Tonotopic Organization
The auditory system converts sound frequency into spatial position at every level from cochlea to cortex.
Basilar Membrane
The basilar membrane in the cochlea varies in stiffness along its length. The base (near the middle ear) is stiff and resonates to high frequencies; the apex is flexible and responds to low frequencies. Georg von Bekesy discovered this frequency-to-place mapping in the 1930s.
Each location along the basilar membrane is “frequency-selective” and has a characteristic tuning curve showing which frequencies excite it most.
Cortical Maps
Tonotopic maps are preserved through the auditory pathway up to primary auditory cortex (A1). In humans and primates, neurons at the anterior end of A1 respond best to low frequencies (~few hundred Hz), while posterior neurons prefer high frequencies (~10 kHz+).
This is analogous to retinotopy in vision and somatotopy in touch. Stimuli close together in frequency are processed by neighboring neurons.
Why This Matters
- STRC Hearing Loss: STRC-related SNHL affects outer hair cells, which are the cochlear amplifiers that sharpen frequency selectivity on the basilar membrane. Damage degrades tonotopic precision.
- Sound Therapy and Hearing Loss: Understanding which frequency bands are impaired helps design targeted sound therapy protocols.
- Singing Bowl Physics: The rich harmonic spectrum of singing bowls activates broad regions of the tonotopic map simultaneously, which may explain their immersive perceptual quality.
- Entrainment: Frequency-specific neural populations may be differentially susceptible to rhythmic entrainment depending on their tonotopic position.