Are there any differences between the spherical coordinates provided by CSD toolbox or EEGlab?
Apart from a -90° rotation of spherical angle Theta, EEGlab places standard and extended 10-20-system electrode sites at different locations than CSD toolbox. In EEGlab, for instance, all electrodes on the x-y plane (i.e., the circle combining the locations Fpz, T7, Oz, and T8) are located -2° below the sphere’s equator. In contrast, the CSD toolbox places these locations directly on the equator (i.e., where they belong if one directly follows the logic of the 10-20-system; e.g., cf. Jurcak et al., 2007). Whereas these differences may be of minor importance when mapping volume-conducted scalp potentials, these differences will have a greater impact for the computation of CSD estimates, because the CSD computation is based on relative differences between scalp sites. For example, a prominent visual ERP component at occipital will be differently represented if, let’s say, sites P9, P7, P5, P07, and P09 have a different spatial representation. This issue will ultimately also impede on the
