The Cochlear Implant (CI)
The sound that we can perceive is the results
of a pressure wave travelling through the air and reaching our ears. The
vibration is then transmitted to the ear drum and then the ossicles. The last
element of these ossicles, named ‘stapes’, conveys this vibration to the
entrance of the tiny coiled organ called the cochlea. The cochlea is made up
with tubes coiled around the main axis and separated by a flexible membrane
called the basilar membrane.
Movements of the stapes generate a displacement of the fluid filling the cochlea which make the basilar membrane vibrate. This vibration excites very specific cells, named hair cells, distributed along the membrane. These specific cells are responsible for the conversion of the mechanical vibration into electric signals travelling through the neural fibers to the central nervous system (CNS).
The pitch perception is mainly ruled by the location of the vibration along the basilar membrane, that is to say, the location of the excited hair cells. A high-pitched sound is due to a vibration at the base of the membrane (see figure below) while a low-pitched sound is due to a vibration at the end of the membrane (called the apex). This frequency organization of the cochlea is known as tonotopy.
Cochlear implants enable to restore an auditory
perception in case of severe to profound hearing loss resulting from the
absence or the deterioration of hair cells. Even if the rest of the auditory
system remains intact, without those hair cells, no message is convey to the
CNS.
Cochlear implant replaces the entire peripheral system, from the ear to the excitation of neural fibers (see figure below). To do so, it is made up with an external part, located on the temporal lobe which converts the sound received by a microphone into an electrical stimulation code. An implanted part receives the informations of the stimulation code and activate the corresponding electrodes.
The activation of different electrodes aims to excite certain neural populations to reproduce the tonotopic repartition of the neural excitation that would results from acoustic stimulation.
Results and limitations
Recent cochlear implant users are able to
achieve a remarkable good performance in speech recognition in silence. Those
impressing results can explain the quick development of cochlear implant in the
past ten years. Nowadays, more than 200,000 patients received a CI. Unfortunately
its performance appear to be dramatically lower in the presence of noise or for
the music appreciation. One must be aware that CI do not restore a perfect
audition and that CI users who managed to yield impressive performance had to
practice a long re-education.
Even though only four electrodes can be used to achieve rather good performance in silence, one can reasonably think that more electrodes could provide better performance in the presence of noise or for music appreciation or speakers discrimination. Indeed the more electrodes are available, the finer the signal analysis is. However the number of implanted electrodes is limited (12 to 22 contact depending on the models). This number remains lower than the number of independent neural population in a normal cochlea.
Unfortunately the electric fields generated by the electrodes spread within the conductive medium and thus each electrode excites a broad neural population.
When more and more electrodes are used, those neural populations begin to overlap. These interactions are commonly pointed out as a main limitation for the good performance of CIs.
Research
Why?
The main goal of research projects on cochlear implants is to enable an optimal auditory perception of their environment to CI users, whatever the conditions.
Despite the technologic and technical breakthroughs, to improve cochlear implants one must improve our knowledge on the human auditory system. One main issue being the understanding of how the human neural fiber respond to an electrical stimulus.
How?
To improve our knowledge on the functioning of the human auditory system both for normal and implanted ears, several ways are possible.
Psychophysics : Psychophysics is based on the analysis of subjects’ perception. By strictly controlling stimuli properties (duration, level, frequency, quality etc.) and then analyzing how those parameters are perceived by subjects, one can make progress in the understating of the human auditory system.
To study some features specific to the working of cochlear implant one can use acoustic simulation. We are currently working on the design of a new acoustic simulator, more realistic than those used in previous studies. The aim of those simulations is to reproduce and control the different stages operated by CI processors and electrodes. While signal processing parts can easily be reproduced on a computer, simulating the response of neural fibers to an electrical stimulus necessitates the use of simplified models and hypothesis. Indeed, subjects who participate these experiments are normal-hearing subjects. Hence, one must create an acoustic stimulus whose perception is comparable to what CI users would hear through their device. Many features can be tested with those simulators and their influence can be investigated with a speech recognition task.
Where?
Some specific tests can be done in hospital department, however, most tests can be done in the laboratory. Normal hearing people as well as CI users regularly come to our lab (Laboratoire de Mécanique et d’Acoustique) to participate experiments from different research projects.
Facilities: -anechoic chamber
-sound insulated booth
-processor and emulator of the external part of a CI device
-electrical stimuli control software
-numeric oscilloscope to control electrical stimuli