Figure 2.

Steerability of TI, Probed via Both Computational Modeling and a Tissue Phantom

For each condition (A)–(E), we simulated the interferential electric field envelope modulation (projected along: i, x-direction, ii, y-direction) that would result from electrodes at the locations indicated by the rectangles (the gray electrodes forming a pair, with an alternating current $I_1$ applied at 1 kHz, and the black electrodes forming a second pair, with an alternating current $I_2$ applied at 1.02 kHz), passing the currents described below in the individual panel caption sections. For exact coordinates of electrodes and numerical values of the peak envelope modulation amplitude, location, and width, see Table 1. We also experimentally measured in a tissue phantom (a non-conductive cylinder of 50 mm diameter and 10 mm height that was filled with a saline solution, with 1 mm diameter silver wire electrodes at various points around the perimeter of the phantom) these two amplitudes (iii, x-direction, iv, y-direction); channels were isolated as described in Figure S3. Finally, we plotted, along line cuts through the simulated (lines) and experimental (dots) datasets, the interferential electric field envelope amplitudes for the x-direction (v) and the y-direction (vi). Simulated and experimental values along the vertical line cut were plotted in gray, and along the horizontal line cut, in black; values were normalized to the peak. Color-maps in i-iv are in V/m. Envelope modulation amplitude maps in iii and iv are a linear interpolation of the measured values. Distances in v and vi were normalized to the phantom’s radius and shown relative to the center of the phantom. Circles in line plots v and vi are measured envelope modulation amplitudes without interpolation.

(A) Electrodes were placed in a trapezoidal geometry with a narrow base, and amplitudes of currents $I_1$ and $I_2$ were set to 1 mA.

(B) Electrodes were placed in a trapezoidal geometry with a wider base, with currents as in (A).

(C) Electrodes were placed in a rectangle, with currents as in (A).

(D) Electrodes as in (C), but now with currents in the ratio $I_1:I_2=1:2.5$ (holding the sum at 2 mA).

(E) Electrodes as in (C), but now with currents in the ratio $I_1:I_2=1:4$ (holding the sum at 2 mA).