Biomedical Engineering Reference
In-Depth Information
FIGURE 6-32
Electric field around
a cochlear electrode
(a) Monopolar
configuration.
(b) Bipolar
configuration.
An important goal of electrode design is to maximize the number of largely nonover-
lapping populations of neurons that can be stimulated by the electrode array. However,
evidence suggests that even monopolar arrays with more than 20 electrodes stimulate only
between four and eight independent sites.
These overlaps are unavoidable for electrode arrays in the tympanic canal as the
electrodes are sitting in the highly conductive perilymph and are relatively far from their
target neurons. If the array could be placed closer to the inner wall of the tympanic canal,
it would be a little closer to the target neural tissue of the spiral ganglion. This can be
achieved to some extent if the electrode array is manufactured with a built-in spiral.
However, even if positioned perfectly the shortest distance between an electrode and a
neuron is about 1 mm. Assuming the inverse square law relationship between the distance
from an electrode and the current density holds, a stimulus of four times the threshold for
neurons in one position would start to trigger neurons 2 mm away from the target neuron
in both directions. For speech perception the critical frequency range is from 500 Hz to
3 kHz, and this represents a distance of less than 14 mm along the basilar membrane. It
is therefore obvious that a maximum of between three and four independent stimulation
sites can be accommodated using monopolar electrodes if a reasonable dynamic range is
to be achieved. This trade-off between the number of independent sites and the dynamic
range is illustrated in Figure 6-33.
With bipolar stimulation the current density gradient is very steep, and it is mostly
constrained between the two electrodes, as shown in Figure 6-32, allowing the number
of independent sites to be increased significantly. Because each neuron must be stim-
ulated by inducing a current along its length, the electrodes must be oriented at right
angles to the long axis of the tympanic canal. The actual number of independent sites
that can be stimulated depends on the condition of the remaining auditory nerve fibers
as these sometimes die back from the organ of Corti. However, it has been found by
experiment that radial bipolar pairs can be positioned about 2 mm apart without signifi-
cant interaction. This spacing makes it possible to achieve eight independent channels to
span the speech frequency sites lying between 10 and 24 mm along the cochlea (Loeb,
1985).
