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Robotic Brain Universe

Hearing and the temporal lobe

How the cochlea analyses sound, how auditory cortex maps frequency, speech and music, what goes wrong, and the implants and gene therapies that restore hearing.

Intermediate · about 10 min · updated 2026-10-02 · awaiting clinical review

See it in 3D:

Illustrative simulation excitatory inhibitory

The cochlea's frequency map and the outer hair cell motor; the cochlear nerve, medial geniculate nucleus and acoustic radiation; tonotopic primary auditory cortex on the transverse temporal gyri; speech, music and the attended talker in non-primary cortex; hidden noise damage, tinnitus and implants in noise; Greenwood's function, interaural time differences and linear decoding; cochlear implants, gene therapy and hearing in later life.

Contents
  1. From air pressure to meaning
  2. What the auditory system is
  3. Why the cortex matters for hearing
  4. How sound becomes a neural code
  5. When hearing changes
  6. When hearing fails
  7. The mathematics of hearing
  8. Technology: implants, decoders and gene therapy
  9. Milestones
  10. Frontiers
  11. Check yourself

From air pressure to meaning

Children born deaf because of mutations in a single gene, OTOF, received one injection of a gene-carrying virus into the inner ear. Over the following months, five of the six recovered hearing, with thresholds improving by 40–57 dB, and their speech perception improved.[1]

Hearing turns tiny pressure waves into language, music and a sense of where things are. The cochlea splits sound into its frequencies, a living amplifier sharpens it, and the superior temporal gyrus turns patterns of sound into words, while picking out the one voice you want to hear in a crowd.[2,3,4,5]

Hearing loss is common: over 430 million people need rehabilitation for disabling hearing loss, and by 2050 nearly 2.5 billion are projected to have some degree of hearing loss. This reading follows sound from ear to cortex, explains its maths and the technologies, from cochlear implants to gene therapy, that restore it.[6]

What the auditory system is

The cochlea. In the inner ear, sound sets up vibrations whose position along the cochlea depends on frequency; von Békésy received the 1961 Nobel Prize in Physiology or Medicine 'for his discoveries of the physical mechanism of stimulation within the cochlea'. Greenwood's frequency–position function describes this map in humans and many other species.[2,7]

Two kinds of hair cell. Outer and inner hair cells do different jobs. Outer hair cells change length when their membrane voltage changes, driven by the motor protein prestin; this is thought to amplify vibrations in the cochlea, which inner hair cells then transduce into nerve signals.[3,8]

To the cortex. Signals travel in the cochlear nerve, part of the atlas's cranial nerve VIII bundle, and reach the medial geniculate nucleus of the thalamus, from which the acoustic radiation carries them to the auditory cortex. Primary auditory cortex lies on the transverse temporal gyri, on the upper surface of the superior temporal gyrus.[9,10,11,12]

Key numbers

People needing rehabilitation for disabling hearing loss
over 430 million[6]
Young adults at risk of avoidable hearing loss from unsafe listening
over 1 billion[6]
Cochlear implants registered worldwide by December 2019
about 736,900[13]
Natural sounds used to map auditory cortex in the music and speech study
165[14]

Why the cortex matters for hearing

Maps of frequency. High-resolution fMRI at 7 T revealed two tonotopic maps in adjacent subdivisions of human primary auditory cortex, in which neighbouring patches respond to similar frequencies; the maps share a low-frequency border, are mirror-symmetric and resemble those of the macaque monkey.[15]

Speech and music. Measuring responses to 165 natural sounds, Norman-Haignere and colleagues found six components of auditory cortical responses: four tuned to acoustic features such as frequency and pitch, and two selective for music and for speech, concentrated in distinct regions of non-primary auditory cortex.[14]

The cocktail party. When two people spoke at once, responses in non-primary auditory cortex represented the speaker the listener attended to, as if that speaker were alone; a simple classifier could decode the attended words and the speaker's identity.[5]

Language. In the dominant hemisphere (usually left), the posterior superior temporal gyrus takes part in speech processing and forms the core of the classical Wernicke area, whose boundaries and role are still debated; see the language reading.[11,16]

How sound becomes a neural code

Amplification. Isolated outer hair cells shorten when depolarised and lengthen when hyperpolarised. Within the organ of Corti, these length changes could influence the mechanics of the cochlear partition and contribute to the exquisite sensitivity of mammalian hearing.[8]

Where is that sound? Jeffress proposed in 1948 a place theory of sound localisation, in which differences in the time a sound reaches the two ears are converted into a place code in the brain.[17]

What the cortex encodes. From intracranial recordings over the human superior temporal gyrus, slow and intermediate fluctuations of speech, such as the syllable rhythm, could be reconstructed with a linear model of the auditory spectrogram, while fast fluctuations such as syllable onsets needed a non-linear representation; individual words could be identified from single trials.[4]

The ascending auditory pathway (simplified)Soundpressure wavesCochlea: frequency mapeach frequency peaks at its ownplace along the cochleaOuter hair cells amplifyprestin changes their lengthInner hair cells transducevibration becomes nerve signalsCochlear nervepart of cranial nerve VIIIMedial geniculate nucleusauditory thalamusPrimary auditory cortextransverse temporal gyri;mirror-symmetric tonotopic mapsNon-primary auditory cortexspeech, music and the attendedtalker
The ascending auditory pathway (simplified). A simplified pathway: the brainstem stages between the cochlear nerve and the thalamus are omitted. The final step from thalamus to cortex runs in the acoustic radiation shown in the atlas.[2,3,5,9,10,11,14,15]
Text version of the diagram
  1. Sound: pressure waves. Leads to Cochlea: frequency map.
  2. Cochlea: frequency map: each frequency peaks at its own place along the cochlea. Leads to Outer hair cells amplify.
  3. Outer hair cells amplify: prestin changes their length. Leads to Inner hair cells transduce.
  4. Inner hair cells transduce: vibration becomes nerve signals. Leads to Cochlear nerve.
  5. Cochlear nerve: part of cranial nerve VIII. Leads to Medial geniculate nucleus.
  6. Medial geniculate nucleus: auditory thalamus. Leads to Primary auditory cortex.
  7. Primary auditory cortex: transverse temporal gyri; mirror-symmetric tonotopic maps. Leads to Non-primary auditory cortex.
  8. Non-primary auditory cortex: speech, music and the attended talker.

When hearing changes

After loud noise. In mice, noise exposure that raised hearing thresholds only temporarily, and left the sensory hair cells intact, nevertheless caused immediate loss of nerve terminals and delayed degeneration of the cochlear nerve. Such hidden damage may add to difficulty hearing in noise and could contribute to tinnitus.[9]

In childhood. Deafness caused by OTOF mutations is congenital or prelingual. In the gene therapy trial, hearing improved over the 26 weeks of follow-up in five of six children aged 1–18.[1]

In later life. Hearing loss is associated with faster cognitive decline and with dementia. In the ACHIEVE trial of 977 adults aged 70–84, hearing aids with counselling did not slow three-year cognitive decline overall, but a prespecified analysis found that the effect differed between a cohort at higher risk of decline and healthy volunteers.[18]

When hearing fails

Hidden hearing loss. Conventional threshold tests can miss noise damage: the cochlear nerve can degenerate even after thresholds recover.[9]

Tinnitus is the perception of sound without an external source. It affects 10–15% of people and severely impairs quality of life in about 1–2%. Deprived auditory input is followed by increased firing, enhanced synchrony and changes in tonotopic organisation in central auditory pathways; counselling is the most widely used treatment, and cognitive behavioural therapy has the best evidence.[19]

Cochlear implants in noise. With current implants the average user can converse in quiet with relative ease but has great difficulty in a noisy workplace, probably because of lost low-frequency fine-structure information, the limited number of effective channels caused by overlapping electrical fields, and central processing deficits.[20]

The mathematics of hearing

A frequency map, a geometry of arrival times and a linear decoder capture three steps of hearing.[4,7,17]

Greenwood's frequency–position function[7]
F(x)=A(10ax−k),human: A=165.4 Hz, a=2.1, k=0.88F(x) = A\left(10^{a x} - k\right), \qquad \text{human: } A = 165.4\ \mathrm{Hz},\ a = 2.1,\ k = 0.88

The characteristic frequency at a point on the cochlea rises almost exponentially with distance from the apex. With the human constants, the apex (x = 0) responds to about 20 Hz and the base (x = 1) to about 20.7 kHz.

Symbols in Greenwood's frequency–position function
SymbolMeaningUnit
FFcharacteristic frequencyHz
xxposition along the cochlea, from apex (0) to base (1)—
A,a,kA, a, kspecies constants: scale, slope and lower-limit correction—
Interaural time difference[17]
Δt≈dc sin⁡θ\Delta t \approx \frac{d}{c}\,\sin\theta

A sound from one side reaches the nearer ear first. In this simple geometry, assuming ears 0.2 m apart and sound travelling at 343 m/s, a source directly to one side (θ = 90°) gives a difference of about 0.58 ms. Jeffress proposed that such tiny delays are converted into a place code.

Symbols in Interaural time difference
SymbolMeaningUnit
Δt\Delta tdifference in arrival time between the earss
dddistance between the earsm
ccspeed of soundm/s
θ\thetaangle of the source from straight ahead—
Reconstructing a spectrogram from the brain[4]
S^(f,t)=∑n∑τg(τ,f,n) R(t−τ,n)\hat{S}(f, t) = \sum_{n}\sum_{\tau} g(\tau, f, n)\, R(t - \tau, n)

A linear decoder estimates the sound's spectrogram at each frequency and time as a weighted sum of recent activity across recording sites. The weights are learned from training data, then applied to new brain recordings.

Symbols in Reconstructing a spectrogram from the brain
SymbolMeaningUnit
S^(f,t)\hat{S}(f,t)reconstructed spectrogram at frequency f and time t—
R(t,n)R(t, n)neural response at electrode n—
g(τ,f,n)g(\tau, f, n)learned weight for electrode n at time lag τ—

Technology: implants, decoders and gene therapy

Cochlear implants stimulate the auditory nerve directly through implanted electrodes, driven by an external processor that converts sound from a microphone into stimulation for each electrode. In 1991 a strategy called continuous interleaved sampling, which delivers brief pulses to the electrodes in a non-overlapping sequence, produced large improvements in speech recognition for all seven users tested.[21]

Better in noise. Bilateral implants and combined electric and acoustic stimulation promise to help users in noise and to restore low-frequency fine structure.[20]

Cortical electrodesTime-lagged responsesSpectrogram estimateWord and speaker
Decoding attended speech (schematic). A schematic of decoding from auditory cortex: activity across electrodes is mapped to an estimated spectrogram, from which a classifier identifies the words and the speaker being attended.[4,5]

Gene therapy. In a trial of AAV1-hOTOF, a single injection through the round window of the cochlea caused no dose-limiting toxicity or serious adverse events in six children; five recovered hearing, with average auditory brainstem response thresholds falling by 40–57 dB.[1]

Ways to restore hearing[1,13,18,20,21]
ApproachHow it worksEvidence here
Hearing aidsAmplify soundACHIEVE trial (with counselling)
Cochlear implantElectrodes stimulate the auditory nerveContinuous interleaved sampling; about 736,900 devices by 2019
Bilateral and hybrid implantsTwo ears, or electric plus acoustic stimulationPromising for listening in noise
Gene therapyDeliver a working copy of a gene to the cochleaOTOF trial: hearing recovered in 5 of 6 children

Milestones

From the cochlea to gene therapy

  1. 1948Jeffress proposes a place theory of sound localisation.[17]
  2. 1961Von Békésy receives the Nobel Prize for the mechanics of the cochlea.[2]
  3. 1985Isolated outer hair cells are shown to move.[8]
  4. 1990Greenwood's frequency–position function is updated across species.[7]
  5. 1991Continuous interleaved sampling improves cochlear implants.[21]
  6. 2000Prestin is identified as the outer hair cell motor.[3]
  7. 2003Mirror-symmetric tonotopic maps in human auditory cortex.[15]
  8. 2009'Temporary' noise damage is shown to degenerate the cochlear nerve.[9]
  9. 2012Speech reconstructed from auditory cortex; the attended talker decoded.[4,5]
  10. 2015Distinct cortical pathways for music and speech.[14]
  11. 2023The ACHIEVE trial tests hearing aids against cognitive decline.[18]
  12. 2024Gene therapy restores hearing in children with OTOF deafness.[1]

Frontiers

Genetic deafness. Before this work no pharmacological treatment was available for congenital deafness; OTOF gene therapy is a novel treatment aimed at its genetic cause. The trial was single-arm, with six children, and is ongoing.[1]

Hearing and the ageing brain. Whether treating hearing loss protects cognition remains open: ACHIEVE found no overall benefit over three years, but its prespecified analysis suggested the effect may depend on a person's risk of decline.[18]

Global need. By 2050, more than 700 million people are projected to need rehabilitation for hearing loss.[6]

Check yourself

Check yourself

  1. What did von Békésy's Nobel Prize recognise?
    Show answer

    His discoveries of the physical mechanism of stimulation within the cochlea.

  2. What does prestin do?
    Show answer

    It is the motor protein that changes the length of outer hair cells, thought to amplify cochlear vibrations.

  3. Which pathway carries auditory signals from the thalamus to the cortex?
    Show answer

    The acoustic radiation, from the medial geniculate nucleus to the auditory cortex.

  4. Where does primary auditory cortex lie?
    Show answer

    On the transverse temporal gyri, on the upper surface of the superior temporal gyrus.

  5. Using Greenwood's human constants, roughly what range of frequencies does the cochlea map?
    Show answer

    From about 20 Hz at the apex to about 20.7 kHz at the base.

  6. Why can 'temporary' noise-induced hearing loss still be harmful?
    Show answer

    In mice it caused loss of nerve terminals and delayed degeneration of the cochlear nerve even though thresholds recovered.

  7. What did continuous interleaved sampling change in cochlear implants?
    Show answer

    It delivered brief pulses to electrodes in a non-overlapping sequence, greatly improving speech recognition.

  8. What did the ACHIEVE trial find overall?
    Show answer

    Hearing aids with counselling did not slow three-year cognitive decline overall.

Glossary[2,3,7,10,11,17,19,21,22]

Cochlea
The inner-ear organ that converts sound into nerve signals, with a map of frequency along its length.
Tonotopy
An orderly map of sound frequency, in the cochlea or the cortex.
Outer hair cell
A cochlear cell that changes length to amplify vibrations.
Inner hair cell
A cochlear cell that transduces vibration into nerve signals.
Prestin
The motor protein of outer hair cells.
Acoustic radiation
The fibre bundle from the medial geniculate nucleus to the auditory cortex.
Heschl's gyrus
Another name for the transverse temporal gyrus, home of primary auditory cortex.
Interaural time difference
The difference in arrival time of a sound at the two ears.
Tinnitus
Perceiving sound without an external source.
Cochlear implant
A device that stimulates the auditory nerve electrically.

References

  1. Lv J, Wang H, Cheng X, Chen Y, Wang D, Zhang L, et al.. AAV1-hOTOF gene therapy for autosomal recessive deafness 9: a single-arm trial. The Lancet 2024;403(10441):2317-2325. doi:10.1016/S0140-6736(23)02874-X
  2. Nobel Prize Outreach. The Nobel Prize in Physiology or Medicine 1961. NobelPrize.org 1961. https://www.nobelprize.org/prizes/medicine/1961/summary/
  3. Zheng J, Shen W, He DZZ, Long KB, Madison LD, Dallos P. Prestin is the motor protein of cochlear outer hair cells. Nature 2000;405(6783):149-155. doi:10.1038/35012009
  4. Pasley BN, David SV, Mesgarani N, Flinker A, Shamma SA, Crone NE, et al.. Reconstructing speech from human auditory cortex. PLoS Biology 2012;10(1):e1001251. doi:10.1371/journal.pbio.1001251
  5. Mesgarani N, Chang EF. Selective cortical representation of attended speaker in multi-talker speech perception. Nature 2012;485(7397):233-236. doi:10.1038/nature11020
  6. World Health Organization. Deafness and hearing loss (fact sheet, 3 March 2026). who.int 2026. https://www.who.int/news-room/fact-sheets/detail/deafness-and-hearing-loss
  7. Greenwood DD. A cochlear frequency-position function for several species—29 years later. The Journal of the Acoustical Society of America 1990;87(6):2592-2605. doi:10.1121/1.399052
  8. Brownell WE, Bader CR, Bertrand D, de Ribaupierre Y. Evoked mechanical responses of isolated cochlear outer hair cells. Science 1985;227(4683):194-196. doi:10.1126/science.3966153
  9. Kujawa SG, Liberman MC. Adding insult to injury: cochlear nerve degeneration after "temporary" noise-induced hearing loss. The Journal of Neuroscience 2009;29(45):14077-14085. doi:10.1523/JNEUROSCI.2845-09.2009
  10. Maffei C, Jovicich J, De Benedictis A, Corsini F, Barbareschi M, Chioffi F, et al.. Topography of the human acoustic radiation as revealed by ex vivo fibers micro-dissection and in vivo diffusion-based tractography. Brain Structure and Function 2018;223(1):449-459. doi:10.1007/s00429-017-1471-6
  11. Hickok G, Poeppel D. The cortical organization of speech processing. Nature Reviews Neuroscience 2007;8(5):393-402. doi:10.1038/nrn2113
  12. Desikan RS, Ségonne F, Fischl B, Quinn BT, Dickerson BC, Blacker D, et al.. An automated labeling system for subdividing the human cerebral cortex on MRI scans into gyral based regions of interest. NeuroImage 2006;31(3):968-980. doi:10.1016/j.neuroimage.2006.01.021
  13. National Institute on Deafness and Other Communication Disorders. Cochlear implants. NIDCD, U.S. National Institutes of Health. https://www.nidcd.nih.gov/health/cochlear-implants
  14. Norman-Haignere S, Kanwisher NG, McDermott JH. Distinct cortical pathways for music and speech revealed by hypothesis-free voxel decomposition. Neuron 2015;88(6):1281-1296. doi:10.1016/j.neuron.2015.11.035
  15. Formisano E, Kim DS, Di Salle F, van de Moortele PF, Ugurbil K, Goebel R. Mirror-symmetric tonotopic maps in human primary auditory cortex. Neuron 2003;40(4):859-869. doi:10.1016/S0896-6273(03)00669-X
  16. Binder JR. The Wernicke area. Neurology 2015;85(24):2170-2175. doi:10.1212/WNL.0000000000002219
  17. Jeffress LA. A place theory of sound localization. Journal of Comparative and Physiological Psychology 1948;41(1):35-39. doi:10.1037/h0061495
  18. Lin FR, Pike JR, Albert MS, Arnold M, Burgard S, Chisolm T, et al.. Hearing intervention versus health education control to reduce cognitive decline in older adults with hearing loss in the USA (ACHIEVE): a multicentre, randomised controlled trial. The Lancet 2023;402(10404):786-797. doi:10.1016/S0140-6736(23)01406-X
  19. Langguth B, Kreuzer PM, Kleinjung T, De Ridder D. Tinnitus: causes and clinical management. The Lancet Neurology 2013;12(9):920-930. doi:10.1016/S1474-4422(13)70160-1
  20. Wilson BS, Dorman MF. Cochlear implants: a remarkable past and a brilliant future. Hearing Research 2008;242(1-2):3-21. doi:10.1016/j.heares.2008.06.005
  21. Wilson BS, Finley CC, Lawson DT, Wolford RD, Eddington DK, Rabinowitz WM. Better speech recognition with cochlear implants. Nature 1991;352(6332):236-238. doi:10.1038/352236a0
  22. Destrieux C, Fischl B, Dale A, Halgren E. Automatic parcellation of human cortical gyri and sulci using standard anatomical nomenclature. NeuroImage 2010;53(1):1-15. doi:10.1016/j.neuroimage.2010.06.010

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Template anatomy for education. Not patient-specific. Not for clinical decision-making.