A generalization of the magnitude squared coherence spectrum for more than two signals: definition, properties and estimation

1. In vagotomized, paralyzed, decerebrate cats, simultaneous recordings were taken from one or more sympathetic nerves [cervical sympathetic (CS), inferior cardiac (IC), splanchnic (SP)] and from medullary neurons in vasomotor-related regions. Coherence analyses were used to ascertain the presence of sympathetic rhythms (2-6 Hz or "3-Hz rhythm," 7-13 Hz or "10-Hz rhythm") that were correlated between different signals. The occurrence of a significant peak at such a frequency in a unit-nerve coherence spectrum allowed the identification of a medullary neuron as sympathetic related. 2. A serendipitous example is given of a rostral ventrolateral medullary neuron that had significant unit-nerve 10-Hz coherence peaks for three sympathetic nerves (CS, IC, SP); but, as revealed by partial coherence analysis, the unit activity's correlation with one nerve's activity could be partially or completely dependent on its correlation with other nerve activities. Thus in this case the unit-CS and unit-IC coherences at 10 Hz were completely dependent on the SP rhythm, whereas the unit-SP coherence was not significantly affected by the CS and IC rhythms. This asymmetry suggests that the neuron was preferentially connected to SP-generating medullary circuits. 3. This example indicates the strength of partial coherence analysis as a means of studying differential connectivity between medullary sympathetic-related neurons and sympathetic output neuron populations.

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Turbulence coherence and its impact on wind-farm power fluctuations

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.713 ◽

2018 ◽

Vol 855 ◽

pp. 1116-1129 ◽

Cited By ~ 8

Author(s):

Nicolas Tobin ◽

Leonardo P. Chamorro

Keyword(s):

Power Spectrum ◽

Atmospheric Turbulence ◽

Turbulent Diffusion ◽

Wind Farm ◽

Spectral Characteristics ◽

Wind Farms ◽

High Frequencies ◽

Power Fluctuations ◽

The Impact ◽

Coherence Spectrum

Using a physics-based approach, we infer the impact of the coherence of atmospheric turbulence on the power fluctuations of wind farms. Application of the random-sweeping hypothesis reveals correlations characterized by advection and turbulent diffusion of coherent motions. Those contribute to local peaks and troughs in the power spectrum of the combined units at frequencies corresponding to the advection time between turbines, which diminish in magnitude at high frequencies. Experimental inspection supports the results from the random-sweeping hypothesis in predicting spectral characteristics, although the magnitude of the coherence spectrum appears to be over-predicted. This deviation is attributed to the presence of turbine wakes, and appears to be a function of the turbulence approaching the first turbine in a pair.

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Comparison between two rat sympathetic pathways activated in cold defense

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.00850.2005 ◽

2006 ◽

Vol 291 (3) ◽

pp. R589-R595 ◽

Cited By ~ 41

Author(s):

Youichirou Ootsuka ◽

Robin M. McAllen

Keyword(s):

Cold Water ◽

Relative Sensitivity ◽

Sympathetic Nerves ◽

Defense Responses ◽

Interscapular Brown Adipose Tissue ◽

Brain Circuits ◽

Brown Adipose ◽

Skin Cooling ◽

Trunk Skin ◽

Coherence Spectrum

In cold defense and fever, activity increases in sympathetic nerves supplying both tail vessels and interscapular brown adipose tissue (iBAT). These mediate cutaneous vasoconstrictor and thermogenic responses, respectively, and both depend upon neurons in the rostral medullary raphé. To examine the commonality of brain circuits driving these two outflows, sympathetic nerve activity (SNA) was recorded simultaneously from sympathetic fibers in the ventral tail artery (tail SNA) and the nerve to iBAT (iBAT SNA) in urethane-anesthetized rats. From a warm baseline, cold-defense responses were evoked by intermittently circulating cold water through a water jacket around the animal's shaved trunk. Repeated episodes of trunk skin cooling decreased core (rectal) temperature. The threshold skin temperature to activate iBAT SNA was 37.3 ± 0.5°C ( n = 7), significantly lower than that to activate tail SNA (40.1 ± 0.4°C; P < 0.01, n = 7). A fall in core temperature always strongly activated tail SNA (threshold 38.3 ± 0.2°C, n = 7), but its effect on iBAT SNA was absent (2 of 7 rats) or weak (threshold 36.9 ± 0.1°C, n = 5). The relative sensitivity to core vs. skin cooling (K-ratio) was significantly greater for tail SNA than for iBAT SNA. Spectral analysis of paired recordings showed significant coherence between tail SNA and iBAT SNA only at 1.0 ± 0.1 Hz. The coherence was due entirely to the modulation of both signals by the ventilatory cycle because it disappeared when the coherence spectrum was partialized with respect to airway pressure. These findings indicate that independent central pathways drive cutaneous vasoconstrictor and thermogenic sympathetic pathways during cold defense.

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