Both of these findings are consistent with predictions by prior modeling studies (Tenke et al., 1993 and Wang et al., 2005). Are volume conduction effects asymmetrical? The tonotopic gradient in macaque A1 is about 1.0 mm/octave DNA Damage inhibitor (Kosaki et al., 1997, Kusmierek and Rauschecker, 2009 and Merzenich and Brugge, 1973), and thus, 6 octaves difference on the tonotopic map is about 6 mm away from a recording
site, for example, more than one-half way along the frequency representation in A1, and the attenuation of LFP amplitude at this distance laterally appears to be about the same as at this distance vertically above A1 (Figure 5). This fits with the observation that neocortical conductivity appears isotropic (Logothetis et al., 2007 and Ranck, 1963). How important is the impact of volume conduction? Our measurements (Figure 5) indicate that in the vertical dimension, the LFP we have studied here shrinks to about
50% of its peak amplitude at 6 mm and then reaches a value of about 5%–10% of peak amplitude at about 12 mm above auditory cortex, continuing to decrease up to the dorsal brain surface. This is consistent with the amplitude of LFP learn more proportional to the inverse of distance, as expected by the forward solution of Poisson’s equation; this quantitative estimate is in reasonable agreement with indications from earlier studies (reviewed by Schroeder et al., 1995). Clearly, the auditory LFP generated in auditory cortex would be strong enough to severely
contaminate an auditory ERP recorded in the overlying secondary somatosensory cortices and presumably also in the underlying visual and multisensory regions in the STS. Importantly, as implied by Poisson’s equation, comparison between conditions where stimulus intensity is near threshold versus well above that value (Figure S5), indicate that volume conduction is determined from by the strength of activation in the generator substrate. Thus, the impact of volume conduction would be relatively greater at sites away from an active LFP generator substrate where local synaptic responses are weak, and the locally generated LFP is negligible. The main motivation for measuring LFPs is that they provide an index of synaptic processes which, albeit less direct than that provided by intracellular recording, is nonetheless practical for routine use in awake behaving animals (Schroeder et al., 1998, Ince et al., 2010 and Scherberger et al., 2005). This information is complementary to that provided by action potentials, since it relates to processes that are causal to generation of action potentials (Rasch et al.