However, selleck chemicals the condition should not be totally insensitive to the variations either, as required by the task. Thus, a criterion is needed for properly choosing the diagonal elements. We have developed a theoretical approach to resolving this issue based on random matrices (see Sec. 3). It is useful to clarify the relation between our approach and several previous matrix-based methods to detect global changes in synchronization.22, 23, 24, 25, 26 The early proposal by Wackermann22 was to examine the Shannon information entropy associated with the spectrum of eigenvalues of the cross-correlation matrix. The method by Allefeld and Kurths23 was based on a matrix whose elements are statistics of various phase differences, which is capable of detecting clusters of phase-synchronization.
Bialonski and Lehnertz proposed to detect phase-synchronization clusters from multivariate time series by using the phase-coherence matrix,24 a matrix whose entries are the values of the mean phase coherence between pairs of time series. They applied the method to EEG recordings from epilepsy patients. The recent method by Schindler et al.25 centered about computing the largest and smallest eigenvalues of the zero-lag (or equal time) correlation matrix, and the method was demonstrated to be able to detect, for instance, statistically significant changes in the correlation structure of focal onset seizures. There was also a method by M��ller et al. on estimating the strength of genuine and random correlations in non-stationary multivariate time series.
27 In all these methods, the matrix elements are quantities derived from some types of correlation measures that typically assume values between zero and one. Our idea of using the APST is motivated by the fact that it can in general be significantly more sensitive to changes in the degree of synchronization than correlations. In particular, as the system becomes more phase coherent, the APST can increase significantly, typically over many orders of magnitude for noisy dynamical systems.19 As we will show in this paper, the synchronization-time matrix, when properly constructed, can indeed be extremely responsive to changes in the degree of synchronization of the underlying noisy system. USE OF RANDOM-MATRIX THEORY TO CHOOSE DIAGONAL ELEMENTS OF SYNCHRONIZATION-TIME MATRIX We have seen that to properly choose the diagonal elements of the synchronization-time matrix �� is the key to our method.
Here we present a sensitivity analysis based on random-matrix theory to find an optimal set of values for the diagonal elements while maximizing sensitivity to changes in synchrony. Multichannel data from a real system are stochastic, as they are corrupted by both internal (e.g., dynamic) and external (e.g., measurement) Cilengitide noises. The APST between any pair of channels can thus be regarded as a random variable, and �� is effectively a random matrix.
Related posts: