Performance with a cochlear implant can be variable, but in general, there are three groups of patients that we think about when we think about how well someone will do with a cochlear implant.
The first group is the child who was born deaf. Children have enormous central nervous system plasticity. And if the children are otherwise healthy, and the have no other comorbidities, in other words, no other medical problems, we try to put the cochlear implant in before the age of one.
And just like English children learn to speak English, and French children learn to speak French, and German children learn to speak German, because of their enormous central nervous system plasticity, these children take this electrical signal, they get speech and language therapy, and by the time they're five, our anticipation is that they will be mainstreamed in a regular school, hearing and speaking like any other child. Nonetheless, if they take their cochlear implant off, they are deaf.
The second group is post-lingually deafened adults. In other words, you're born with hearing, you learn speech and language, you hear for a significant period of your life, and then you subsequently lose your hearing. And you need a cochlear implant.
The main factor in success with a cochlear implant in this group is the duration of deafness. There is some evidence that the shorter the duration of deafness, the better you'll do with your cochlear implant. But the advantage that we have with this group of adults is, we put on the cochlear implant and we can hook it up to a computer and we can say to the adult, 'How does this sound?' And they'll say it's too high, it's too low, it's too squeaky, it's too fuzzy, and we can make adjustments to make it better.
The third group are people who are born deaf, had very little hearing aid use, or no hearing aid use, and they're now in mid-life. They have no memory of sound. They've never heard sound. In this group, if we put in a cochlear implant, and we say, 'How does it sound?', they don't know how it sounds, because they never heard sound. Their central nervous system plasticity is long gone, and they may pick up some awareness of environmental sound, which can be very helpful. They may pick up some awareness of their speech, which may improve their speech if they have speech. But they don't generally perform well with cochlear implants.