Is the polarity of a CSD signal more informative than the polarity of the surface potential?
Yes, it is. The choice for a surface potential reference is quasi-arbitrary, meaning that the polarity can only be interpreted in relative but not absolute terms. For instance, rereferencing the data from a vertex (Cz) recording reference to a linked-mastoids or average reference will change the polarity of the EEG/ERP signal for certain sites (regions) and/or time intervals (components), which, of course, has also implications for spectral data properties (e.g., phase, amplitude). In contrast, a surface Laplacian or CSD transform represents “true” estimates of current activity at scalp, meaning that the signal polarity (negative = sink, positive = source) is unambiguous by itself. CSD estimates are inherently reference-free by virtue of their computation (i.e., second spatial derivative), that is, they depend only on the topography of the EEG signal. As the EEG/ERP topography is not altered by different reference schemes because the subtraction of a constant (i.e., a different referen
