babies

[ronnie sundin <ronnie@xxxxxxxxxx>]

this popped up on the h3o-list recently,  enjoy.

..ronnie



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Babies have a different way of hearing the world by listening to all
frequencies simultaneously. By Joel Schwarz.

The world apparently sounds very different to infants than it does to
adults. Sometimes it's filled with a cacophony of sounds that makes it
difficult for babies to distinguish a single sound from all the surrounding
noise, says a University of Washington scientist.

That's because babies are generalists and hear all frequencies
simultaneously so they can respond to unexpected sounds, reports Lynne
Werner, a UW professor of speech and hearing science, in the May edition of
The Journal of the Acoustical Society of America.

"Babies have a different way of listening to the world," contends
Werner. "In real life we are confronted with a variety of sounds. Somehow
the adult brain takes all sounds we hear and separates them into where they
are coming from and then focuses on the one we want to hear. Adults usually
hear in a narrow band of sound, while babies seem to use a different
approach. They don't have the selective attention of adults and they don't
pay attention all of the time. Instead they always seem to be listening
broadband or to all frequencies simultaneously."

Researchers have known for some time that babies are born with functioning
hearing and that sensitivity to sound improves dramatically during infancy.
Improvements continue through age 10 when the average child's hearing is
comparable to an adult's.

To further understand how babies hear, Werner tested 73 infants ages 7 to 9
months and 40 adults 18 to 30 years old. All had normal hearing. They were
individually exposed to four half-second bursts of a computer-generated
1000-hertz tone and a 1000-hertz broadband noise that sounded like telephone
dial tone or static. Sometimes the tone or the noise was played alone and
sometimes the sounds were masked with background noise. The sounds were
played at different levels of loudness to see if the subjects could detect
them. Subjects heard the sounds through foam earphone inserts that were
placed in the right ear canal.

The computer randomly generated the four different types sound and the
infants were trained to respond when they heard a sound. Infants were seated
in their mother's lap in a test booth and an assistant kept the child
attentive and entertained by manipulating silent toys on a table in front of
the child. The assistant and mother wore headsets and listened to masking
sounds to ensure that they could not hear any of the sounds presented to the
child. An observer outside the booth watched and scored the child's
responses through a window or on a video monitor. Infants were rewarded for
responding to a signal by the activation of a mechanical toy. Typically
babies turned toward the sound or changed their activity level when they
heard something. Infant also responded by pronounced change in their facial
expression or by looking at their mother.

Adults were tested in a similar fashion, sitting alone in the booth.
They were told to raise a hand when they heard a sound that would activate
the mechanical toy.

Werner found that on average babies are relatively better at
detecting noise than tones. In the quiet condition the infant-adult
difference in detecting noise was 14 decibels versus 7 decibels in the
masked trials. A 15-decibel deficit in adults is the equivalent of a minor
hearing loss, she said. With the background masking the infant-adult
difference in detecting the tone was 10 decibels and 5 decibels for
the noise.

In addition, Werner said preliminary examination of 11 infants' psychometric
function for detecting broadband noise does not support the idea proposed by
some scientists that babies pay more attention to broadband noise than to
tone. Like 10 adults also tested, infants produce similar psychometric
functions for broadband noise and tone detection.

"If you are a baby it is sensible to listen broadband, and it was
valuable for our ancient ancestors for survival in the Serengeti (the
Serengeti Plain of Eastern Africa)," says Werner. "But in today's western
culture a baby is at a great disadvantage. All the noise we expose people to
makes it difficult for babies. The practical lesson from this research is,
if you are talking to a baby or reading her a story, background noise can be
a problem. Turn off the television or radio."

Co-author of the study is Kumiko Boike, a UW doctoral student who did
preliminary research as an undergraduate. The study was funded by National
Institute of Deaf and Communicative Disorders, one of the National
Institutes of Health.

Source URL:
http://www.washington.edu/newsroom/news/2001archive/05-01archive/k0529
01.html


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