COCHLEAR
IMPLANT
SIMULATION
version 2.0
Copyright: ATV, MBM, RTV, MSQ, University of Granada
Granada, December 2004
All rights reserved.
A cochlear implant is an electronic system that is used to provide hearing to
subjects affected by severe or profound hearing loss. The system consists of two
main elements, an external processor and an internal element that is implanted
into the patient by means of a surgical operation. The implanted element has
one electrode array which is allocated into the cochlea, in order to provide
stimulation of the auditory nerve by means of electrical stimuli.
The basic operation of the cochlear implant is the following: The processor
has a microphone to get the sound. The processor analyzes the sound and
determines the stimulation level to be sent at each electrode (and its evolution
in time). The stimulation pattern is sent into the internal part of the system
by a radio transmission, and the internal part generates the electrical pulses,
that are presented at each intra-cochlear electrode of the implant. The pulses at
each electrode cause the activation of the neural ends of the auditory nerve, and
this activity is transmitted to the central nervous system, providing a hearing
sensation to the patient.
Currently, cochlear implant is the only effective solution for most of the
severe and profound hearing losses for which hearing aid is not enough. Today
there is enough experience with this treatment (there are about 50.000 implanted
patients around the world) and its efficiency is not questioned. This way, there
is a considerable percentage of implanted patients who are able to communicate
through telephone using a cochlear implant (that is, they are able to understand
speech from the hearing provided by their implant, without any visual aid and
under moderate noise levels).
Knowing with some precision how the sound is perceived with a cochlear implant
is extremely difficult, because of the amount of involved factors:
- Technological factors: technical features of the cochlear implant system
(stimulation rate, disposition and number of electrodes in the implant,
stimulation mode), signal processing performed from acquisition of audio
to generation of stimulation, coding strategy, etc.
- Surgical factors: allocation of the electrode array, its insertion depth, etc.
- Physiological factors: index of surviving neurons, refractory period, dynamic
range for electrical stimulation, intensity resolution, etc., and the
variation of such parameters for each cochlear portion.
- Other factors: appropriate programming map (fitted to the patient) in the
processor, duration of deafness, age of beginning of deafness related to the
auditory and language development, auditory experience before deafness,
ability and disposition for learning and auditory training, etc.
It is well known that all these factors affect in any way to sound perception
by implanted subjects, but it is difficult to evaluate the influence of each one
(in relation with the others) or which one will condition the hearing perception
quality with the highest influence. In order to know how a cochlear implant
wearer perceives the sound we have some possibilities:
- Testimony from patients. The information provided by patients about the
quality of their sound perception is of great interest for understanding how
the implant affects to hearing quality. This information has the disadvantages
that it is very subjective and also that it is affected by the patient
auditory experience (both, previous and following the cochlear implant
activation).
- Indirect observations. By means of tests designed for evaluating certain
aspects of hearing quality of implanted patients, obtaining additional specific
information is possible.
- Analysis of audio signal transformations. By applying Signal Processing
techniques we can simulate the transformations that take place in the
audio signal from its acquisition to the neural ends activation. This way,
the information loss associated to both, the analysis process (due to the
processor, the implant and the coding strategy) and to the interaction
between electrodes and neural ends, can be represented.
The purpose of “Cochlear Implant Simulation” project has been the combination
of both, the experience acquired through patients testimony and observation
of their abilities, and the knowledge about technological aspects in cochlear
implant systems and physiology of hearing, in order to develop an application
that simulates hearing perception through the cochlear implant. This simulation
system aims to verify the following features:
- Representing the main factors conditioning the quality of hearing perception
through the cochlear implant.
- Providing a system to perform simulations with audio files in order to allow
normal hearing people to hear the sound like perceived by an implanted
subject.
- The simulation system is intended to be a generic simulation platform,
in order to allow simulations according to the features of the different
cochlear implant systems available in the market, or even according to
features of fictitious systems not available in the market.
- The simulation system is intended to work under a user-friendly interface,
allowing its usage by all the parts involved in the treatment of hearingloss
with cochlear implant (the patient’s parents and relatives, teachers,
speech therapists and professionals involved in rehabilitation, psychologists,
technicians involved in fitting, ENT specialists, etc.).