These results can be explained by volume conduction. Tones at BFMUA evoke strong MUA and CSD responses (Fig. 3C). CSD responses accompanied with MUA more likely reflect local activity than CSD responses without MUA concomitants (e.g. near the foot of tuning curve), and these are strong enough to generate similarly strong (and local) LFP responses like those to low frequency tones shown in Figure 4A. Tones that are away from the BFMUA may still evoke weaker CSD responses. However, considering the tonotopic organization of auditory cortex, concurrent strong CSD responses must occur somewhere else in either ascent or descent positions along the tonotopic gradient. In such cases, due to volume conduction, the LFP would still be strong. However, the LFPs generated by remote loci do not have correspondingly strong local responses in the CSD profile. In such cases, LFPcal should and does differ from LFPobs. Accordingly, LFPobs responses to tones more than 1 octave away from BFMUA could not be accounted for solely by electrical potentials generated by the CSD responses derived from LFPobs themselves. This conclusion is consistent with the idea that LFPobs responses are generated by a mixture of local and non-local electrophysiological events.
