Analysis Of Time-varying Coherence Based On Wavelet Transform

Classical coherence analysis reflects the synchronization between two random processes in certain frequency under the assumption that the signals are stationary. In EEG study, the coherence analysis is mainly used to analyze the coupling and drive-response relation about the brain activities in different regions. Classical coherence analysis can extract the rhythmic consistency of the EEG activities in different regains. But it can not extract the transient characteristics of brain activities. In order to track the spatial-temporal characteristics of EEG activities during cognitive conception, the time-varying coherence analysis is proposed.This thesis first reviewed the development of coherence analysis form coherence analysis of stationary signal to coherence analysis of non-stationary signal. The time-varying coherence based on Morlet wavelet and Harmonic wavelet transform is mainly introduced for the time and frequency resolution in different parameters. The results showed that the time and frequency resolution hold the line in different frequency when using Harmonic wavelet transform. If the analysis is focused on certain frequency band, we can satisfy the demand on frequency resolution through adjusting the parameters of Morlet wavelet functions. We introduce the estimation performance of WOSA algorithm, and then derived the estimation performance of Morlet wavelet coherence from the statistical properties of the WOSA estimate. Finally, we analyzed the influence of estimation performance of Morlet wavelet coherence in different parameters. The selection of the length of integral window and the threshold of coherent coefficients which indicate that two signals are statistic independent were also discussed. Some simulation signals were designed to test the validity of Morlet wavelet coherence analysis, and compared the estimation performance of Morlet wavelet coherence which changed the length of integral window according to different frequency and Harmonic wavelet coherence which has fixed length of integral window in every frequency, then indexed the irrationality about comparing the results of two wavelet coherence analysis using fixed integral window.Moreover, the event-related EEG was investigated by using Morlet wavelet coherence. The results showed that there are obvious differences in visual cortex and other regains according to different attention ranges cue. It is validated the conclusion that visual information processing is...