visual cortices on the brain surface, and have analyzed the spread of LFPs only in the “lateral” dimension. This is but one of the relevant dimensions that need to be considered, especially given that models of the underlying generators of scalp ERP/EEG components often contain directional terms (Ingber and Nunez, 2011;Srinivasan et al., 2006;Winter et al., 2007). Spread of LFPs along 'vertical' dimensions creates apparent similarity and coherence between depths (Maier et al., 2010), though it could be just due to the volume conduction (Kocsis et al., 1999). To provide a more general assessment of the spatial spread of LFPs, we examined the issue in the context of tonotopic mapping in primary auditory cortex (A1). Corresponding to the precise mapping of the retinal receptor surface in V1 as examined by recent studies (Katzner et al., 2009;Xing et al., 2009), A1 contains a precise spatial map of the cochlear surface (Kosaki et al., 1997;Merzenich and Brugge, 1973), which allows examination of the lateral spread of LFPs as was done in V1. Moreover, due to A1's placement in the inferior bank of the lateral sulcus, vertical penetrations through A1 could examine the spatial spread of LFPs in the vertical dimension as well.
