The results described above reveal apparent volume conduction of LFP over relatively large distances traveling parallel to the cortical sheet, lateral to their site of generation. To get at volume conduction perpendicular to the cortical sheet in A1, we examined the spatial spread of the P30 component described in Figure 1 above. Figure 5A shows LFP responses to broad-band noise (BBN) recorded at recording depths with 200 μm intervals from the depth of A1 to the dura at the dorsal brain surface in one penetration. Near the bottom of the column, there is a polarity inversion of this component in supragranular A1, like that shown for the tone-evoked P30 in Figure 1. Above the inversion, the component is gradually attenuated over distance. Figure 5B shows the amplitude distribution of the P30 component in the LFP and CSD signals at the same timing. Insets in both columns show a magnified view of the top one third of depths, and one can see that the peak remained observable up to the dural surface of the brain, about 18 mm above A1. The sink and source of CSD, however, were clearly confined to the proximity of inversion.
