Animation 6.4: Mapping Auditory Frequencies
In the inner ear, high-frequency sounds maximally excite sensory hair cells at the base of the cochlea, while low-frequency sounds maximally excite hair cells at the apical end. At each level of the auditory system, from the cochlear nucleus to the auditory cortex, sound frequencies continue to be mapped in an orderly succession called tonotopic mapping.
In this tutorial, we will review the auditory system pathways of the brain and describe how the 2-deoxyglucose technique can be used to map the tonotopic organization of auditory brain regions.
The fact that tonotopy is conserved throughout much of the auditory system suggests that it is important for our interpretation of sound. Tonotopic organization also has implications for the treatment of sensorineural hearing loss where no amount of mechanical amplification (e.g., via a hearing aid) can compensate for the inability to generate or convey a neural impulse from the cochlea. If the VIIIth (vestibulocochlear) cranial nerve is intact, cochlear implants can be used to partially restore hearing. A cochlear implant consists of a peripherally mounted microphone and a digital signal processor that transforms a sound into its spectral components. The cochlear implant includes an electrode array that is positioned along the length of the tonotopically organized basilar membrane and the VIIIth nerve endings. This placement enables electrical stimulation of the nerve in a manner that mimics some aspects of the spectral decomposition naturally performed by the cochlea. Although cochlear implants cannot help patients with VIIIth nerve damage, brainstem implants are being developed that use a conceptually similar approach to stimulate the tonotopically organized cochlear nuclei directly, bypassing the auditory periphery altogether.
Textbook Reference: Auditory System Pathways Run from the Brainstem to the Cortex, p. 154