Nationwide Children's Hospital is holding a Hearing Team Support Series which discusses important topics for families of children with hearing loss. I attended the Individualized Education Plan session in July and this past week I attended the session on Cochlear Implant Mapping. The presentation was excellent, and I learned a lot of technical information regarding implant mapping, specifically for young children, that I did not know. Cochlear Implant Mapping is very technical, and even a year later, I still have a difficult time understanding it all. As parents, we play a crucial role in the mapping process, so it is important that we understand as much as possible about this technology.
The information I will provide was taught at the presentation and is correct, to the best of my understanding, and is specific to the Cochlear Brand of implants. There are slight differences between the three manufacturers, so the terminology used may vary slightly from one manufacturer to the next. If you feel that any information provided is inaccurate, please leave a comment so I can research further.
There are three parameters of sound that the audiologist is programming during the mapping session that allows the implant to translate external sound into the electrical signal that stimulates the auditory nerve, allowing for perception of sound. These three parameters include pitch, loudness and tempo.
With normal, acoustic hearing pitch is heard from 20 - 20,000 hertz (Hz). Cochlear Implant maps provide pitch across 120 - 7200 Hz, which are the primary speech frequencies. The hertz distribution is coded among the available electrodes providing electrical stimulus for the cochlear implant recipients. Individuals with cochlear implants do not hear the entire distribution of sound like a person with normal hearing. For example, the Middle C of a piano is at a pitch of 250 Hz. As you move down the piano, the hertz get lower and lower, so an individual with electrical hearing can not tell the difference in the pitch of the sound outside of the parameters of their map. Programming the pitch across the electrodes is critical for hearing all of the nuances of speech. Consider the "eeee" sound, for example. "Eeee" is not just one sound, but a collection of sound across different frequencies. In order to develop speech equivalent to hearing peers, pitch programming is essential.
The loudness, or intensity of the sound affects the voltage required to run the map. With acoustical hearing there is a loudness range of about 120 decibels (db). With electrical hearing, there is a range of roughly 40 db's. This is called the dynamic range and it is the difference between the threshold level, T-level and Comfort level, C-level. The T-level is lowest level of stimulation needed to hear. The C-level is the maximum stimulation of sound that can be comfortably tolerated. The difference in the dynamic range is no more than 40 db's for a person with a cochlear implant. For our children, patient participation is not practical, so the audiologist must find a different way to establish the T and C levels. Neural Response Telemetry is software provided by Cochlear that provides the audiologist with with the levels need for an individual to hear. There can be some minimal variability in what the computer suggests as T and C levels for the patient, so the audiologist with also use their past experience, updated information from parents, teachers and therapists and hearing tests (in the sound booth, with conditioned response when age appropriate) in order to program the processor.
I found this area of the presentation to be the most interesting. When looking at the dynamic range, this means that the map actually takes really soft sounds and increases them to the T-level, and it takes very loud sounds and decreases them to the C-level. This finally explains why Drew is so loud sometimes! He has no idea how loud his screams actually sound to me, with my natural acoustic hearing. We can roughly estimate, knowing that the goal for cochlear implant recipients is to hear at 20 db, that Drew is hearing his loud screams at what we would know 60 db's to sound like. With a dynamic range of only 40 db's (versus my range of 120 db's), Drew has no idea how loud he sounds! (I don't know why I didn't understand this prior to this course, but finally his behavior is making sense to me! As a side note, if any parents with older cochlear implant children have any suggestions on how to help Drew understand that he is actually louder than what he hears, please leave a comment.)
The final area of the programming revolves around tempo. This is referring to how fast the speech is coded. With the newest cochlear implant technology, speech can be coded between 250 - 1800 Hz. In layman's (my kind of) terms, this means how many times per second sound is coded through the electrodes in order to stimulate the auditory nerve fibers. This provides the recipient with detailed information on the timing of sound. The default is 900 Hz, which means that the processor codes sound 900 times per second. This can be tweaked for each individual, but apparently faster is not better, because if the sound is coded too fast, the recipient can have a difficult time interpreting sound. For children have little participation in the mapping process, most audiologists use the default settings.
During the time of activation, and shortly thereafter, mapping sessions are quite frequent. As a map is established, there are several ways that a parent can identify the need for a mapping session:
Decrease in child's auditory reaction or alertness to sound
Decrease in vocalizations and/or vocal play
"Slushy" production of previously mastered speech sounds
Any sign of physical discomfort, such as eye or facial twitches
After one month of use, the child should tolerate their everyday program all day. If they require a quieter map to start the day, consult with the audiologist.
Difficulty operating external equipment
Irritation to the skin under the transmitting coil/magnet
Refusal to wear sound processor all day
Child complains of difficulty of hearing
I have found the following on-line resources to be helpful when trying to learn more, and understand, cochlear implant mapping for children: