Development of a Model of the Electrically Stimulated Cochlea - Biomedical Technology - page 148

2.4 Individual Model of a Degenerate Nerve Fiber

It has been shown that in persons with profound hearing loss, the dendrites are almost
lost but the somas and axons survive, this is known as retrograde neural degeneration.
It has been also shown that the degree of retrograde concerns only Type I auditory
nerve fibers [ 20 ]. A degenerate version of the auditory nerve model similar to the
one presented in [ 20 ] was used to simulate its effect. This was modelled by removing
the first four nodes of each nerve presented in Fig. 4 a.

3 Results

3.1 Experiment 1: Simulation of the Voltage Distribution
for Different Electrode Positions

In the first part of the experiment we simulated the intracochlear voltage at different
electrode positions. Current CI devices are equipped with measurement circuitries
that provide enough resolution to capture the intracochlear potential at the electrode
contacts. This objective measurement is known as electrical field imaging (EFI) [ 12 ].
In an EFI measurement current is typically flowing between one intracochlear elec-
trode contact and an extracochlear reference located near the housing of the implanted
stimulator (although the reference electrode position depends on cochlear implant
manufacturers). This monopolar stimulation forces the current to spread through all
cochlear tissues. The details of the intracochlear potential profiles, therefore, depend
on the anatomy and conductivities of the tissues and on the properties of the electrode
contacts. These factors differ from subject to subject and may vary over time [ 11 ].
The finite element model of the electrically stimulated cochlea was used to simulate
an EFI measure. Monopolar stimulation was simulated stimulating one active elec-
trode within the array in the cochlea, and defining the reference electrode as a cube
of size 50 50mm that houses the cochlear implant. Three different electrode

insertions/positions in the scala tympani were simulated (Fig. 5 ): (1) Modiolar elec-
trode position where the electrode is placed close to the nerve, (2) midscalar electrode
position where the electrode array is placed in the middle of the scala tympani, and
(3) lateral electrode position where the electrode array is placed close to the lateral
wall of the cochlea.
Each electrode was stimulated individually and the resulting voltage was mea-
sured at the stimulating and non-stimulating electrode contacts. Figure 6 presents the
results of the simulations for the three electrode positions. As expected, the model
simulations show two distinct areas around each electrode being stimulated [ 2 ]. The
first being the reactive component originating beyond the stimulated electrode. This
region shows voltage distribution that decays exponentially with electrode distance.
The second region corresponds to the reactive component originating at the electrode
nerve interface of the electrode being stimulated. It has been shown that the voltage

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