© Borgis - New Medicine 3/1999, s. 33-34
Artur Lorens, Lech Śliwa, Adam Walkowiak
Principles of speech processor fitting in the programme of rehabilitation of children after cochlear implantation
Institute of Physiology and Pathology of Hearing, Warsaw, Poland
Director: Prof. Henryk Skarżyński, MD, DSc
In order for the child to achieve optimal benefit from rehabilitation after cochlear implantation, speech processor fitting must be performed repeatedly.
The initial objective of programming is to obtain a behavioural and comfortable loudness level for each electrode. The authors present methods of adjusting the speech processor electronically to the child´s individual electric current requirements, which have been developed and implemented in the Institute of Physiology and Pathology of Hearing. Knowledge of these problems might be useful for otolaryngologists to understand the needs of implanted children better.
A cochlear implant is applied in cases of profound and total deafness, when a patient has no benefit from conventional hearing aids. The cochlear implant provides direct electrical stimulation of the auditory nerve. Thus it is possible to induce hearing sensations by-passing the damaged inner ear and enable perception of environmental sounds using the remaining efficient part of the auditory pathway. In order to realise this task, the implant system must convert acoustic information into a suitable electrical signal. The conversion is performed within an integral sub-unit of the system, the speech processor. The results that the patient can achieve in the post-operative rehabilitation period depend to a great extent on appropriate fitting of the processor, and its parameters must be carefully adapted to the individual needs of a patient.
Speech processor function
The speech processor provides continuous electrical stimulation of the auditory nerve. The elicited hearing sensations should be similar to those which would result from the perceived original acoustic signal. In modern cochlear implant systems, several techniques are employed to meet this objective. Generally, the hardware of a processor can be of two different types, consisting of analogue or digital circuits.
As far as the signal processing method is concerned, there are several different algorithms, called speech coding strategies, that can be used.
Speech processor fitting
Fitting of a speech processor includes selection of an appropriate coding strategy, and loading specific parameters determining the form of the output signal most suitable for the particular patient. In digital processors, the parameters include stimulus amplitudes referring to hearing sensation threshold THRµA1 and comfort hearing level MCLµA, stimulus pulse duration, the compression coefficient of acoustic-to-electrical signal conversion, stimulation polarity mode (bipolar or monopolar), etc.
In analogue systems, the transmitting circuit´s frequency characteristics are varied.
Parameters such as THRµA and MCLµA, are determined based on psychoacoustic tests. Accurate measurement of the parameter values is very important, because this determines the range of the stimulating current and the dynamics of the auditory nerve reaction to electrical stimulation (Lorens 1998). The auditory nerve reaction dynamic is typically not much more than 10dB, while the speech signal is over 30dB. Therefore, an appropriate signal compression must be applied along with signal processing. Relatively small errors in determining the values of parameters THRµA and MCLµA may lead to a situation in which the patient will not perceive hearing sensations on an appropriate level, assessing the subjective hearing level as too high or too low, or will not able to distinguish changes in acoustic signal magnitude. The above parameters determine the compression characteristic of the speech processor. Therefore, precise measurement of THRµA and MCLµA parameters is critical for speech understanding (Lehnhardt, 1992).
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