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. The tuning bandwidth of the auditory cortical LFP response to tones appears equivalent to that of the EPSP over intensities ranging from threshold

to 70 dB, covering the intensity (60 dB) used in the present study (Kaur et al., 2004), and consistent with the idea that the LFP is a reflection of local synaptic events (Kaur et al., 2004, Nicholson, 1973 and Nicholson and Freeman, 1975). Given this, the LFP’s broader tuning relative to MUA is consistent with that of subthreshold excitatory synaptic potentials (EPSPs) relative to that of action potentials (Ojima and Murakami, 2002, De Ribaupierre et al., 1972, Veliparib Tan et al., 2004 and Volkov and Galazjuk, SAR405838 nmr 1991). Not surprisingly then, our results agree with

prior ones showing that in auditory cortex, the tuning bandwidth of the LFP is generally wider than that of neuronal firing (Eggermont, 1998, Eggermont et al., 2011, Noreña and Eggermont, 2002 and Kaur et al., 2004). It is not clear exactly why the conclusions of Xing et al. (2009) differ from those of most other studies, save that of Katzner et al. (2009) (discussed below). One noteworthy point is that the LFP that Xing et al. (2009) observed was nearly always a negative deflection, regardless of the depth in V1. Like the fact that the LFP and neuronal firing measures reported by Xing et al. (2009) gave the same readout, despite being generated by well-recognized and distinct underlying neuronal processes, this polarity-depth invariance

in the LFP is in stark contrast with most other reports; for active cortical regions, transcortical (surface-depth) polarity MTMR9 inversions of “locally generated” LFPs are ubiquitous across sensory areas and independent of stimulus type (Givre et al., 1994, Maier et al., 2011, Mitzdorf and Singer, 1978, Peterson et al., 1995 and Steinschneider et al., 2008). It is possible that specific anesthesia effects (e.g., a suppression of normal ambient excitability and variability) may contribute to the findings of Xing et al., even though anesthesia per se is a common factor in many of the experiments considered above. Similarly the very small dimensions of electrical contact area of the electrodes could be a reason for the difference between the findings of Xing et al., and those of other studies (however, see Nelson and Pouget, 2010), though similar contact dimensions were used in other studies (e.g., Kreiman et al., 2006) that clearly show spread of LFPs over much greater distances than Xing et al. A final possibility we consider is the areal size of the activated substrate.