We investigated the spatial spread of the LFP in comparison to well-localized indices of neuronal ensemble activity, current source density (CSD) and multiunit activity (MUA) in primary auditory cortex. We show that the signals differ significantly in their spatial spread with an order of LFP > CSD > MUA, and that LFPs in particular, exhibit a far larger spatial spread than that predicted by some of the recent reports on this topic (Katzner et al., 2009;Xing et al., 2009). In fact, LFPs clearly spread well beyond the boundaries of activated tissue, in that auditory cortical LFPs can be traced up to the dorsal surface of the brain. Thus, these earlier studies do not appear to provide a general context for understanding either the spatial spread of the LFP or the scope of neuronal activity measured by an LFP. There are a number of interrelated physiological and technical considerations that bear on the interpretation of our findings and their relations to earlier ones.
