cochlear implant

Meaning: Normally, sound vibrations move the eardrum, the ossicles and the fluid of the cochlea. This bends hair cells on the basilar membrane, and the hair cells trigger action potentials in the auditory nerve. A cochlear implant replaces the hair cells' job. An external microphone and speech processor pick up sound and send a signal to an internal electrode array in the cochlea. The electrodes stimulate the auditory nerve directly. Electrodes near the base of the cochlea excite high-frequency regions and those near the apex excite low-frequency regions, which matches place theory. It works best when the auditory nerve and brain pathways are intact.

On the MCAT: Passages contrast conductive hearing loss (a problem in the outer or middle ear, such as the eardrum or ossicles) with sensorineural hearing loss (damage to hair cells or the auditory nerve). You may be asked which device suits which problem. Amplification often helps conductive loss, while an implant targets hair cell damage. Questions may also bring in the sociological debate over implants and Deaf culture, or language development and early intervention.

Common confusion: An implant is not just a stronger hearing aid, and it does not restore normal hearing. Hearing aids make sound louder for hair cells that still work. An implant skips the hair cells, and the signal it gives is coarser, so users usually need training to interpret it.