LESSON 85

Cochlear damage and recruitment

Name: Cochlear damage and recruitment
Uploaded: 2026-11-04T03:30:00+03:00
Duration: 12 min 3 s
Description: Cochlear damage does more than raise the threshold. In this lesson we look at the abnormally rapid growth of loudness known as recruitment, the reduced dynamic range, reduced frequency selectivity, temporal changes, and how they affect understanding in noise and hearing aid fitting.

Cochlear damage does more than raise the threshold. In this lesson we look at the abnormally rapid growth of loudness known as recruitment, the reduced dynamic range, reduced frequency selectivity, temporal changes, and how they affect understanding in noise and hearing aid fitting.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn how cochlear damage changes not only the threshold but also the way sound is perceived.
We will cover recruitment, the reduced dynamic range, reduced frequency selectivity and difficulty understanding in noise.
In the cochlear amplifier lesson, we saw how outer hair cells boost the vibration of the basilar membrane.
In most sensorineural losses, the outer hair cells are the first and most affected structures.
When outer hair cells are damaged, the gain for soft sounds and the natural compression are lost.
The peak of the traveling wave also loses its sharpness, so frequency tuning becomes blurred.
Inner hair cells convert the vibration into a nerve signal; their damage reduces the transfer of information to the nerve.
In this lesson we will focus on the perceptual traces of outer hair cell damage, and in the next lesson on inner hair cell loss.
We can group the perceptual consequences of cochlear damage under four headings.
The first, and the only one seen on the audiogram, is the rise in threshold.
The second is that loudness grows faster than normal above threshold; this is called recruitment.
The third is a reduction in the ear's ability to separate nearby frequencies, that is, in frequency selectivity.
The fourth is changes in the processing of fine timing information in sound.
Loudness recruitment is the abnormally rapid growth of loudness above threshold in an ear with an elevated threshold.
Soft sounds are not heard, but as sound gets louder, loudness grows quickly and catches up with the normal ear at high levels.
The cause is the loss of the natural compression provided by outer hair cells; the basilar membrane responds more linearly to intensity.
A normal ear amplifies soft sounds a lot and strong sounds little; in a damaged ear, this balancing effect is gone.
The phenomenon was described by Fowler in the nineteen thirties by balancing loudness between the two ears.
This method is known today as the alternate binaural loudness balance test; we will cover it in the lesson on suprathreshold tests.
An ear with recruitment also easily notices small increases in intensity; the short increment sensitivity index, or SISI, is based on this.
Recruitment is a different concept from hyperacusis, in which tolerance to everyday sounds is reduced regardless of hearing threshold.
Let us consider an example with one normal ear and a cochlear loss in the other ear, with a threshold around fifty decibels HL.
At twenty decibels, the normal ear hears the sound as soft, while the damaged ear does not hear it at all.
At fifty decibels, the sound is comfortable for the normal ear, while the damaged ear is only just noticing it.
At seventy decibels, the sound has reached a comfortable level in the damaged ear; it went from threshold to comfortable in twenty decibels.
At ninety decibels and above, both ears hear the sound as similarly loud; loudness has caught up with normal.
The values in this example are for teaching; the actual rate of growth varies from person to person.
Loudness can grow in different patterns.
In complete recruitment, the damaged ear hears high levels as loud as the normal ear; this is common in cochlear losses.
In partial recruitment, loudness approaches normal but does not fully catch up.
In over-recruitment, the damaged ear hears loud sounds as even louder than the normal ear; it has been reported in some cochlear disorders.
In decruitment, loudness grows more slowly than normal; this pattern has been linked with some retrocochlear conditions.
These distinctions were used historically to identify the site of lesion; today they matter more for hearing aid fitting.
The range between threshold and the uncomfortable loudness level is called the dynamic range.
In a normal-hearing ear, this range is about one hundred decibels.
In cochlear loss, the threshold rises, but the uncomfortable loudness level, or UCL, often changes little.
For example, if the threshold is sixty and the UCL is one hundred decibels HL, the dynamic range drops to forty decibels.
Speech itself spans a range of about thirty decibels, from weak consonants to strong vowels.
Fitting this speech and everyday sounds into a narrowed dynamic range is one of the core problems of hearing aid fitting.
A fixed gain that makes soft sounds audible can easily push strong sounds to an uncomfortable level.
Frequency selectivity is the ear's ability to separate a sound into its frequency components.
In a healthy cochlea, each frequency forms a sharp peak in a narrow region of the basilar membrane.
When outer hair cells are damaged, this peak flattens out, and a tone stimulates a wider region.
Psychoacoustic measurements show that auditory filters can broaden to several times normal in moderate cochlear loss.
Psychophysical tuning curves also lose the sharp V shape of the normal ear and become shallow and broad.
As filters broaden, more noise falls into each filter, which increases masking.
In a normal ear, the formant peaks that distinguish vowels are represented as clear peaks in the inner ear.
Narrow filters keep most of the noise around the signal out.
The upward spread of masking also stays limited.
In cochlear damage, broad filters flatten the formant peaks; similar vowels and consonants may be confused.
Because more noise enters each filter, the same noise covers speech more easily.
The masking effect of low-frequency noise spreading to high-frequency regions also increases clearly.
As we saw in the previous lesson, in cochlear loss the thresholds of short and long tones come closer; temporal integration decreases.
Gap detection, on the other hand, depends largely on how audible the sound is.
When the sound is made audible enough, the gap threshold of many people with cochlear loss comes close to normal.
In contrast, the ability to use timing information in the fine structure of the wave has been reported to weaken in cochlear loss.
This weakness can affect pitch perception, music listening and separating speech from noise.
Since cochlear implants largely cannot convey fine structure, preserving any remaining acoustic hearing is valuable in this respect.
In nineteen seventy eight, Plomp explained the effect of cochlear loss on speech perception with two components.
The first component is attenuation: speech is heard as softer, and in quiet a louder voice is needed.
The second component is distortion: even when sound is loud enough, frequency and timing details are processed in a blurred way.
A hearing aid can largely compensate for the attenuation component by making sound louder.
The distortion component, however, is not fixed by amplification and shows up especially as difficulty understanding in noise.
So people with cochlear loss need a better signal-to-noise ratio than normal listeners for the same level of understanding in noise.
These mechanisms explain a complaint we often hear in the clinic: I can hear, but I can't understand.
The patient notices that someone is speaking, but cannot pick out the words because consonants and details sound blurred.
When the family raises their voice, recruitment makes the sound uncomfortable quickly, and the patient says do not shout, it bothers me.
In crowded places, speech gets lost in the noise because of broad filters and upward spread of masking.
Some patients also describe being clearly bothered by sudden loud sounds, such as a door slamming or clattering dishes.
Explaining these complaints helps the patient and family understand the situation and keep their expectations realistic.
In the clinic, besides thresholds, we first measure the uncomfortable loudness level and speech performance in noise.
Wide dynamic range compression gives more gain to soft sounds and less to strong ones, fitting sound into the narrowed range.
The maximum output of the hearing aid is limited so that sound does not reach an uncomfortable level in an ear with recruitment.
Since amplification does not fix distortion, the signal-to-noise ratio is improved with directional and remote microphone systems.
Finally, counseling that clearly explains the limits of hearing aids in noise keeps expectations realistic.
A tip from the clinic: communication advice for the family of a patient with recruitment is very useful.
Shouting quickly pushes sound to an uncomfortable level and does not make consonants clearer.
Instead, advise them to speak face to face, with light on their face, clearly and a little slowly.
Getting the patient's attention before speaking and reducing background noise also make understanding much easier.
With outer hair cell damage, compression is lost; loudness grows abnormally fast above threshold, which is called recruitment.
Since the UCL often stays the same while the threshold rises, the dynamic range narrows.
Auditory filters broaden; formants blur and noise covers speech more easily.
A hearing aid compensates for attenuation, but the distortion component may persist as difficulty understanding in noise.
Test yourself: what is the main cause of recruitment?
The correct answer is B: loss of the natural compression provided by the outer hair cells.
The basilar membrane responds more linearly to intensity, so loudness grows fast above threshold and catches up with normal at high levels.
In the next lesson we will learn about regions where inner hair cells are lost, called cochlear dead regions, and the TEN test.
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