How important is the impact of volume conduction? Our measurements (Fig. 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 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 by the strength of activation in the generator substrate. Thus, the
