Field patterns in a 3D tapered spiral model of the electrically stimulatedcochlea

Despite the fact that cochlear implants are widely and successfully used in clinical practice, relatively little is known to date about the electric f ield patterns they set up in the cochlea. Based upon the available measurem ents and modelling results, the scala tympani is usually considered to be a preferential current pathway that acts like a leaky transmission line. The refore, most authors assume the current thresholds to decay exponentially a long the length of the scala tympani. Here we present potential distributio ns calculated with a fully three-dimensional, spiralling volume conduction model of the guinea pig cochlea, and try to identify its preferential curre nt pathways. The relatively well conducting scala tympani turns out to be t he main one indeed, but the exponential decay (J similar to e(-z)) of curre nt is only a good description of the far-field behaviour. In the vicinity o f the electrodes, i.e. near the fibres that are most easily excited, higher current densities are found, that are best described by a spherical spread of the current (J similar to 1/R-2) The results are compared with those ob tained with a variant of our previous, rotationally symmetric, model and wi th measurements in the literature. The implications of the findings are dis cussed in the light of simulated neural responses. (C) 2000 Elsevier Scienc e B.V. All rights reserved.