Programming of the cochlear implant, or what is commonly called a “Mapping”, refers to the setting of the electrical stimulation limits necessary for the cochlear implant user to “hear” soft and comfortably loud sound. Since the Cochlear Implant can only process sounds in a very narrow range (6-15 dB), it must be able to compress the natural range of sounds (normal speech is usually between 40-60 dB) into this small band.
Due to this small range of sound that a cochlear implant is limited to, CI users are more sensitive to loudness changes. Therefore, “stimulation levels” must be set so the sounds processed by the cochlear implant should not cause discomfort to the CI user. These stimulation levels correspond to psychophysical (i.e. sensory responses to outside stimuli) measurements known as Threshold (T) and Comfort (C or M) Levels. During the mapping process, the “T” and “C” levels of each electrode on the cochlear implant’s internal electrode array (the spiral located inside the inner ear or “cochlea”) are adjusted for the user to hear the wide range of sounds that one is typically exposed to (soft to loud).
Threshold levels are set to allow the user access to soft speech and outside sounds. Comfort Levels refers to the amount of electrical current one needs for perception of a comfortably loud ‘beeping’ signal. These measurements are downloaded into the speech processor and then incorporated into a coding strategy, which the processor uses to send the electric signals to the internal implant in an organized manner. The processor will not allow the signals to enter it to exceed these set measurements.
Each MAP is individual to its user and is constantly changing. Over time, MAPs may become weak, softer, or less clear. This may be the result of tissue growth, or the user becomes used to their MAP and ‘adapts’ to it, finding that over time, it may no longer be a loud enough signal. Re-programming the device will compensate for the changes that may occur due to adaptation.
Threshold (T-levels) measurements may be obtained in several ways, depending on the age or functioning level of the patient. For children over age six, a hand raising method is used to determine T-Levels (similar to a hearing test). For children under three, a different method is used, where the child is trained to look at a moving toy in response to sound. The stimulation is then reduced until the lowest level that the child can detect the sound. Typically, two audiologists are used; one training the child and one manipulating the computer controls.
For children between the ages of three and six, play audiometry is used. The child is trained to place a block in a box in response to electrical stimulation. The stimulation is reduced to the lowest detectable level.
Comfort level measurements may also be obtained in several ways, depending on the age or functioning level of the patient. For children over 5, a loudness scaling chart or verbal response is used to determine Comfort Levels. For children under age five, the comfort levels may have to be estimated, using the threshold levels or other objective measures as a reference.
The mapping report provides printed out information on map parameters, threshold and comfort level settings. Parents’ understanding of this report is not necessary for the child’s progress with the device. It is merely a print-out of the settings created during the mapping session. It should be used only to be used as a reference for the mapping audiologist.
What may be helpful to the parent is a report of what is in each program slot. The parents should be given instructions as to when or whether to change programs; which program may be designated for an FM System; which may be a back-up program or an old program; which may be a noise program, etc.
When a mapping is performed efficiently, the user will have a hearing within a normal to near-normal hearing range. Parents may use the mapping sessions to discuss progress at home, school, and therapy. Questions and problem issues may also be addressed. The mapping audiologist will give recommendations on if and when to change programs as well as when a follow-up appointment should be made.