The influence of static correlations on multivariate correlation analysis of the EEG.

The choice of the EEG reference strongly influences the results derived from different correlation measures. Such a dependence may easily mislead the interpretation of the correlation structure of the brain activity. We provide a systematic study of the influence of the choice of reference on linear multivariate EEG correlation patterns as determined by sensitive correlation measures derived from the equal-time correlation matrix. In addition, an effective algorithm to extract the effect of static correlations is developed. The eigenvalues of the correlation matrix and their spacing statistics are studied for artificial time series with known correlation structure and for an epileptic EEG in various montages. The correction method proposed in this paper works with varying quality for different choices of the EEG reference. Furthermore, the optimal choice of the reference depends also on the correlation structure of the underlying system.