COCHLEAR IMPLANT SIMULATION       version 2.0              

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Sound perception through a cochlear implant

What is a cochlear implant?

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).

How does a patient with a cochlear implant perceive sound?

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.

Purpose of “Cochlear Implant Simulation” project

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.).