Cochlear Implant fitting parameters
 
The following is a list of the most commonly varied parameters. This nomenclature used is focussed on Nucleus® implants but similar parameters exist in most cochlear implant systems.
 
Channel: a combination of electrodes (sources and sinks) that typically is associated with the output of one of the spectral analysis bandfilters.
T level: Threshold level – the minimum stimulation level (current) that the subject can detect (i.e. hear) on a channel.
C level:  Comfort level – the loudest comfortable stimulation level (current) on a channel.
Number of channels: The total number or stimulation channels available (depends on the device and also on the number of electrodes that are usable for any given Implant recipient).
Number of maxima: The number of stimulated channels in each cycle for n-of-m strategies such as ACE™
Rate:  The rate of sequential stimulation.
Update rate: The rate at which the spectral information used to modulate the electrode output is updated.
C-SPL: Determines the input dB SPL value at which the infinite compression starts to operate and the implant stimulates at C-level. Input sound levels above C-SPL will be stimulated at C-level.
T-SPL: The input SPL level where the implant starts to stimulate at T-level. Note that C-SPL is equivalent to sensitivity and C-SPL minus T-SPL is equal to IIDR. Input sound levels below T-SPL will, generally, not lead to stimulation output.
Compression curve: The steepness of the infinite instantaneous compression curve that maps filterbank output to electrical dynamic range (Q-value in Nucleus® terminology).
FAT The Frequency Allocation Table determines which frequency range is connected to which electrode/channel.
Stimulation mode: Defines how the channels are formed, for instance we can stimulate between two neighbouring intra-cochlear electrodes (bipolar mode) or between an intra-cochlear and an extra-cochlear electrode (monopolar). Monopolar stimulation is the default for Nucleus systems and by far the most commonly-used mode..
One of the main problems in implant fitting, unlike the fitting of hearing aids, is that the inter-subject variation of T and C levels is much larger than the average dynamic range (C-T) for an individual subject. This means that a map for one recipient may be either inaudible, or far too loud for another individual. When a map is far too loud, this may even cause painful stimulation of non-auditory nerve fibres, hence it is something to avoid at all times. The latter remark is especially true for children because over-stimulation can create anxiety about the implant/hospital/audiologist and in this way hamper the habilitation process. This means there is no way one can start with a ‘default’ fitting in all subjects. As an example: the average T level can vary between roughly 100 and 1000 m A, a 20 dB difference [1] , while the average dynamic range is only about 4 dB.
T and C levels are by far the most important map parameters, since this is what makes a map audible/comfortable. All the other parameters can be considered fine-tuning for audio quality. But we do know that some recipients do much better with one setting of for instance rate, while others may do better with a different setting, and yet others do equally well in both settings.
This part of the portal describes the classical fitting method and after that some new ways that are being used today . It will focus primarily on T and C levels setting. While the examples refer to work using the Cochlear Corporation devices, the principles are widely applicable. Other CI fitting research forming part of the HEARCOM project is outlined here .
Read here for more information on the relation between CL and current in Nucleus series implants.
Example of the parameter window of Customd Sound™, the clinical fitting software of Cochlear.

Example of the parameter window

Back to Cochlear Implant Fitting.


[1] The Nucleus implants use the logarithmic CL scale: 1 CL is about 2% increase in current, 6 CLs is about 1 dB. Other manufacturers use similar logarithmic current scales but with different reference values.