Research stage: Early human evidence

Central Gain

Evidence level

Human (non-randomized) evidence, with supporting animal work

What is it?

When the ear is damaged and sends fewer signals to the brain, the central auditory system turns up its own amplification, its 'central gain', to keep its average activity stable, so that as the ear delivers less, activity in the central auditory brain paradoxically increases. The compensation is genuinely useful and can even restore basic hearing. But turning up the amplifier also amplifies the system's own background noise, and when it overshoots the result is sound that isn't there (tinnitus) and everyday sounds that feel unbearably loud (hyperacusis), and this can happen even when a standard hearing test looks perfectly normal ('hidden hearing loss'). In animals, perceived loudness and sound-evoked brain activity even rose in lockstep, direct evidence that the turned-up gain is what makes sounds too loud.

Reduced cochlear input
Fewer signals reach the brain
Central gain increases
A homeostatic amplification response
Loudness increases
Perceived as hyperacusis
Neural noise increases
Perceived as tinnitus
Audiogram can look normal
The loss is 'hidden' at the auditory nerve

Mechanism

Cochlear injury reduces auditory-nerve firing. Neurons downstream respond homeostatically, raising their gain so that mean firing rate and coding efficiency are preserved, which is why sound-evoked activity becomes paradoxically enhanced at virtually every level of the central auditory pathway despite reduced cochlear output. The price of that homeostasis is that spontaneous 'neural noise' is amplified along with the signal, which can be perceived as tinnitus, and that sound-evoked responses are over-amplified, producing excessive loudness growth (hyperacusis). Human recordings show this can occur with a normal audiogram: auditory-nerve output (ABR wave I) is reduced while the more centrally generated wave V is normal, the periphery sends less, the centre has renormalised ('hidden hearing loss'). Central gain is one proposed explanation for the broader, directly-observed phenomenon of central hyperactivity, elevated spontaneous and sound-evoked firing in central auditory structures after cochlear damage, first characterized by researchers such as Kaltenbach and Mulders & Robertson; PV interneuron loss, DCN fusiform-cell hypersynchrony, Kv7/KCNQ channel loss, and cortical tonotopic map reorganization are other proposed mechanisms for that same broader phenomenon. This shared-amplification account is also supported by human epidemiology: in a population study of 3,645 Swedish adults, hyperacusis was strongly associated with tinnitus (odds ratio 3.5), and among people with severe tinnitus, as many as 80% also reported hyperacusis, consistent with both symptoms arising from the same turned-up central gain.

Homeostatic Plasticity Central Gain Enhancement Hidden Hearing Loss Neural Noise Amplification Loudness Growth

Behind the science

What is 'central gain', in everyday terms?

Think of a hearing aid that the brain runs on itself. When the microphone (the cochlea) starts sending a weaker signal, the brain turns up its internal amplifier so that what it hears stays roughly as loud as before. That keeps hearing usable, but an amplifier cannot tell signal from noise. Turning it up also turns up the system's own background hiss, and if it overshoots, quiet sounds become uncomfortably loud and the amplified internal noise can be heard as a ringing that has no external source.

How can someone have tinnitus with a perfectly normal hearing test?

A standard audiogram only measures the softest tone you can detect. You can lose a substantial share of the auditory-nerve fibres and synapses that carry sound to the brain and still detect those soft tones, so-called 'hidden hearing loss'. Researchers showed this directly: in tinnitus patients with normal audiograms, the first wave of the auditory brainstem response (wave I, generated by the auditory nerve) is reduced, while the later, more central wave V is normal. Less is coming in, yet the brain's output is back to normal, exactly what you would expect if the centre had turned up its gain.

How do you prove the brain's amplification is what makes sounds too loud?

You have to link the neural change to the perception in the same individual, otherwise the correlation could be coincidence. Researchers trained animals on a task whose reaction times give a reliable read-out of how loud a sound seems, while chronically recording from the auditory cortex and midbrain of those same animals. When a drug (salicylate) temporarily induced hearing loss and hyperacusis, perceived loudness and sound-evoked brain activity shifted in parallel, and the two tracked each other animal by animal. That within-subject link is the strongest evidence the gain model has.

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Evidence (8)

Central Gain Restores Auditory Processing following Near-Complete Cochlear Denervation.

Neuron · 2016

After near-complete cochlear denervation, the cortex ramps up 'central gain' to restore basic sound detection, compensation that can overshoot into hyperactivity.

Animal study (in vivo)Supports Central GainMouse

An integrative model of tinnitus based on a central gain controlling neural sensitivity.

Neuroscience and biobehavioral reviews · 2011

The seminal model: hearing loss triggers a compensatory rise in central gain that stabilises neural activity but amplifies 'neural noise', producing tinnitus.

ModelSupports Central GainN/A

Central gain control in tinnitus and hyperacusis.

Frontiers in neurology · 2014

The canonical review of central gain enhancement: why activity rises centrally even as the cochlea sends less, and how that produces tinnitus and hyperacusis.

ReviewSupports Central GainN/A

Testing the Central Gain Model: Loudness Growth Correlates with Central Auditory Gain Enhancement in a Rodent Model of Hyperacusis.

Neuroscience · 2019

A direct test: within the same animals, drug-induced changes in loudness perception tracked changes in sound-evoked central activity, the link the gain model required.

Animal study (in vivo)Supports Central GainRat

Tinnitus with a normal audiogram: physiological evidence for hidden hearing loss and computational model.

The Journal of neuroscience : the official journal of the Society for Neuroscience · 2011

Human evidence: in tinnitus patients with normal audiograms the auditory nerve sends less (reduced ABR wave I) while brainstem responses stay normal, hidden hearing loss plus central renormalisation.

Human study (non-randomized / first-in-human)Supports Central GainHuman

Association between Hyperacusis and Tinnitus.

Journal of clinical medicine · 2020

In 3,645 Swedish adults, hyperacusis was strongly linked to tinnitus, and up to 80% of people with severe tinnitus also reported hyperacusis.

Human study (non-randomized / first-in-human)Supports Central GainHuman

Depolarization shift in the resting membrane potential of inferior colliculus neurons explains their hyperactivity induced by an acoustic trauma.

Frontiers in neuroscience · 2023

A specific cellular mechanism for inferior colliculus hyperactivity after acoustic trauma: neurons' resting membrane potential shifts, making them fire more easily.

Animal study (in vivo)Supports Central GainMouse

Synaptic zinc plasticity shapes adaptive and maladaptive cortical plasticity following cochlear injury.

Science advances · 2026

Synaptic zinc plasticity in auditory cortex drives both the adaptive recovery and the maladaptive hyperactivity that follow cochlear injury.

Animal study (in vivo)Supports Central GainMouse

Timeline

2011
The seminal model: hearing loss triggers a compensatory rise in central gain that stabilises neural activity but amplifies 'neural noise', producing tinnitus.
2011
Human evidence: in tinnitus patients with normal audiograms the auditory nerve sends less (reduced ABR wave I) while brainstem responses stay normal, hidden hearing loss plus central renormalisation.
2014
The canonical review of central gain enhancement: why activity rises centrally even as the cochlea sends less, and how that produces tinnitus and hyperacusis.
2016
After near-complete cochlear denervation, the cortex ramps up 'central gain' to restore basic sound detection, compensation that can overshoot into hyperactivity.
2019
A direct test: within the same animals, drug-induced changes in loudness perception tracked changes in sound-evoked central activity, the link the gain model required.
2020
In 3,645 Swedish adults, hyperacusis was strongly linked to tinnitus, and up to 80% of people with severe tinnitus also reported hyperacusis.
2023
A specific cellular mechanism for inferior colliculus hyperactivity after acoustic trauma: neurons' resting membrane potential shifts, making them fire more easily.
2026
Synaptic zinc plasticity in auditory cortex drives both the adaptive recovery and the maladaptive hyperactivity that follow cochlear injury.

Related Hypotheses

PV Interneuron Dysfunction

PV interneuron dysfunction is a proposed cellular mechanism FOR central gain: losing PV-mediated cortical inhibition is one way the system's gain runs away after peripheral injury. Chambers & Polley (2016), which supports both, directly demonstrates the compensatory gain increase after cochlear denervation.

Dorsal Cochlear Nucleus Origin

Central gain is the general homeostatic account that activity rises at virtually every level of the auditory pathway after reduced cochlear input; the DCN model localises tinnitus generation to one of those levels, hypersynchronous fusiform cells in the brainstem.

Interacting Brain Networks

This model explicitly sets out to incorporate the deafferentation-based neurophysiological models, of which central gain is one, with the dysfunctional noise-cancelling model. Where the central gain account stops at increased amplification after reduced cochlear input, this framework adds the oscillatory (thalamocortical dysrhythmia) and network mechanisms proposed to turn that altered activity into a consciously heard percept.

Basis: PubMed 23597755

Cochlear Synaptopathy

Cochlear synaptopathy, nerve damage invisible on the audiogram, is a leading candidate source of the reduced peripheral input that the central gain model proposes triggers compensatory, and potentially runaway, central amplification. Both describe consequences of hidden cochlear damage that a standard hearing test misses.

Potassium Channel Dysfunction

Reduced Kv7.2/3 channel activity is a specific ion-channel mechanism for the broader rise in central excitability ('central gain') this hypothesis's papers describe after noise exposure, a candidate molecular substrate for why gain increases centrally.

Cortical Tonotopic Map Reorganization

Cortical map reorganization is the spatial/topographic account of cortex-level hyperactivity after hearing loss; central gain is the amplification account of the same broader phenomenon.

Basis: PubMed 15474168

Central Neuroinflammation

The Central Neuroinflammation hypothesis proposes a specific molecular driver of the cortical hyperexcitability underlying central gain: TNF-α-mediated inflammation that increases excitatory and reduces inhibitory synaptic input in the auditory cortex after hearing loss.

Open Questions

If central gain is an adaptive, homeostatic response, why do only some people with hearing loss develop tinnitus or hyperacusis?
Can central gain be measured reliably in an individual patient, well enough to guide treatment?
Which level of the pathway (brainstem, midbrain, cortex) contributes most to the gain increase, and can it be selectively turned down?