In a normal-hearing ear, 25,000 inner and outer hair cells transfer the sound information to the auditory centre in the brain. In a CI recipient with inner ear hearing loss, only 12 to 120 stimulation sites are available for this process. However, it is not the number of electrodes that is crucial, but the different possibilities for stimulation of the auditory nerve fibres. Although the considerable compression has a negative influence on sound quality, this is where the brain comes in. It is able to make optimum use of this sparse and fragmented information to form an adequate overall sound impression and, eventually, to recognise speech.
The quality of sound perceived varies greatly from person to person and changes over time. Once the CI has been initially activated, a whole range of sounds can be perceived. Only in rare cases does a CI user understand every word straight away and express satisfaction with the sound quality, even describing it as being almost the same as natural hearing. Others report that the voices seem to come from a very long way away and sound hollow or “like Mickey Mouse”.
After the CI is first activated, most patients merely hear a jingling sound, like that of many bells ringing, although they can distinguish individual word fragments. Auditory training with speech therapists is necessary to learn how to define individual sounds in order to understand speech. Fairly rapid progress is usually made. As a rule, CI recipients are able to follow a conversation in quiet surroundings at the end of their first week following initial activation. Speech understanding in background noise is always more difficult, especially for recipients with only one CI.