Why is there a variation between the outcomes for some Cochlear Implantees?
First of all about >90% of people experience outcomes of 90%+ in understanding speech when they have a cochlear implant. That's amazing isn't it, but for the other 10% or so of people who don’t get a good outcome why is this? (1)
A proviso - I'm not a doctor and this is not a medical explanation, but rather my understanding of the process. I recognise some of the things I say may be controversial. It's not meant to be... but rather my understanding and experiences.
There are anatomical issues which I'm not going into here, as well as the underlying reason for deafness in the first place. But assuming there are no other issues except sensorineural deafness then here are some of the reasons why a cochlear implant works better for some than for others.
Anyone who has had a baby will know that when it is born it cannot see/focus, cannot speak, walk, eat and cannot hear/understand. It is during the first couple of years of life that all these functions develop. Just supposing we kept a normal healthy baby in a pram and wouldn't let it learn to walk. Then 20 years later we tried to teach it.... the chances are high it could never learn to walk or at best walk poorly. This is because the crucial time for developing this walking skill is in the first couple of years of life. If this skill isn't developed the brain reallocates those pathways which would lead to the skill of walking to some other function and it is particularly difficult to activate them and train the muscles for walking after the developmental years.
So too our hearing. It is in the first 2-3 years of life that we learn to hear and understand and learn to speak. If we don't hear sound in this crucial period then our brain doesn't lay down and develop the hearing pathways and these resources are allocated to some other skill.
This is at least some part of why a cochlear implant works so well for someone like me who has, at one stage in my life, had perfect hearing and perhaps less well for someone who has never heard sound. But the brain is amazing and throughout life can create new connections and even people, profoundly deaf from birth, have had major improvements in their understanding of sound.
Another reason is: If you take me to China and put me in a classroom full of students with the teacher talking in Mandarin I won't have a clue what is being said. I will hear everything, but not understand it. This is because I have never learned nor heard Mandarin being spoken. It's going to take me quite a long time to become familiar with these sounds and start to understand this language. It is the same for someone who is deaf. If they have never heard the sound, how can they understand it when they do hear it? I was asked in one of my hearing tests recently to identify, by sound alone, a snare drum roll. I couldn't because I don't think I had ever heard that sound. So if someone has never heard speech or never heard it well, why would they suddenly understand it because they have had a cochlear implant? And even if it is explained to them what the word or sound is, the brain hasn't the strength in the pathways to understand it every time.
Most people’s hearing understanding will improve with time as they become more familiar with the sounds, as their brain strengthens and lays down new pathways. But for some it may be they have been too deaf for too long and these new neurons won't grow well.
Then we have a sound ‘memory bank’. My mother taught me that the noise the dog made was a dog barking. She pointed it out, along with lots of other sounds like cows mooing and birds chirping, over and over again. When I first had my implant, even though I knew what a dog barking sounded like, I still couldn't easily identify these sounds and I spent some time familiarising myself with them all again – matching the noise I had to the memory of the sound. My brain took the new impulses from my cochlear implant and as I heard them 'catalogued' them and put them into my memory bank using the new stimulus I was receiving. Because I already had a large memory bank of sounds I was able to match them up so hearing and understanding again was relatively quick for me. But if you have to build that memory bank from scratch it's going to take a long time - especially if the 'technology' is faulty to start with. (eg you have some underlying medical condition making it harder to transmit and store the sound.)
I often get asked why some long-term deaf people who have had a cochlear implant don't experience improved speech. I asked my audiologist about this and he told me that it is almost impossible to change speech characteristics after adolescence. As an example, I'm sure some of you know someone who has come from a foreign country to your country and they've been here decades but still speak with a strong accent. It's not because they don't want to change their accent, but rather they can't because the speech processes were laid down strongly in their childhood and these do not change a great deal as we age.
Naomi, a mother of a cochlear implantee on our Cochlear Implant forum pointed out, “Clear speech relies a lot on the muscles being developed enough to form the shape to make the speech sound. So sometimes in children who have issues that cause muscle weaknesses, their speech is often not clear. So similarly in adults if they haven't had hearing as a young child and then try to approximate what they hear using their mouth, it is likely the muscles needed to make those sounds are not fully developed or accustomed to the way they need to work in order to make the sound. Again it is a really difficult thing to re-train them and that can be another reason why speech development is often not seen at the same rate for different people with an implant.”
All of these things factor in when someone has a Cochlear Implant. Most people who have a cochlear implant get great speech understanding improvement. Some don’t, but what I do know is that even those with 'poor' outcomes still experience improvement over hearing aids. They can hear environmental sounds, it helps with lip reading and most would refuse to take off their processor and consider their implant a success.
(1) CIC research – contact the author for more details if required.