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As printed in Hearing Health, volume 19:1,
Spring 2003
By Ruth Litovsky, Ph.D.
Why do we have two ears anyway? According to the Roman
philosopher Epictetus, “We have two ears and one
mouth so that we can listen twice as much as we speak.”
Notwithstanding the philosophical merits of this statement,
we ask ourselves this question when considering how
hearing aids and cochlear implants (CIs) might best
be used by people with hearing loss.
Let’s first take a situation we face in our
everyday lives: we are in a room full of people and
while they are talking simultaneously in several locations
around us, we can ignore some voices and pay attention
to others. Our ability to hear in such an environment
is usually severely compromised without the use of both
ears.
Among the advantages of having input from both ears
is the ability of the one nearest to a stimulus to pick
up and focus on that specific sound rather than others.
This is known as the “better ear” effect.
Another benefit of having input from both ears is that
specialized neurons in the auditory pathway are sensitive
to cues that help us identify the location of a sound
source, a phenomenon called binaural hearing. These
important cues include differences between the two ears
in the arrival time and intensity of sounds from various
directions. For instance, a sound that arrives from
the right will reach the right ear first and have greater
intensity in the right than the left ear. The reverse
is true for sounds arriving from the left.
Binaural hearing also provides an important subjective
component known as externalization whereby sounds appear
to come from locations at a distance from the head rather
than near the ear or inside the head, as is the case
with one ear.
Research involving people without hearing loss shows
that binaural hearing provides listeners improved ability
to extract meaning from speech in the presence of multiple
interfering sounds. Further, sound localization is severely
compromised if listeners are not allowed to utilize
one of their two ears. These findings may have significant
relevance for single-ear hearing aid wearers and CI
users who have difficulty in noisy environments despite
their ability to function well in quiet spaces.
Studies of binaural input have shown that in various
situations, bilateral hearing aids offer a significant
advantage over a single unit. Much less is known about
the effectiveness of bilateral CIs or “binaural
hybrid hearing,” the combined use of a hearing
aid on one ear and implant technology on the other.
To evaluate the benefits, it is necessary to simulate
aspects of complex environments that might influence
the ability of hearing aid and CI users to extract information
from their surroundings. Research in this area is only
beginning.
It is interesting to note, however, that many people
who have received bilateral implants are delighted with
the results. They report that they perceive sounds as
coming from outside the head and that they have less
difficulty in many scenarios, including noisy, multi-talker
environments. And some studies suggest that bilateral
CIs result in improved sound localization abilities
and better performance on speech-in-noise tasks.
Many questions remain. For example, to what extent
and under what circumstances is a second CI more beneficial
than wearing a hearing aid in the non-implanted ear?
How much of a two-ear benefit should a second device
offer in order to justify the added cost? How does one
quantify “benefit” to include subjective
impressions, objective measurements from experimental
studies and general quality of life outcomes? Finally,
how does one address these issues with children?
The next few years of research are likely to provide
answers to some of these questions and open up the doors
to improved understanding of the emerging world of binaural
hybrid hearing.
Ruth Litovsky, Ph.D., is the director
of the Binaural Hearing and Speech Lab in the Waisman
Center and Department of Communicative Disorders at
the University of Wisconsin, Madison.
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