Fig. 2.

Original data X and Y and its cross correlation function over delay (Left panel) and surrogate data of X and Y and its cross correlation function over delay (Right panel) in uni- (Fig. 2a) and bi-variate surrogate tests (Fig. 2b). When the uni-variate surrogate test has been applied to original data X and Y cross correlation function of original data and surrogate data are different from each other (Fig. 2a.). However, when the bi-variate surrogate test has been applied to original data X and Y, cross correlation function of original data and surrogate data are identical (Fig. 2b.).