The Cochlea Sits at a Critical Point

01The evidence

• The interplay between active hair bundle motility and electromotility has been modelled as a cooperative amplification mechanism, with the two active processes combining rather than one driving the other.Cochlear active mechanics

02The tension

The motor account says outer hair cells push the basilar membrane and amplify motion. It explains gain but not the shape of the response: the compression, the sharp tuning that degrades with level, and the spontaneous otoacoustic emissions that a quiet ear produces. A motor that pushes would ring only if it were unstable, which is a strange design for a sensor.

03The theory

A system poised near a Hopf bifurcation responds to a weak drive with a large, compressive, frequency-selective amplification, and it emits spontaneously when the drive is removed. That is the observed behaviour of the cochlea, including the emissions.

On this account the outer hair cells do not supply the amplified motion. They adjust the distance to the critical point, setting sensitivity and tuning by moving the system closer to or further from instability. The power comes from the same metabolic pool that keeps the system poised.

The prediction is that the signature of healthy hearing is a critical exponent, measurable as the scaling of response amplitude with stimulus level, and that damage shifts the system away from the critical point rather than simply reducing gain. That distinguishes it from the motor account, which predicts a proportional loss.

The clinical consequence is that hearing loss should be characterised by a change in the exponent, not only by a threshold shift, and that otoacoustic emissions should be treated as a readout of distance to criticality rather than as a byproduct.

04What this is not

05Predictions

• P1Response amplitude should scale with stimulus level through a critical exponent that changes with damage.
• P2Reducing outer hair cell function should shift the operating point rather than scale gain uniformly.
• P3Spontaneous emissions should track distance to the critical point.

06What would refute it

Gain that scales proportionally with outer hair cell force, with no change in exponent, would restore the motor account.

07What it would change

It would change how hearing loss is measured, and make criticality a diagnostic quantity.