How far does electrical stimulation activate white matter tracts? A computational modeling study

The aim of this study was to model how the different parameters of electrical stimulation (intensity, pulse shape, probe geometry) influence the extent of white matter activation.

The electrical potentials generated by the stimulating electrodes were determined by solving Laplace equation. The temporal evolution of membrane potentials at each nodes of Ranvier of an axon was then computed by solving the coupled system of differential equations describing membrane dynamics and cable propagation.

Regions of unilateral propagation were observed for monophasic pulses delivered with a bipolar probe aligned along the tract. For biphasic pulses, the largest activation areas and depths were found with a high inter-electrode-distance (IED) bipolar probe, oriented orthogonally to the tract. The smallest activation areas and depths were found for bipolar stimulations with the probe aligned parallel to the tract and low IED. For isotropic white matter regions, the activation area and depth were three times larger than for anisotropic white matter tracts.

Bipolar probes with biphasic pulses offer the greatest versatility: an orthogonal orientation acts as two monopolars (increased sensitivity when searching for a tract), whereas a parallel orientation corresponds to a single monopolar (increased specificity). Activation is more superficial when stimulating highly anisotropic tracts.

This knowledge is essential for interpreting the behavorial effects of stimulation and the recordings of axono-cortical evoked potentials.