Abstract:
The resistivity of bone is the most variable of all the tissues in the human body, ranging from 312 U cm to
84,745 U cm. Volume conduction models of cochlear implants have generally used a resistivity value of
641 U cm for the bone surrounding the cochlea. This study investigated the effect that bone resistivity
has on modelled neural thresholds and intracochlear potentials using user-specific volume conduction
models of implanted cochleae applying monopolar stimulation. The complexity of the description of the
head volume enveloping the cochlea was varied between a simple infinite bone volume and a detailed
skull containing a brain volume, scalp and accurate return electrode position. It was found that,
depending on the structure of the head model and implementation of the return electrode, different
bone resistivity values are necessary to match model predictions to data from literature. Modelled
forward-masked spatial tuning curve (fmSTC) widths and slopes and intracochlear electric field profile
length constants were obtained for a range of bone resistivity values for the various head models. The
predictions were compared to measurements found in literature. It was concluded that, depending on
the head model, a bone resistivity value between 3500 U cm and 10,500 U cm allows prediction of neural
and electrical responses that match measured data. A general recommendation is made to use a resistivity
value of approximately 10,000 U cm for bone volumes in conduction models of the implanted
cochlea when neural excitation is predicted and a value of approximately 6500 U cm when predicting
electric fields inside the cochlear duct.
