Chunk #17 — 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
Fig. 2A exemplifies the cortical dipole produced by the inversion of the earliest cortical responses to clicks (N8 of Steinschneider et al., 1992; P12 and P24 of Arezzo et al., 1975) within primary auditory cortex in the monkey (cf. Liégeois-Chauvel et al., 1994, for suspected homologs in human). The depths indicated are displacements below the ipsilateral frontal dura during an intracranial penetration of the superior temporal plane from a trajectory approximately orthogonal to the cortical surface within the Sylvian fissure (adapted from Tenke et al., 1987; cf. Steinschneider et al., 1992). At a depth of 5 mm, a dual positivity (P12/24) is seen following the auditory brainstem response (ABR) and an initial negativity (N). At great distances from the generator, the auditory evoked potential (AEP) is a low amplitude response, with a very shallow gradient (e.g., from 5–17 mm) and a stable morphology. Although the field potential amplitude noticeably increases with increased proximity to primary auditory cortex (at about 22 mm), it is only within the generating tissue (i.e., 23–24 mm) that amplitudes and spatial gradients become quite large, whereupon
