Current focussing in cochlear implants: an analysis of neural recruitment in a computational model.

Several multipolar current focussing strategies are examined in a computational model of the implanted human cochlea. The model includes a realistic spatial distribution of cell bodies of the auditory neurons throughout Rosenthal's canal. Simulations are performed of monopolar, (partial) tripolar and phased array stimulation. Excitation patterns, estimated thresholds, electrical dynamic range, excitation density and neural recruitment curves are determined and compared. The main findings are: (I) Current focussing requires electrical field interaction to induce spatially restricted excitation patterns. For perimodiolar electrodes the distance to the neurons is too small to have sufficient electrical field interaction, which results in neural excitation near non-centre contacts. (II) Current focussing only produces spatially restricted excitation patterns when there is little or no excitation occurring in the peripheral processes, either because of geometrical factors or due to neural degeneration. (III) The model predicts that neural recruitment with electrical stimulation is a three-dimensional process; regions of excitation not only expand in apical and basal directions, but also by penetrating deeper into the spiral ganglion. (IV) At equal loudness certain differences between the spatial excitation patterns of various multipoles cannot be simulated in a model containing linearly aligned neurons of identical morphology. Introducing a form of variability in the neurons, such as the spatial distribution of cell bodies in the spiral ganglion used in this study, is therefore essential in the modelling of spread of excitation. This article is part of a Special Issue entitled <Lasker Award>.