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November 19, 2002

Loss of hair cells can cause distortion in perceived hearing

From: Kingsport Times News, TN
Nov. 19, 2002

By Danielle Tousinau

Most people with sensorineural hearing loss experience a phenomenon called recruitment. They perceive that sounds become too loud too fast.

In other words, for a set amount of increase in sound intensity that normal hearing listeners would find comfortable, people with sensorineural hearing loss might find the same amount of increase in sound intensity uncomfortable.

This happens because of the way the ear works once you have hearing loss.

To understand how and why recruitment occurs, it is easiest to compare the hearing organ to a piano keyboard.

The hearing organ known as the cochlea contains thousands of sensory hair cells.

In this comparison, the hair cells are equivalent to the keys on the piano keyboard. The piano keyboard is divided into several octaves, with each octave containing eight white keys that correspond to certain pitches.

Similarly, the hair cells in our inner ears are divided into critical bands, with each critical band containing a certain number of hair cells that correspond to certain pitches.

Each key on the piano belongs to a specific octave, and each hair cell belongs to a specific critical band.

When a hair cell in a critical band is stimulated, the entire critical band sends a signal to the brain that we hear as one unit of sound at the pitch that critical band is sensitive to.

When you have a sensorineural hearing loss, some of the hair cells become damaged and stop functioning or die. The hair cells do not grow back.

Now, each critical band has hair cells that are missing.

This is like a piano that is missing keys so that each octave would no longer have eight keys. To play a song you may have to substitute a different key for one that is missing.

The hearing organ works the same way.

Hair cells in nearby critical bands are "recruited" and activated in place of ones that are damaged or missing.

Now hair cells will be activated for more than one critical band.

This recruitment of hair cells is what makes some sounds seem much louder than they normally would.

That's because existing hair cells must function not only in their original critical band, but also in nearby critical bands that they have been recruited to.

Now the brain receives a signal from the hair cell's original entire critical band as well as from the nearby critical band.

This results in a perception of the sound being loud.

When more hair cells are lost and more critical bands are stimulated, the situation becomes worse as sounds become even louder.

Although one result of recruitment is that sounds may seem louder than they would to the average listener, another result of recruitment is that sounds seem distorted and unclear to the listener.

This happens because instead of a specific frequency region being stimulated for a single sound, many frequency regions are recruited and stimulated for that single sound.

Thus, it becomes difficult to distinguish similar sounding words, and the listener's ability to understand speech will greatly diminish.

Before you are fitted with hearing aids, testing should be done to determine your understanding of speech and your comfort levels with loud sounds.

For people with sensorineural hearing loss who experience recruitment, hearing aids must be selected and fitted appropriately to compensate for this phenomenon so that amplified sounds are not uncomfortably loud.

Circuits in modern hearing aids help control sounds so that they remain comfortable for the listener.

While hearing aids cannot fix distortion problems, modern hearing aids can reproduce sound cleanly so that more distortion is not created for the listener.

Copyright 2002 Kingsport Times-News. All rights reserved.