Importantly, all CSD topographies are reference-free estimates of radial current flow at scalp, their sign (polarity) reflecting the direction of radial current flow with respect the scalp surface (positive values or sources represent current flow towards the scalp, negative values or sinks represent current flow exiting the scalp; e.g., Nunez, 1981; Nunez and Srinivasan, 2006). For this reason, the CSD pattern of sources and sinks provides a representation of current generators underlying an ERP topography (e.g., Tenke and Kayser, 2012), which can be obtained from any EEG recording reference scheme. In case of the auditory N1, the underlying current generator pattern consists of bihemispheric dipole activity involving both auditory cortices, which can be readily appreciated from the CSD topographies, particularly for those with intermediate spline flexibility (m = 3 or m = 4). In contrast, this pattern is blurred for surface potentials, resulting in a diffuse midline maximum. Furthermore, despite preserving the relative topographic distribution of ERP values when using different reference schemes, in case of the auditory N1, a linked-mastoids reference will largely eliminate positive values across the recording
