Mapping the Cochlear Implant
A Parent's Guide to Understanding a Cochlear Implant Programming
Nicole C. Sislian, MA, CCC/A
One of the routine events in the life of a Cochlear Implant child is the many visits to their cochlear implant center to reprogram the implant – or to get a new Map set. What is a Mapping, and what information should parents come away with that would help them in the overall development of their child? Nicole Sislian, supervisory audiologist at the New York Eye & Ear/Beth Israel Cochlear Implant Center, gives us a detailed explanation of the MAP and what it accomplishes.
The Need for a MAP
Reprogramming of the cochlear implant, or what is commonly called a “MAP”, refers to the setting of the electrical stimulation limits necessary for the cochlear implant user to perceive soft and comfortably loud sound. Normal acoustic hearing can process sounds within a 120dB range. Normal speech ranges anywhere between 40 and 60dB (the shaded area of this audiogram). Cochlear implant recipients have a dynamic range of only 6-15dB in electrical current. Therefore, in cochlear implant speech processors, a 120dB acoustic range must be compressed into an electric range of 6-15dB.
Due to the small electrical range that a cochlear implant is limited to, CI users are more sensitive to loudness changes, and loudness growth differs from those with ordinary hearing. Cochlear implant processors must compress acoustic amplitudes (i.e. the range of all the different levels of sound) into this small range. 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 threshold and comfort levels of each individual electrode on the cochlear implant’s internal electrode array that is located inside the ear are adjusted in order for the user to hear a wide range of sounds in ‘everyday life’ (soft to loud).
Threshold levels are set to allow the user access to soft speech and environmental sound. Comfort Levels refers to the amount of electrical current a user 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 entering it to exceed these set parameters.
Updating the MAP
A cochlear implant user needs to have their MAP frequently updated. This is because each MAP is individual to its user and is constantly changing. Over time, MAPs may become weak, softer, or less clear. This is the result of several factors including adaptation and tissue growth. Fibrous tissue continues to grow over the internally implanted device (electrodes and receiver) during the months following surgery. These changes may change the amount of electrical stimulation needed to stabilize the signal. The greater the tissue growth over the implant device, the more power is required to stimulate it. If tissue growth occurs without mapping, the signal will begin to fade.
Also, when the MAPs are new, they tend to sound louder. With experience, the user becomes accustomed to the stimulation and ‘adapts’ to it, finding that over time, it may no longer be a loud enough signal. This is called adaptation. This is similar to when one listens to music at a set volume level for a long period of time. It sounds comfortably loud at first, but as the minutes pass we find ourselves raising the volume. Reprogramming the speech processor compensates for any changes that occur due to adaptation.
How a MAP is Performed
Threshold measurements may be obtained in several ways, depending the age or functioning level of the patient. For adults and children over age six, a hand raising method is used to determine T-Levels (similar to a hearing test). This is done by having the child trained to raise their hand in response to hearing a sound until the threshold is reached. Typically, one audiologist is used. Electrical stimulation is reduced until the user detects a sound 100% of the time.
For children under three years of age, the Visual Response Audiometry (VRA) method is used. 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 is able to detect the sound. Typically, two audiologists are used; one training the child and one manipulating the computer controls.
Play audiometry is used for children three years to six years old. The child holds a block or other toy in their hand, usually at the ear of the side that is being MAPed, and is trained to place it into the box when they perceive the sound. The stimulation is reduced to the lowest detectable level. Again, two audiologists are used.
Comfort level measurements may also be obtained in several ways, depending the age or functioning level of the patient. For adults, a loudness scaling chart (giving a sound a value from one to ten) or verbal response is used to determine Comfort Levels. For children over the age of five, this method may also be used. For children under age five, the comfort levels may have to be estimated, using the threshold levels or objective measures (see below) as a reference.
Objective methods are when the audiologist monitors the brain’s response to sound input, such as Neural Response Monitoring or Electrically Evoked Stapedial Reflexes. These methods allow the audiologist to set threshold and comfort levels without patient input.
After every MAP, the user is asked to repeat various speech sounds and words. The MAP may be altered for sounds that are missed or confused.
In addition to regular MAPs, the cochlear implant patient must also be periodically evaluated. A soundfield audiogram and speech perception performance must be tested using the implant in the sound booth. These results assist in the MAPing process. Any speech frequency tested which is not being perceived within the typical hearing range may be boosted during the mapping. These tests are done less frequently than a MAP, typically twice a year. Younger children and those recently implanted may have these tests performed more often.
The mapping report provides printed out information on map parameters, threshold and comfort level settings. It is not essential for parents to understand the information contained in this report. It is merely a print-out of the settings created during the mapping session. It should be used only 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 clear instructions from the audiologist performing the MAP on the following: a schedule of when or whether to change the programs; which programs 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.
The MAP procedure is programming the speech processor so that the normal acoustic dynamic range may be compressed into the small electrical range of the cochlear implant device. When a MAP is performed efficiently, the user will have hearing within a normal to near-normal range. Parents should use the mapping sessions to discuss their child’s progress at home, school, and in therapy. Questions and problem issues may also be addressed. The audiologist will give recommendations on if and when to change programs as well as when a follow-up appointment should be made.
Nicole Sislian, M.S. is the Supervisory Audiologist at the NY Eye & Ear / Beth Israel Cochlear Implant Center in New York City. She performs MAPs and Auditory Evaluations on children with cochlear implants.