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Recruitment from Hearing Loss Explained

© June 2001 by Neil Bauman, Ph.D.
Question: Some people can't wear hearing aids because of severe recruitment. Recruitment seems to be such a weird word for this problem. The word means "to enlist." The dictionary doesn't describe anything to do with hearing loss and hearing aids. So what really is recruitment and how did it get this strange name?—C. H.
Answer: Good questions. No wonder you are confused! Even many of the hearing health care professionals don't understand this condition that goes by the strange name of recruitment. There is a lot of misunderstanding about recruitment. Actually, recruitment really is a good word to describe this phenomenon—once we understand what goes on in our inner ears.

What is Recruitment?

Very simply, recruitment is when we perceive sounds as getting too loud too fast.
Before we look at how recruitment got its name, there are two things we need to know about recruitment.
First, recruitment is always a by-product of a sensorineural hearing loss. If you do not have a sensorineural hearing loss, you cannot have recruitment.
Second, there are two other phenomena that often get confused with recruitment. These are hyperacusis (super-sensitivity to normal sounds) and phonophobia (fear of normal sounds resulting in super-sensitivity to them). Both hyperacusis and phonophobia can occur whether you have normal hearing or are hard of hearing. In fact, if you have a sensorineural hearing loss, you could suffer from all three conditions at once!

How Recruitment Got Its Name

Now let's look at how recruitment "works" and how it got its name. Perhaps the easiest way to understand recruitment is to make an analogy between the keys on a piano and the hair cells in a cochlea.
The piano keyboard contains a number of white keys while our inner ears contain thousands of "hair cells." Think of each hair cell as being analogous to a white key on the piano.
The piano keyboard is divided into several octaves. Each octave contains 8 white keys. Similarly, the hair cells in our inner ears are thought to be divided into a number of "critical bands" with each critical band having a given number of hair cells. Each critical band is thus analogous to an octave on the piano.
Just as every key on the piano belongs to one octave or another, so also, each hair cell belongs to a critical band.
When you play a chord on the piano—you press two or more keys together but they send one sound signal to your brain. Similarly—but yet different—when any hair cell in a given critical band is stimulated, that entire critical band sends a signal to our brains which we "hear" as one unit of sound at the frequency that critical band is sensitive to. This is the situation when a person has normal hearing.
However, when we have a sensorineural hearing loss, some of the hair cells die or cease to function. When this happens, each "critical band" no longer has a full complement of hair cells. This would be analogous to a piano with some of the white keys yanked out. The result would be that some octaves wouldn't have 8 keys any more.
Our brains don't like this condition at all. They require each critical band to have a full complement of hair cells. Therefore, just as our government, when it runs short of military personnel, puts on a recruitment drive, so too, our brains do the same thing. However, since all the hair cells are already in service, there are no spares to recruit.
What our brains do is rather ingenious. They simply recruit some hair cells from adjacent critical bands. (Here is that word recruit or recruitment.) These hair cells now have to do double duty or worse. They are still members of their original critical band and now are also members of one or more additional critical bands.
If only relatively few hair cells die, then adjacent hair cells may just do double duty. However, if many/most hair cells have died, then in order to have a full complement of hair cells in each critical band, any given hair cell may be recruited into several different critical bands.

The Result of Recruitment

The result of this recruitment causes us two basic problems.
First, the sounds reaching our brains appear to be much louder that normal. This is because the recruited hair cells still function in their original critical bands and also in the adjacent one(s) they have been recruited into.
Remember that when any hair cell in a critical band is stimulated, the whole critical band sends a signal to our brains. So the original critical band sends one unit of sound to our brains, and at the same time, since the same hair cell is now recruited to an adjacent critical band, it stimulates that critical band also. Thus, another unit of sound is sent to our brains. Hence, we perceive the sound as twice as loud as normal.
If our hearing loss is severe, a given hair cell may be recruited into several critical bands at the same time. Thus our ears could be sending, for example, eight units of sound to our brains and we now perceive that sound as eight times louder than normal. You can readily see how sounds can get painfully loud very fast! This is when we complain of our recruitment.
In fact, if you have severe recruitment, when a sound becomes loud enough for you to hear, it is already too loud for you to stand.
The second result of recruitment is "fuzzy" hearing. Since each critical band sends one signal at the frequency of that critical band, when hair cells get recruited into adjacent bands, they stimulate each critical band they are a member of to send their signals also. Consequently, instead of hearing just one frequency for a given syllable of sound, for example, perhaps our brains now receive eight signals at the same time—each one at a different frequency.
The result is that we now often cannot distinguish similar sounding words from each other. They all sound about the same to us. We are not sure if the person said the word "run" or was it "dumb," or "thumb," or "done," or "sun," or? In other words, we have problems with discrimination as well as with volume. If our recruitment is bad, our discrimination scores likely will go way down.
When this happens, basically all we hear is either silence or loud noise with little intelligence in it. Speech, when it is loud enough for us to even hear it, becomes just so much meaningless noise.
This is why many people with severe recruitment cannot successfully wear hearing aids. Their hearing aids make all sounds too loud—so that they hurt. Also, hearing aids cannot correct the results of our poor discrimination. We still "hear" meaningless gibberish.
However, people with lesser recruitment problems will find much help from properly adjusted hearing aids. Most modern hearing aids have some sort of "compression" circuits in them. When the compression is adjusted properly for our ears, these hearing aids can do a remarkable job of compensating for our recruitment problems.