Tonotopy

The cochlea as frequency map, piano keyboard of the ear

Overview

Tonotopy is the orderly mapping of sound frequency onto position in the auditory system. It begins in the cochlea, where the basilar membrane is narrow and stiff at the base, so it vibrates most to high frequencies, and wider and more flexible at the apex, where it responds to low frequencies. Georg von Bekesy described this travelling wave, work recognized with the Nobel Prize in Physiology or Medicine in 1961. The frequency map is preserved through the auditory nerve, cochlear nucleus, superior olive, inferior colliculus, medial geniculate body, and primary auditory cortex, so the identity of the neurons firing indicates frequency. At lower frequencies, the timing of nerve firing, phase locking, provides additional pitch information, which is described by temporal and volley theories. Because damage is often localized along the cochlea, hearing loss is frequently frequency specific, and the tonotopic arrangement is the basis for allocating frequencies to electrodes in cochlear implants.