All About Cochlear Implant Testing

If you received your cochlear implant(s) when you were older or maybe you were told that you are a candidate for cochlear implantation, you would have had a traditional hearing test (as described in the last installment of the Audiologist's Corner by Marisa DeFrancesco) that identified you with a severe to profound hearing loss. 

Both hearing aids and cochlear implants provide a way for sound to travel from a microphone on up to the brain. In the case of cochlear implants, sound is picked up from the microphone on the speech processor and the speech processor converts sound into a digital signal. This signal travels to the coil that is connected via a magnet to the head and the coil transmits the digital signal to the internal device that was surgically implanted. The internal device sends the signal to the  electrode array that was inserted into the cochlea or inner ear and from there, the signal travels up to the brain. It’s an amazing process that happens almost  instantaneously! 

Those of you who have had your cochlear implant(s) for quite some time should be  seeing your cochlear implant audiologist at Sick Kids at least once a year. It’s the same as making sure you have your eyes, teeth and general health checked regularly. There is a good chance that the “MAP” on your cochlear implant(s) has changed. 

What is a “MAP”? It is the measured threshold (“T”) level and comfort (“C”) level on each electrode, along the electrode array that was implanted inside your  cochlea. Just like keys on a piano, each electrode is “tuned” to a different frequency or pitch and these pitches range from low pitch to high pitch. Although we can hear many different pitches, the pitches on the electrodes are the ones that are most important for hearing speech. 

How are these “T” and “C” levels measured on each electrode?

Measuring “T” levels is similar to having a hearing test. Your audiologist asks you to  “raise your hand”, “say yes” or something similar whenever you hear even the softest “beeping” sound. This softest level is called the threshold or “T” level for that electrode (marked in green on the graph). T levels are measured on several electrodes (as many as 22). The cochlear implant audiologist will also measure comfort or “C” levels on the electrodes. She will raise the volume on a particular electrode and you will tell her when the volume sounds comfortable or “just right” (marked in red on the graph). 

Once your speech processor is “MAPped”, your cochlear implant audiologist may also do an NRT (Neural Response Telemetry) test. This might not be your favourite test. You listen to a “clicking” or “snapping” type noise that starts soft and gets pretty loud. However, it is an important test as it helps to ensure that your “T”  and “C” levels are set properly and that your internal device is working well. 

Next, your cochlear implant audiologist will take you to a sound booth and do a standard hearing test while wearing your speech processor(s). You will be asked again to listen to sounds and this time they will come from a speaker in the sound booth. You will “raise your hand”, “say yes” or “push a button” whenever you hear the softest sound. This test is to make sure you are hearing all the different  pitches at very soft levels. She will also ask you to repeat words to see how softly you can hear them. These very soft levels for both pitch and speech are what a typically hearing person can hear. Finally, you will be asked to repeat back words at a comfortable loudness level in both quiet and in noise. Your cochlear implant  audiologist does all of these tests to make sure you are hearing your very best in  both easy and difficult listening situations. 

It’s important to remember to take good care of your speech processor(s) and to change the microphone filter at least once a year… twice a year is even better. You can remind yourself by putting a note on your calendar in January and June so you don’t forget. 

Your cochlear implant audiologist is always happy to answer any of your questions and never hesitate to ask. After all, these visits to Sick Kids are about you and your hearing.