Cochlear implants differ in the way that they process sound and how they present electricity to the hearing nerve. Other than the speech processing strategies discussed below, there are two different ways of encoding sound information. The first form, analog coding, involves continuous coding of the sound signal with subsequent transfer to the receiver in multiple radio-frequency channels. Electrodes are continuously stimulated. The second form, digital coding, requires sampling of the sound waveform and assigning a number to these “bits” of information. These bits of information are then transferred to the receiver where they are decoded. Electrodes are stimulated in a pulse fashion. Interestingly, neither approach is 100% effective for all implant users. Recently, combining the two schemes has seen some success.
Cochlear implant can also be distinguished by their use of single vs. multiple channels, the number of electrodes, and their use of either monopolar or bipolar stimulation. The number of electrodes stimulated with different electrical stimuli determines the “channels” used. In other words, an implant may have multiple electrodes, but if the same information is presented to all the electrodes at one time they are essentially functioning as a single channel system. In contrast, multi-channel devices provide different information to several electrodes or groups of electrodes. Early implants had only one electrode (and one channel); recent advances have lead to the development of implants with multiple electrodes (22) and multiple channels (usually 4-8). Having more electrodes means that multiple channels can be localized to areas of the cochlea that are most responsive, and stray current that is stimulating adjacent structures (facial nerve, vestibular nerve) can be rerouted. Cochlear implant surgery is the essential way to put it in patients’ hearing.
