Fig. 4.

Schematics of dipole optimization procedure using translated lead fields of a moving tetrode. (a) During data collection, the tetrode moves down (down arrows) the path of penetration in steps of size Δz_step, yielding measurements of the same source at 4n_step spatial points (black dots). From the probe’s point of view used in the analysis, real movement of the tetrode is equivalent to virtual movement of the source in the opposite direction (up arrow). x_s and x_i, i={0,1,2,3}, denotes the Cartesian coordinates of the position of the source and the ith lead in the first step, respectively, relative to the (moving) tetrode tip (x₀). (b) The tetrode lead registers the extracellular action potential of a single unit that is characterized by a single dipole current source with moment p. The dipole moment vector is translated to a new position relative to the probe at each step (x_s + kΔz_step). At each step, the model prediction of the probe potential is the scalar (dot) product of the dipole moment vector of the source, p, and the lead field vector of the probe at that relative position, L_i(x). Thus for a fixed physical source position, the dipole interacts with the lead field in a set of translated virtual positions. See text for the details of dipole optimization