Chunk #19 — 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
An intracortical view of the AEP inversion, shown in Fig. 2B, reveals the limitations of the cortical dipole as an explanatory concept at high local resolutions. At this scale, the simplistic model of dipole sheet or layer must be recast as the superposition of layers and/or regions composed of extracellular current sources and sinks, each element contributing to the field potential throughout the conductive medium. Much of the activity is locally cancelled (i.e., closed) and cannot be precisely anticipated from distant recordings; however, the resultant field potential typically inverts in polarity across the cortical mantle. In the case shown in Fig. 2B, the initial negativity of the AEP increases with depth through the middle cortical layers, inverting in subgranular layers and white matter (23.8–24.0 mm; cf. Steinschneider et al., 1992). A comparison of concurrent AEP, CSD and multiunit activity (MUA) 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
