Introduction to fitting for Cochlear Implants
There are four major manufacturers of cochlear implants. The Australian Cochlear™ Corporation
, of which the partner BE-CTC (Cochlear Technology Centre Europe) is part, has the largest market share. The other two major suppliers are the US-based Advanced Bionics Corporation
, and the Austrian-based MED-EL
is the smallest player on the market. While these manufacturers have, in the past, advocated rather different speech processing methods, their current products all use broadly similar processing methods, based on the Continuous-Interleaved-Sampling (CIS) method first described by Wilson and colleagues (Wilson et al, 1991). This processing is intended to simulate the temporo-spatial coding of the acoustically-stimulated cochlea. The speech processor implements a bank of band-pass filters, covering a frequency range bounded at 100 to 300 Hz at the low frequency end, and extending to an upper limit at 5 to 8 kHz.
The Cochlear Corporation device uses 22 electrodes spaced along its array, the current Advanced Bionics implant has a 16 element array, while the MED-EL electrode has 12 pairs of electrodes (each pair sharing the same position along the array). The speech processor filter-bank generates amplitude envelope signals representing the output of each of the filter-bands. In the standard CIS method, the resulting set of amplitude envelopes are used to modulate the electrical currents delivered to the individual electrode sites, such that the lower frequency speech processor filters control apical electrode levels, and the higher frequency filters control basal electrode levels. The Cochlear Corporation and MED-EL also use, as an alternative to standard CIS, an “n-of-m” strategy (Wilson, 1993) which differs from CIS in one important respect. Here there are m filters (where m is typically the same as the number of usable electrodes) but only the n channels showing the highest signal levels at that instant of time are selected for stimulation in any given analysis time frame. For the Cochlear Corporation device, m is maximally 22, and n is a fitting parameter, typically set between 8 and 12. The n-of-m strategy implemented in Nucleus® cochlear implants is called ACE™.
A Cochlear implant generally stimulates the auditory nerve with series of short biphasic electrical pulses  . The pulses are biphasic because the net current through the tissue should be zero to avoid unwanted long-term electrochemical effects. Since stimulating multiple electrodes at the same time can give an unpredictable loudness percept because of channel interactions (addition of stimulus voltage fields) most current commercial coding strategies use sequential stimulation. The rate of pulse stimulation to an electrode depends on processing strategy. The slowest pulse rates in use are 200 pulses/s (pps). A pulse rate of around 800 pps is common to several strategies, while higher rates of up to 5000 pps can be used in some recent strategies. With pulsatile stimulation within these ranges of rate, the percept is not of a burst of pulses, but rather as a continuous signal. The most crucial aspect of fitting for a cochlear implant is to establish the lowest and highest usable stimulation level for each electrode in the array, and this is a common feature of all cochlear implants.
Since the auditory nerve may have only about 6-12 dB dynamic range for electrical stimulation, we need to compress the +100 dB acoustical window available to normal hearing considerably. This is done by first ‘selecting’ a small part of the acoustical window (normally about 30 dB; the speech dynamic range) by means of an Automatic Gain control. The width of this window is referred to as the instantaneous input dynamic range (IIDR). The IIDR is further compressed in an instantaneous non-linear compression to match the very small dynamic range for electrical stimulation.
Fitting methods can only be implemented through tools provided by the manufacturer. This is essential for device safety. It also ensures that the basic fitting methods use the same parameters are in every country. The most important fitting parameters are listed below. It must be noted that these parameters are somewhat strategy and device dependent, some parameters will occur in different devices under different names. Almost all of these parameters are used in the fitting of all three manufacturers; implants.
An example screenshot of Custom Sound™, the clinical fitting software of Cochlear.
 A few alternative schemes exist although these are not commonly used in current devices. For example, the SAS processing scheme of the Clarion implant presented continuously varying current waveforms to each electrode. Other schemes are in development that consider triphasic or other more complex pulse patterns. In general, however, the fitting approaches for these forms of stimulation are similar to those for biphasic pulses.
Nucleus is a registered trademark of Cochlear Limited
ACE is a trademark of Cochlear Limited