neurons responded to superimposed displacements in the same direction with additional transient currents, whose decay could be fitted by two exponentials with time constants of approximately 10 and 100 ms. In contrast, displacement in the opposite direction caused small "outward" currents without obvious adaptation. This behavior persisted with increasing background displacements, suggesting a shift in the displacement-response curve along the displacement axis. 4. White noise stimulation supported the step data and confirmed that the receptor's sensitivity was independent of mean slit membrane displacement. When the relative displacement of the stimulus (i.e., strain) was held constant at different maintained backgrounds, the SNR of the neurons remained fairly constant at approximately 2-10 over the frequency range from 4 to 450 Hz. The receptor current frequency responses showed high-pass characteristics, with a two- to sevenfold enhancement of the response amplitude and a phase lag relative to the stimulus of 90 degrees at 300 Hz. Low coherence values in the frequency range of 0.5-125 Hz were explained by nonlinear adaptation. 5. We conclude that, by rapidly adapting to the mean displacement of the slit membrane, slit organ mechanoreceptor neurons maintain a high sensitivity and SNR that allow the detection of small and rapid changes in cuticular strain.

Gravity map of Brazil: 2. Regional and residual isostatic anomalies and their correlation with major tectonic provinces

An isostatic residual gravity map of Brazil has been computed by removing from a 0.5° × 0.5° Bouguer anomaly grid a regional gravity field calculated for compensating masses of surface topography. The coherence function, a statistical measure of the correlation between Bouguer anomaly and topography, was first computed in order to constrain the compensation mechanism within Brazil. Similar to results for North America and Australia, the coherence function of South America has a broad transition between high and low coherence values, suggesting a combination of tectonic provinces with different flexural rigidities and/or loading processes. In view of this result, we have considered, as a first approximation, a model in which the surface topography is the only load acting on a nonrigid lithosphere. A regional gravity field has been computed assuming Airy‐Heiskanen isostasy with compensation at the crust‐mantle boundary. The residual gravity map, which was obtained by removing the computed regional gravity field from the observed Bouguer anomaly, shows a long‐wavelength N‐S trending negative anomaly over most of Brazil. This gravity feature of approximately 3000 km width is the southern continuation of the western North Atlantic negative geoid/gravity anomaly and reaches at least ∼15 mGal in the northern portion of Brazil. Using the upward continued isostatic residual gravity field at 300 km, this long‐wavelength component, which may be dynamically induced, has been removed to first approximation. The final isostatic residual gravity anomaly map depicts anomalies with wavelengths between 100 and 1000 km which correlate with major tectonic provinces. Negative anomalies occur mainly over Paleozoic intracratonic and Cretaceous rift‐type sedimentary basins, and granitic intrusions and along Proterozoic thrust belts. Positive residual anomalies are generally observed over regions affected by igneous activity and volcanism such as in the Amazon basin and the Paraná flood basalt province. Positive anomalies are also associated with overthrust crustal plates which define a suture zone in central Brazil and over sub‐Andean Tertiary foreland basins.

On the efficacy of linear system analysis of renal autoregulation in rats.

In order to assess the linearity of the mechanisms subserving renal blood flow autoregulation, broad-band arterial pressure fluctuations at three different power levels were induced experimentally and the resulting renal blood flow responses were recorded. Linear system analysis methods were applied in both the time and frequency domain. In the frequency domain, spectral estimates employing FFT, autoregressive moving average (ARMA) and moving average (MA) methods were used; only the MA model showed two vascular control mechanisms active at 0.02-0.05 Hz and 0.1-0.18 Hz consistent with previous experimental findings [Holstein-Rathlou et al., Amer. J. Physiol., vol. 258, 1990.]. In the time domain, impulse response functions obtained from the MA model indicated likewise the presence of these two vascular control mechanisms, but the ARMA model failed to show any vascular control mechanism at 0.02-0.05 Hz. The residuals (i.e., model prediction errors) of the MA model were smaller than the ARMA model for all levels of arterial pressure forcings. The observed low coherence values and the significant model residuals in the 0.02-0.05 Hz frequency range suggest that the tubuloglomerular feedback (TGF) active in this frequency range is a nonlinear vascular control mechanism. In addition, experimental results suggest that the operation of the TGF mechanism is more evident at low/moderate pressure fluctuations and becomes overwhelmed when the arterial pressure forcing is too high.

Seismic noise in northcentral North Carolina

Ambient seismic noise in the frequency range 0.5 to 7 Hz is investigated for a portion of northcentral North Carolina. Three-component, digital data were collected as part of the state's bid for the Superconducting Super Collider. Stations were established along the proposed collider ring position and were intentionally located near sources of transient vibration. The absence of transient vibrations over all or a significant portion of the recording interval at six stations allowed seismic noise to be investigated. Noise spectra display similarities in shape; all spectra decay with approximately constant slope (roughly f −3) below 1 Hz and all flatten in the frequency range 2 to 3 Hz. Vertical-component spectra decay with roughly constant slope ( f −2.1) for f > 4 Hz. At most stations, spectral levels for horizontal components are greater than vertical-component levels for f > 4 Hz. Vertical-component spectra are similar in shape and magnitude to vertical spectra computed for RSTN station RSCP. The most likely source for the ambient seismic noise is higher-mode surface waves produced by cultural activity. Ambient noise cannot be attributed to system noise or to the direct effects of traffic. Higher-mode surface waves are suggested by low coherence values computed for orthogonal components and extremely complex particle motion diagrams. Large horizontal particle motions and high spectral levels for horizontal components suggest that a significant portion of the noise, particularly for f > 4 Hz, is being produced by Love waves. Rayleigh waves contribute to the noise in the band 2 to 3 Hz.

Intramuscular EMG coherence in health and in neuromuscular disorders

The electromyogram (EMG) interference pattern (EMGIP) is the summed activities of many single fiber action potentials (SFAPs) as they propagate past the recording electrodes, and reflects their temporal and spatial relationships and their propagation speed. Comparing EMGIPs from two locations along the same muscle fibers should give insight into changes that were introduced between these two locations. These changes are expected to correlate with normal and abnormal muscle physiology. In this study, the coherence function was used as the comparison tool. This study was designed to measure the coherence between two EMGIPs recorded intramuscularly by two concentric electrodes separated by 1 cm along the same muscle fibers. The intent was to find coherence values in—and differences among—healthy and diseased muscles. There were 191 subjects: 33 healthy, 85 neuropathic, 28 myasthenic, 13 myotonic, and 32 myopathic. The two main features of this coherence analysis were the significant differences in coherence values between some of the groups in the frequency range of 0 to 70 Hz, and the marked lack of coherence between the two recording electrode EMGIPs in the frequency range above 300 Hz in all of the groups. Also, statistical analysis based on coherence values indicated two significantly distinct clusters in the frequency range of 0 to 70 Hz: the myopathy/myasthenic cluster, with relatively high coherence values, and the normal/neuropathy/myotonic cluster, with relatively low coherence values. Coherence-based classification was correct in 67% of subjects. On theoretical grounds, it was expected that increased dispersion of muscle fiber conduction velocities in myopathy will translate into high-frequency coherence differences between the groups, but this was not found. Instead, significant differences were found in the low-frequency range. It is postulated that the latter resulted from changes in the muscle volume conductor differences in packing of the muscle fibers and differences in fibrous tissue that resulted in differences of impedance within the muscles. This study emphasizes the need for further research into the role and possible utilization of the coherence function in electromyography.

Relationship between Pretreatment Electroencephalographic Coherence Measures and Subsequent Response to Electroconvulsive Therapy: A Preliminary Study

Quantitative analyses of EEG recordings taken prior to a course of ECT in elderly depressed patients showed that normal anterior interhemispheric coherence in the delta frequency band was associated with more favorable clinical response to ECT, whereas lower coherence values were associated with incomplete therapeutic response. Clinical implications of these findings are discussed.

Transverse relationships of the alpha rhythm on the scalp

Using auto- and cross-spectral analyses on data of 3 normal adult subjects, two components of rhythmic alpha activity were distinguished with respect to their spatial distribution and spatial relationships on the scalp. Derivations from 21 scalp points vs. linked ears were analyzed. The alpha activity with power maxima in the bilateral occipital areas (component A) showed high coherence with activity in the anterior areas and somewhat lower coherence with activity in the central/parietal areas. The other component (B) appeared dominantly in the central areas, showing extremely low values of coherence between the anterior and posterior regions, and an unstable phase relation among recording points. Components A and B appear to correspond to the ‘generalized’ and ‘localized’ alpha components described earlier. It is suggested that these two components account for the major properties of the rhythmic alpha activity.

BEACH GROUND-WATER OSCILLATIONS

Oscillations of ground-water table were measured in perforated piezometers distributed across the Baltic beach at the coastal research station of Lubiatowo, Prom spectral and correlation analysis it appears that the most pronounced wind-wave oscillations having a period of about 6 seconds have not been detected in the beach, while longwave oscillations, dominant in the beach spectrum, correlate well with nearshore long waves. Among the longwave oscillations, it is only those having periods about or slightly longer than 100 seconds that obey the Boussinesq law. However, some longer waves may also be generated by the interaction of shorter waves. Oscillations with periods from 50 to 100 s are very coherent throughout the beach. In the absence of stochastic noise and variety of inputs, low values of coherence for other periods indicate presence of nonlinear effects. More light should be shed on the origin of this nonlinearity and theoretical tools should be tried to complement the Boussinesq model.