Biomedical Engineering Reference
In-Depth Information
FIGURE 6-33
Relationship
between
independent
stimulation sites and
the signal dynamic
range of a
monopolar cochlear
implant.
If further improvement in the number of independent sites is required to improve
speech fidelity, it may require a fundamentally new type of electrode or a different place-
ment method (Wilson and Dorman, 2008).
A cochlear implant relies on the fact that many of the auditory nerve fibers remain
intact in patients with sensorineural deafness. These neurons can be made to fire using
electrical stimulation of the appropriate strength, duration, and orientation. These induced
responses arrive at the brain looking just like the impulses generated by acoustic signals
triggered by hair cells.
The perceived intensity of the sound is determined by the number of neurons acti-
vated and their firing rate, and these are both dependent on the amplitude of the stimulus
current. The pitch is related to the place on the basilar membrane from which those nerve
fibers originally derived their acoustic input, in agreement with the place-pitch theory. In
principle one should be able to recreate the normal neural response to stimuli if enough in-
dependent spectral channels could be excited, and this would result in the patient “hearing”
the sounds (Loeb, 1985).
New research conducted by Claus-Peter Richter at Northwestern University in Chicago
confirmed, using deaf guinea pigs, that illumination of the neurons of the inner ear using
infrared light stimulated activity in the interior colliculus (a neural relay point between
the inner ear and the auditory cortex in the brain). Spatial frequency maps of this region
were as sharp as those produced by real sounds, in contrast to the blurred maps produced
using electrical stimulation (Nowak, 2008).
At present, the mechanism that makes neurons sensitive to infrared light is unknown,
though it is speculated that it might be due to a rise in temperature. However, that notwith-
standing, if this technique proves to be safe then it opens the way to cochlear implants
with hundreds or even thousands of individual points of excitation.
