Fitting and mapping of the Cochlear implant system


This stage is most serious with regard to the function and the performance of the cochlear implant. The programming and the mapping of the C.I. are always executed by doctors or audiologists, but under medical supervision, not forgetting that we send electric current directly in the acoustic nerve, so it is a “medical act”.
It requires patience and insistence firstly on the part of the doctor who makes periodically the mappings of the device trying to find for each patient the most excellent intensity and configuration with which the sound messages should be channelled so that the patient has a pleasant and normal hearing. Secondly when we deal with children on the part of the speech therapist, who will learn the child, that had never heard in his life, to translate the acoustic signals to speech with the help of playing audiometry and acoustic behaviour audiometry and thirdly on the part of the patient and his environment.
The objective is, by using the knowledge of the prosthetics audiometry, of the hearing neurophysiology and the paediatric clinic audiology, to map the C.I., so as the user to acquire such hearing threshold that it allows him speech recognition, with sufficient ability of discrimination.
Through the duration of session when the exterior speech processor is programmed in order to provide comfortable levels of hearing for speech, receiving reliable correspondence from a very small child can be time-consuming and difficult. The implants allow the use of objective measurement, the Electrically Evoked Compound Action Potentials (ECAPs), that can be recorded via the processor and via the Neural Response Telemetry (NRT) technique. These recordings are used in order to supplement the behaviouristic answers and they provide much better mappings, therefore and hearing, to the very young children.
An additional aspect of programming is which strategy of speech processing is adopted. This influences factors such as the rate with which the information is sent in the channels and if the channels are stimulated together or separately, as well as various other technical characteristics. The more important adjustment, in each strategy, are the rate of stimulation of the nerve in each individual electrode (rate), the range of each vibration of the stimulation (pulse) and the way of irritation. Other more specialised adjustments are the sensitivity of the microphone, the gain of input for each electrode, the automatic control of input gain (AGC) for the total signal, the starting level of the stimulation (Base Level) and the value Q that fixes, in the algorithm of the strategy, the nonlinear relation curve of input/output stimulation.
The each new planning is evaluated by the record of the patient with regard to the acoustic perception of sounds and speech and the discrimination of individual elements of phonation based on a protocol that we have created for the needs of Greek language.