The larger, more obvious LFP, the positivity peaking at ~30 ms, and the negativity peaking at ~50 ms (P30/N50, Fig. 1 A, B, and C) appear to arise mainly from processes in the supragranular layers. The superficial P30 extends upwards from a supragranular current source that we interpret as a “passive” CSD feature reflecting current return to the “active” current source, itself representing the initial activation of supragranular pyramidal cells (by granule cell afferents from Layer 4). Passive current return happens because of the conservation of net electrical currents and electrical neutrality. N50 extends vertically from a superficial current sink (an asterisk in Fig. 1C), whose physiological significance is less clear. As discussed below, we use the P30 to track LFP spread vertically. To get at lateral spread of LFPs, we focused analysis on the initial negativity associated with the frequency-selective responses in Layer 4/lower Layer 3 (“1” and “2” Fig. 1); this negativity extends in a ventral direction from the current sinks in these locations, particularly the lower (Layer 4) one. Figure 2 shows Layer 4 MUA, CSD and
