Early Cortical Somatosensory and N1 Auditory Evoked Responses: Analysis with Potential Maps, Scalp Current Density Maps and Three-Concentric-Shell Head Models

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Scalp current density (SCD) mapping shows scalp areas where current either emerges (sources) from the brain into the scalp or enters (sinks) from the scalp into the brain (Nunez 1981). By definition the SCD of a point of the scalp is equal to the amount of current which flows (radially to the scalp surface) from the brain into a scalp unit volume surrounding the scalp point. SCD is reference-independent, the peaks and troughs of its distribution are sharper than those of the scalp potential (SP) and SCD, in contrast to SP, reflects mainly the activity of cortical generators. SCD maps may be estimated (Perrin et al. 1987b) by taking the spatial derivatives of SP maps. To obtain differentiable SP maps a bidimensional spline interpolation method was used (Perrin etal. 1987 a).