Partial Coherence Analysis Research Articles

Characterization of neurons of the supramammillary nucleus and mammillary body that discharge rhythmically with the hippocampal theta rhythm in the rat

We examined the activity of single cells of the supramammillary nucleus (SUM), the mammillary body (MB), and adjacent regions of the diencephalon with respect to the hippocampal electroencephalogram (EEG) in urethane-anesthetized rats. Twenty-nine of 170 cells were found to discharge synchronously with the theta rhythm of the hippocampus (theta-related neurons). All of the 29 theta-related cells were localized to the SUM or MB. A subset of theta-related cells of SUM and MB discharged in short-duration bursts comparable to the pyramidal complex spike cells of the hippocampus. In contrast to hippocampal complex spikes, however, which predominantly exhibit this mode of firing during non-theta states, the burst firing of SUM/MB cells was strongly correlated with the theta rhythm. The proportion of bursting neurons was higher in MB than in SUM. Using partial coherence analysis, we examined the relationship between SUM/MB theta-related cells and the two generators of theta of the dorsal hippocampus. The theta-related cells of MB showed a stronger correlation with "CA1" than with "dentate" theta, whereas no such asymmetry was found in the relationship between neuronal firing of SUM cells and the two generators of theta in the hippocampus. The foregoing suggests that the theta-related cells of MB are driven by descending projections from the hippocampal formation (CA1), whereas those of the SUM are not. The SUM and MB are intimately connected with the hippocampal formation--the SUM mainly via ascending projections to the dentate gyrus, and the MB via direct descending projections from the subiculum. Theta-related SUM/MB cells may be directly involved in the generation of theta and/or the transfer of theta rhythmicity to various parts of the limbic system and forebrain.

Separation of hippocampal theta dipoles by partial coherence analysis in the rat

In order to separate the effect of different theta generators in the hippocampus and to characterize the pattern of relationships between them, in this study, we calculated the coherence that remains between EEG signals, recorded (1) in the stratum oriens of the CA1 region and (2) close to the hippocampal fissure in the dentate gyrus of the right or left hippocampus, after the variations, common also for a third recording site is eliminated (partialization). We found that in both anesthetized and freely moving rats, there is a selective high correlation (coherence) between theta rhythmic activities of contralateral homonymous sites of the hippocampus. The coherence between field potentials recorded in ipsilateral superficial and deep layers was eliminated when allowance was made for any of the contralateral hippocampal recordings. On the other hand, coherence between contralateral homonymous theta dipoles did not decrease when partialized by a heteronymous hippocampal EEG signal. The present results support earlier findings on multiple hippocampal theta dipoles and indicate that they can be separated using partial coherence analysis. The left and right superficial and deep dipoles oscillate as if they formed two separate systems one extending over the superficial CA1 layers on both sides and the other consisting of the left and right deep hippocampal theta dipoles. The results also suggest an important role of the commissural projections in interhemispheric theta synchronization.

Differential relationships among the 10-Hz rhythmic discharges of sympathetic nerves with different targets

Partial coherence analysis was used to remove the influences of the central circuits controlling a sympathetic nerve (as reflected by its discharges) on the coherence of the 10-Hz discharges of other sympathetic nerves in unanesthetized decerebrate or urethan-anesthetized cats. In many cases, partialization reduced but did not eliminate the sharp peak near 10 Hz in the coherence functions relating the discharges of sympathetic nerve pairs. This observation implies that the central sources of the 10-Hz rhythmic discharges of any nerve are not identical to those responsible for the rhythm recorded from any other nerve. Partial coherence analysis also revealed differential relationships among the 10-Hz rhythmic discharges of sympathetic nerves with different targets. Importantly, the pattern of differential relationships observed in one experiment could be the reverse of that in the next. Although the basis for the differential relationships is not yet clear, nonuniform coupling of multiple brain stem 10-Hz oscillators and/or nonuniform cross talk between spinal circuits controlling different sympathetic nerves may be involved.

Coordination of the cardiac-related discharges of sympathetic nerves with different targets

Partial coherence analysis was used to remove the influences of pulse-synchronous baroreceptor nerve activity (as reflected by the arterial pulse) on the coherence of the cardiac-related discharges of sympathetic nerve pairs in unanesthetized decerebrate cats. It can be predicted that the peak at the heart rate frequency in the ordinary coherence function relating the discharges of two nerves will be eliminated by either partialization using the arterial pulse or surgical baroreceptor denervation, if the central circuits controlling the nerves share baroreceptor inputs but are not interconnected. Contrary to this prediction, in many experiments the peak was not eliminated by partialization using the arterial pulse. Moreover, partialization often nonuniformly reduced the peaks at the heart rate frequency in the coherence functions for different nerve pairs. These results are consistent with a model of multiple routes over which baroreceptor influences are distributed to the central circuits controlling different sympathetic nerves. Specifically, we propose that the direct route from the baroreceptors to each of the central circuits is complemented by cross talk among the central circuits.

Extended Partial and Multiple Coherence Analyses and Their Application to Reactor Noise Investigation

Method of partial and multiple coherence analyses was studied in an attempt to develop an effective method for multivariable noise analysis at power reactors. The conventional method of coherence analyses was reviewed by following its mathematical formulation, which revealed rather strict limitations in its applicability to practical problems if there exist feedback loops among measured variables. In order to obviate the limitations inherent in the conventional method, this paper proposed a new method of extended partial and multiple coherence analyses on the basis of a multivariable autoregressive modeling, which permits, even for systems with feedback, the correct evaluation of the partial and multiple coherence for variables of concern. Both the conventional and the newly introduced coherence analysis methods were applied to simulation data as well as to JPDR-II (Japan Power Demonstration Reactor-II) noise data. The results of the analysis suggested that the extended partial and multiple coherence analyses can be an effective tool for investigation of at-power reactor noise where various feedback effects are thought to be significant.

Extended partial and multiple coherence analyses and their application to reactor noise investigation.

Method of partial and multiple coherence analyses was studied in an attempt to develop an effective method for multivariable noise analysis at power reactors. The conventional method of coherence analyses was reviewed by following its mathematical formulation, which revealed rather strict limitations in its applicability to practical problems if there exist feedback loops among measured variables. In order to obviate the limitations inherent in the conventional method, this paper proposed a new method of extended partial and multiple coherence analyses on the basis of a multivariable autoregressive modeling, which permits, even for systems with feedback, the correct evaluation of the partial and multiple coherence for variables of concern. Both the conventional and the newly introduced coherence analysis methods were applied to simulation data as well as to JPDR-II (Japan Power Demonstration Reactor-II) noise data. The results of the analysis suggested that the extended partial and multiple coherence anal...

Coherence analysis of systems with feedback and its application to a BWR noise investigation

Two new methods for coherence analysis are presented with the aim of analyzing dynamic characteristics of systems with feedback which include closed-loops among measured variables. In order to investigate feedback effects implicit in measured ordinary coherence, this paper proposes a method of evaluating system dynamics by comparison of the ordinary coherence and noise power contribution ratio derived from an autoregressive modeling. It is theoretically shown that the comparison of these two functions enables us to evaluate the property of a feedback effect in the system if it exists. Concerning the coherence analysis of multivariable systems, this paper discusses, through numerical examples, some difficulties which are encountered in the conventional partial coherence analysis if there exists feedback loops connecting the output to the inputs. In order to obviate these difficulties this paper introduces a new method called extended partial coherence analysis which ensures the evaluation of direct correlation existing between two variables of interest by eliminating the effect of the remaining variables, regardless of whether or not the system contains feedback loops. Through application to simulation data as well as to actual BWR noise data, it is shown that the methods presented here have a potential applicability to various studies of at-power reactor noise, especially of its generation and propagation mechanisms.

Relative contributions of intracortical and thalamo-cortical processes in the generation of alpha rhythms, revealed by partial coherence analysis

The thalamo-cortical relationships of alpha rhythms have been analysed in dogs using partial coherence function analysis. The objective was to clarify how far the large intracortical coherence commonly recorded between different cortical sites could depend on a common thalamic site. It was found that the alpha rhythm of the LGN influenced only moderately the coherence between cortical alpha rhythms whereas that of the pulvinar had much more influence, at least in relation to some cortical areas. Significant phase shifts between thalamus and cortex were also measured. The results demonstrate that there are thalamo-cortical and cortico-cortical components which interact in the generation of cortical alpha rhythms.

PARTIAL COHERENCE MATCHING OF SYNTHETIC SEISMOGRAMS WITH SEISMIC TRACES*

AbstractA crucial step in the use of synthetic seismograms is the estimation of the filtering needed to convert the synthetic reflection spike sequence into a clearly recognizable approximation of a given seismic trace. In the past the filtering has been effected by a single wavelet, usually found by trial and error, and evaluated by eye. Matching can be made more precise than this by using spectral estimation procedures to determine the contribution of primaries and other reflection components to the seismic trace. The wavelet or wavelets that give the least squares best fit to the trace can be found, the errors of fit estimated, and statistics developed for testing whether a valid match can be made.If the composition of the seismogram is assumed to be known (e.g. that it consists solely of primaries and internal multiples) the frequency response of the best fit wavelet is simply the ratio of the cross spectrum between the synthetic spike sequence and the seismic trace to the power spectrum of the synthetic spike sequence, and the statistics of the match are related to the ordinary coherence function. Usually the composition cannot be assumed to be known (e.g. multiples of unknown relative amplitude may be present), and the synthetic sequence has to be split into components that contribute in different ways to the seismic trace. The matching problem is then to determine what filters should be applied to these components, regarded as inputs to a multichannel filter, in order to best fit the seismic trace, regarded as a noisy output. Partial coherence analysis is intended for just this problem. It provides fundamental statistics for the match, and it cannot be properly applied without interpreting these statistics.A useful and concise statistic is the ratio of the power in the total filtered synthetic trace to the power in the errors of fit. This measures the overall goodness‐of‐fit of the least squares match. It corresponds to a coherent (signal) to incoherent (noise) power ratio. Two limits can be set on it: an upper one equal to the signal‐to‐noise ratio estimated from the seismic data themselves, and a lower one defined from the distribution of the goodness‐of‐fit ratios yielded by matching with random noise of the same bandwidth and duration as the seismic trace segment. A match can be considered completely successful if its goodness‐of‐fit reaches the upper limit; it is rejected if the goodness‐of‐fit falls below the lower one.

Application of partial coherence methods for identification of interior noise sources in aircraft

Effective interior noise control procedures require identification of the noise sources and the noise transmission paths. Recent developments in computational procedures have led to increased interest in partial coherence analyses for source/path determination. The theoretical aspects of this method have been explored by Bendat and also by Dodds and Robson; however, only a few reports of applications to practical problems are available. Previous authors reported only partial success in identifying sources. Therefore, improvements of the approach are felt to be needed. This paper describes the latest results of an ongoing effort to develop partial coherence techniques for interior noise source/path determination in aircraft. The paper includes a summary of the computational techniques as developed by Bendat [J. Sound Vib. 49(3), 293–308 (1976)] and illustrates the application to a two‐input, single‐output system with coherence between the inputs. Solution of this system by previously used methods illustrat...

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.