5.2 Tonotopy is preserved

Each inner hair cell sits at a specific place along the basilar membrane and is tuned to that place’s characteristic frequency. The 10–30 fibers contacting a given IHC all inherit that fiber’s characteristic frequency (CF). The auditory nerve is therefore tonotopically organized: fibers carrying low-frequency information are spatially grouped (they all come from apical IHCs), and fibers carrying high-frequency information are grouped on the other side (basal IHCs).

This tonotopy persists. The cochlear nucleus in the brainstem is tonotopic. The inferior colliculus is tonotopic. The medial geniculate is tonotopic. The primary auditory cortex is tonotopic — you can map it with electrodes or with fMRI and find an orderly progression of CF across its surface. The Greenwood-function map of 5.4 echoes all the way up to the cerebral cortex.