CI Atlas · When Hearing Aids Aren't Enough · Module 05

5Why recruitment happens

Recruitment is not a mysterious quirk; it follows directly from how the cochlea normally controls loudness. In a healthy ear the outer hair cells apply active, level-dependent tuning that compresses a huge range of input levels into a manageable response and sharpens the frequency tuning of each nerve fibre. When outer hair cells are damaged, that compression and tuning are lost: the nerve-fibre tuning curves widen and lose their sensitive tips, so a rising stimulus quickly spreads to excite fibres in the broadened tails, and the total neural response surges. That surge is heard as abnormally rapid loudness growth — recruitment. The same logic explains a hopeful fact: electrical stimulation, which drives the nerve directly and grows in a near-linear way, produces no recruitment at all.

TWhere the steep growth comes from

Recruitment is, at bottom, a change in how the auditory nerve's response grows with level. To see why, recall the cochlear amplifier (Chapter 2): the outer hair cells actively tune and compressthe response, boosting faint sounds and reining in loud ones, and sharpening each fibre's frequency tuning.

CThe lost cochlear compression

When hearing loss occurs through outer-hair-cell damage, that active process fails. The cochlea loses its compressionand its gain control, and the tuning curves of the nerve fibres lose their sharp, sensitive tips. The fine, level-dependent control that kept loudness growing gently is gone.[2003]

CRecruiting the tails

With the tips of the tuning curves flattened and widened, near threshold only the few neurons tuned to the stimulus frequency respond. But as the stimulus level rises, the broadened tails of neighbouring fibres are rapidly brought in — recruited — so the total number of responding fibres climbs steeply. That abnormally fast growth in neural activity is what the listener perceives as the explosive loudness growth of recruitment.

CWhy electric hearing has none

Here is the payoff for the implant. A cochlear implant bypasses the hair cellsand drives the auditory nerve directly, and the nerve's response to electrical current grows in a roughly linear way — there is no broadened tuning curve to recruit. So electrical hearing shows no recruitment, and the usable range can even be widened by adding controlled randomness (stochastic stimulation). Recruitment, the bane of the powerful hearing aid, simply does not arise for the implant — a recurring reason the implant succeeds where amplification fails.

A healthy fibre's tuning curve has a sharp, sensitive tip at its characteristic frequency — it fires to a whisper there and ignores other frequencies. Outer-hair-cell damage abolishes the tip: the threshold at CF rises and the curve flattens into a broad bowl. Now a rising stimulus quickly excites the wide, shallow region, sweeping in many fibres at once — the steep loudness growth of recruitment, and the broadened frequency resolution of the next module, are two faces of this one change. Schematic.

Underneath the loudness curve is a mechanical one: the basilar membrane's input/output function. In a healthy cochlea the outer hair cells make it compressive — a 100 dB range of input is squeezed into a far smaller range of motion (the shallow mid-level slope), which is how the ear packs the whole world into a manageable response. Damage removes the active gain and the function goes linear and steep: every decibel of input now produces a full decibel of response, so loudness shoots up. Recruitment, seen at the level of the membrane itself. Schematic.

Case 10.5 · Why electric hearing is steadier

A patient with severe recruitment that made hearing aids unbearable is implanted, and afterwards reports that loudness feels far more controlled and comfortable across a wide range of sounds.

Why does electrical hearing avoid the recruitment that plagued the hearing aid?

Self-assessment — Module 52 questions

Question 1 · Trainee

What mechanism produces loudness recruitment?

Question 2 · Clinician

Why does electrical (implant) hearing show no recruitment?

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