Coding of sound frequency

The nervous system has two ways of encoding the frequencies of a sound. Tonotopic mapping, in which the frequency maps to location beside the cochlea, takes place as afferents with successively lower CFs are found closer to its apex. This is most significant for frequencies above 1–3 kHz. For lower frequencies, coding uses the property which afferents fire with greatest probability during a specific phase of a sound wave, phase-locking. It is only essential that a separate afferent fires during some cycles when a group of cells is included. Furthermore, if various group phase-lock onto distinct sections of the cycle then entire population of cells acting in concert can encode frequency.