Guide to Cochlear Implants
After all T-levels have been obtained, the programmer then works on setting the comfort level or C-level. (This is sometimes referred to as the M-level, as in the most comfortable level.) The C-level represents the level of current that creates a sensation of sound that is comfortably loud. This is an important value since the distance between the T-level and C-level for each electrode will provide the dynamic range of the implant. The dynamic range is the amount that sound is allowed to grow from its softest to its loudest. A good dynamic range is one that permits sound to grow evenly from soft to loud. If the dynamic range is very narrow, sound does not grow in the appropriate manner. This restricts auditory perception. It is not unusual, however, for children to show narrow ranges at the time of the initial switch-on. Parents can be assured that this range will grow with time as the child adapts to the implant and the sound it delivers.
When each electrode has been assessed for a T- and a C-level, the computer then generates a MAP. The MAP records the information detailing the current levels for each electrode. It is unique to each child with a cochlear implant. No child should ever use another child’s processor because the MAPs within it are different. MAPs are stored on a computer chip located in the speech processor using a retrieval system similar to a file cabinet. Each drawer in the file cabinet is called a program. One MAP is assigned to a single program. Over time, the audiologist will create new MAPs, erase old MAP from the program slot, and replace them with the newer versions. All current implants are able to store more than one MAP on the speech processor. Return trips to the implant center may be less frequent because of this feature. Access to multiple MAPs enables the parent to make comparisons between and among the different programs that are stored in the speech processor.
Once all T- and C-levels have been found and a MAP has been generated, the child is finally given an opportunity to listen to speech. This is the moment that parents have anticipated and played out in their minds. It is difficult to predict what any given child’s first response to speech through the implant will be. A child’s reaction is related to his or her age and prior experience with listening through hearing aids. It is not unusual for some children to show very little response, especially if they are very young. In some cases, the child’s first response to sound may disappoint the parents. Some children may cry or attempt to remove the headset. Parents should know that any crying is more likely to be a reaction to an unfamiliar stimulus and not to pain. Nonetheless, parents will often find the experience unsettling and may begin to second-guess their decision about implantation. Parents should trust the judgments of the implant center team; the team has experience with the range of responses seen at initial switch-on and should continue their work even if a negative response occurs.
Assessing the Initial Auditory Responses with the Implant
Most children show observable responses to speech during the switch-on session. Parents should understand that these responses do not generalize to everyday activities until wear time and training has occurred. The audiologist may perform some simple post-switch-on listening assessments to determine which auditory skills are present. The audiologist can then identify those skills that should be targeted by the school program or therapist working with the child. Parents are encouraged to view the initial switch-on as the starting point in the child’s auditory development. It is a beginning, not an end.