Chunk #20 — 3. Impact of spatial scale on CSD implementations — 3.1. Empirical considerations for linear (one-dimensional) intracranial recordings — 3.1.1. The cortical dipole and field closure
implicates direct contributions to the initial negativity from the thalamocortical afferent volley, immediately followed by postsynaptic activation. This is evidenced by early CSD sinks in middle cortical layers (lamina IV waveforms; 23.3–23.4 mm), corresponding in timing and location to multiple high amplitude MUA peaks. Despite the prominence of the early sink and its time-locking with local MUA, its impact on the volume-conducted field potential is largely cancelled by adjoining, field-closing sources. These sources are found both in supra- and subgranular laminae (immediately above and below the channels labeled as lamina IV), reminiscent of the closed-field pattern observed for the flash-VEP in lamina 4 of striate cortex (Kraut et al., 1985; Schroeder et al., 1995; Tenke et al., 1993). In the same manner, the large source that follows the early sink (lamina IV CSD waveforms from 20 to 55 ms) occurs in association with a MUA reduction below baseline, and is likely to reflect local postsynaptic inhibition (i.e., it is an active source), with somewhat shallower, circuit-closing sinks above and below (immediately above and below channels labeled as lamina IV). In contrast to the early negativity, the positive components invert more superficially, corresponding to sequential supragranular activation (CSD sinks at 22.8–23.0
