Modelling surface potentials from intracochlear electrical stimulation

Volume conduction models were used qualitatively to model surface potentials from cochlear implant patients recorded earlier by the authors. These recorded potentials reflected the equivalent dipole orientation in the head in patients who are deaf due to otosclerosis, but increased uniformly with the distance between the stimulating electrodes along the basilar membrane in other patients, which suggested a low and high resistivity of the cochlear bone, respectively. Several models of the head were constructed, with compartments representing the skin, skull, brain, cochlea, internal and external ear canal. In the "petrous bone" model, the cochlea was modelled as a cavity in a bony layer surrounded by the brain compartment. Of all models, the petrous bone model using a high resistivity ratio (1:100) between the bony and the other compartments was the only one that produced outcomes similar to the potentials observed in non-otosclerosis patients. In conclusion, the results suggested that the surface potentials observed in non-otosclerosis patients are sufficiently explained by a high impedance between cochlear turns and a non-specific return of current via the wall of the petrous bone into the larger brain compartment.