Influence of the heterogeneity of the tissue conductivity on the potential field and threshold currents.

<p><b>A</b>: In the originally homogeneous tissue (with <i>σ</i> = 0.1 S/m), ten 20-µm-sided cubes were added, in which the conductivity was alternatively set to 0.01 S/m and 1 S/m. The neuron model 3 was positioned at the edge of these cubes to maximize the effect of the heterogeneity. <b>B</b>: The extracellular potential field (centered on its spatial mean) is plotted along the neuron model for distances of 500 µm (left) and 3 mm (right) from the stimulation electrode. At 500 µm, the profile of <i>V_ext</i> (in red) displays larger variations than that obtained with the homogeneous model (in blue), with large gradients in the regions of low conductivity (light gray bands) and small gradients in the regions of high conductivity (dark gray bands). These variations result in smaller threshold currents than in the homogeneous model (2.1 mA vs 3.8 mA, respectively). At 3 mm, these field variations are even smaller leading to little decrease of the threshold currents (87.4 mA vs 100.0 mA).