**2. Structure and mechanism of action**

Sensory hair cells within the cochlea have the responsibility for transforming sound vibration to neural signal in healthy individuals; then, the signal continues its way to auditory cortex through cochlear nerve. Cochlear implants take the place of these cells using electrodes which stimulate the nerve fiber electrically. **Figure 1** illustrates a cochlear implant device. Common cochlear implants have two parts: external component as a hearing aid and internal component which is surgically inserted in mastoid [16].

The external part is consisted of three parts: a microphone for gathering sounds, a speech processor analyzing and encoding sound into a digital code, and a magnetic headpiece which transmits coded signals by a transcutaneous radiofrequency link to the internal part.

The internal part has a receiver stimulator which receives and decodes the data and conducts decoded signal to the electrode array. In the next step, there is a flexible silicone carrier, which has variable number of electrodes. The remaining cochlear nerve fibers are stimulated by the electrode array, which is surgically implanted in scala tympani of the cochlea.

**Figure 1.** Different parts of cochlear implant (source: NIH/NIDCD https://www.nidcd.nih.gov/health/cochlear-implants).
