The averaged, nose-referenced ERP waveforms were transformed into reference-free current source density (CSD) estimates (µV/cm2 units) using a spherical spline surface Laplacian (Perrin et al., 1989; for details, see Kayser, 2009; Kayser & Tenke, 2006a; Kayser et al., 2007). CSD estimates are based on the second spatial derivative of the recorded surface potentials and represent the magnitude of the radial (transcranial) current flow entering (sinks) and leaving (sources) the scalp, thereby mapping the direction, location and intensity of current generators that underlie an ERP topography (Mitzdorf, 1985; Nicholson, 1973; Nunez & Srinivasan, 2006). Apart from representing a common bridge between scalp-recorded EEG and intracranial local field potential recordings (see Tenke & Kayser, 2012, for a review), a distinct advantage of surface Laplacian methodology is that any EEG reference will yield the same, that is, unique CSD waveforms with reduced volume-conducted contributions, yielding more focused components with unambiguous polarity and sharper topographies compared to ERPs (Kayser et al., 2009).
