The factors of change of psychophysical parameters at cochlear implant users
Abstract
The operation of the speech processor plays a major role in the process of hearing with a cochlear implant. The speech processor must be programmed so that the hearing sensation that arises by electrical stimulation must be as close to the normal hearing sensation as possible. In the course of programming the speech processor one must define the stimulation parameters, the stimulation mode, the speech coding strategy and the frequency table. During the fitting process it is the most important to determine the exact electrical threshold and comfort levels, which assumes a series of psychophysical measurements to be taken. The intensity at which just a hearing sensation is evoked (threshold level) and the intensity that causes a bearable loudness (the comfort level) must carefully be determined on each electrode. The speech processor will then transform the external sound stimuli – with the use of the appropriate speech coding strategy – to this dynamical range.
Psychophysical parameters show a temporal change which can be traced back to numerous reasons. The electrodes implanted into the inner ear will be covered with connective tissue which changes the electrodes’ impedance. This way to reach the same stimulation one must set different amplitudes for the stimuli. During an adaptation period the central nervous system gets used to the louder sounds (especially in the case of praelingual and long lasting post lingual hearing loss). The comfort level rises, the hearing threshold drops, the dynamical range widens. As the result of continuous stimulation some regeneration processes occur at the periphery of the hearing nerve. This causes the change of thresholds (sometimes drastically). Numerous reasons can cause the electrodes to dislocate inside the cochlea. Because of the small sizes a sub-millimeter dislocation can cause a significant change in the threshold levels. Having the above in mind we must say that the regular programming of the device is very important.
