I am calculating the non orthogonalized envelope correlation of the Hilbert signal in the alpha range between the STN LFPs and the source-reconstructed cortical signal. I have read that you should not expect negative values, and this is the case when I do env corr between cortical patches or between sensors; yet, when I do it between LFPs and cortex, negative correlation values appear. How come? How should I interpret those?
This is true, only for the case when orthogonalization.
This is because the overall magnitude of the correlation between x and y is computed as the mean of the correlation of the envelop of x and the envelop of y orthogonalized with x (i.e., y_ort_x), and the correlation of y and x orthogonalized with y: