The computation of the spatial derivative for a linear (or stepwise) penetration of tissue originally proposed by Freeman and Nicholson (1975) was a local “slope-of-slope” measure, estimated as the potential at any location minus half the potential at each of the two neighboring sites (an alternative, 5-point smoothed estimate was also suggested). As the method was developed, the anisotropy of the tissue impedance was also explored (Nicholson, 1973). However, for one-dimensional applications, small impedance variations between laminae were generally ignored, while efforts to preserve the available spatial resolution without introducing computational noise proved to be a greater concern (Freeman and Nicholson, 1975; Mitzdorf, 1985; Tenke et al., 1993). Even though small sampling-related irregularities may be magnified in local CSD estimates, smoothing carries the risk of obscuring or misrepresenting reproducible, physiologically meaningful laminar processes.
