Membrane polarization is well predicted by the <i>mirror</i> estimate, and is small at large distances.

<p>Longitudinal profiles of extracellular potential <i>V_ext</i> (<b>A</b>) and membrane potential <i>V_m</i> (<b>B</b>) are plotted for the neuron model 3 (illustrated at the top of the figure), located at 500 µm from the stimulation electrode, for a current of −/+ 442 µA, a level that elicits a spike in experiments. Both profiles are the <i>mirror</i> image of each other. The inset in B shows that the membrane potential at the end of the cathodic phase of the stimulus is well predicted by the <i>mirror</i> estimate (Eq. 6). It can further be noted that the extracellular potential plotted in panel A displays very small variations around its spatial mean (dashed line). Similarly, the variations of <i>V_m</i> around the resting potential are only of a few mV (B), which is not enough to reach the activation threshold and elicit a spike. At this distance, a current of 6080 µA is actually required to elicit an action potential in the modeled neuron, about 15 times higher than the experimental thresholds.