tutes chronic acoustic trauma and may result in permanent hearing
loss. Time will tell whether the vastly increased use of portable music
players will lead to an increase in the prevalence of early-onset hear-
ing loss.
What is the connection between exposure to loud sounds
and hearing loss? Although the mechanisms are not understood,
something injurious happens from intense overstimulation of hair
cells. It appears that hair cells may die from a kind of excitotoxic over-
stimulation, perhaps similar to what can happen in brain neurons
when seizures occur (see Chapter 8). As far as we know, hair cells once
permanently damaged are not repaired or replaced. And there are only
about thirty-five hundred inner hair cells (the ones most responsible
for hearing) in each ear—that’s not very many. Each retina has more
than one hundred million photoreceptor cells—lose a few thousand
photoreceptor cells for some reason, and you would probably not even
notice it. But lose a few thousand hair cells, and you are essentially
deaf.
What can be done to improve hearing in folks who have sustained
hearing loss? The most common kind of therapeutic device used to
help people who have suffered hearing loss is the acoustic amplifier
hearing aid, which has been around for many years in different forms.
In its original form it would have been the ear trumpet, a device to
increase the intensity of sound by collecting more of it—essentially
a pinna extension offering passive amplification. Some ear trumpets
were quite fancy and could be fashion statements in the same way
that eyeglasses can be.
The twentieth century saw the development of electronics that
could produce a miniature amplifier, powered by battery, that actively
magnifies the intensity of incoming sound. Most contemporary