- Exposure to continuous white noise sabotages the development
of the auditory region of the brain, which may ultimately impair hearing
and language acquisition, according to researchers from the University
of California, San Francisco.
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- According to the scientists, the young rats used in their
study were exposed to constant white noise that is relevant to the increasing,
random noise encountered by humans in today's environment. They theorize
that their findings could aid in explaining the increase in language-impairment
developmental disorders over the last few decades.
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- The researchers, which included Howard Hughes Medical
Institute medical student fellow Edward Chang and otolaryngology professor
Michael Merzenich at the University of California at San Francisco, published
their findings in the April 18, 2003, issue of the journal Science.
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- "While the rat is not a perfect model of human auditory
development, it does allow us to investigate the fundamental role of early
sensory experience in mammalian auditory development," said Chang.
"For example, we do know that exposing infant rats to specific sound
stimuli can induce long-standing representational changes in the brain.
Other researchers have shown that there are striking parallels in humans
and other animals."
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- Although past experiments have demonstrated the important
effects that visual experience can have on brain development in animals
and humans, Chang said very few comparable experiments have been reported
that explore the effects of patterned early auditory experience on cortical
development.
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- "Auditory experience is clearly an important factor
in humans for learning language," he said. "We learn to speak
and read through our sensitivities to speech sounds that are heard during
early life."
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- Thus, Chang and Merzenich designed experiments in which
they reared rat pups in an environment of moderate continuous background
noise, which, while not injurious to their peripheral hearing, was loud
enough to mask normal environmental sounds. They then used electrophysiological
methods to gauge the organization of the auditory cortex in those animals,
as well as in control animals raised in a normal auditory environment.
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- The mapping technique consisted of recording the responses
of auditory cortex neurons to a variety of sounds presented to anesthetized
animals.
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- "We knew from previous work that the rat auditory
cortex normally undergoes a very dramatic, specific, and progressive development,"
said Chang. "During the first month of life, it becomes much more
specific and well tuned to different frequencies and temporal patterns
of sound. The brains of animals reared in noise, however, did not achieve
the basic benchmarks of auditory development until they were three or four
times older than normal animals," he said.
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- Additional tests on the maturing noise-reared rats showed
that their auditory regions continued to be plastic -- they continued to
reorganize their neural circuitry in response to exposure to sound stimuli
alone, long after the brains of normal rats had ceased rewiring. This suggested
that a "critical period" for exposure-based plasticity in the
brain had been extended.
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- They performed supplementary long-term experiments that
showed that although auditory development was delayed in the noise-exposed
rats, it did mature to normal adult levels once the animals were removed
from the noisy environment. And furthermore, they observed those plasticity
effects consolidated during the extended critical period persisted into
the future, suggesting that this exposure were indeed "critical."
Chang summarized, "it's like the brain is waiting for some clearly
patterned sounds in order to continue its development. And when it finally
gets them, it is heavily influenced them, even when the animal is physically
older."
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- Chang said that the findings "suggest that there
are two sides to the coin. "On the negative side, these findings suggest
that noise can have devastating effects on the rate of development of the
brain. They emphasize the importance that children, especially those at
risk, be exposed to salient features in speech sounds in order for their
auditory development to be normal. On the positive side, our findings may
mean that the time frame may be longer in which treatment of such children
will allow them to catch up." According to Chang, the need for exposure
to structured sounds underscores the importance of special therapy for
children with disorders that might affect auditory processing.
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- "There are many linkages between neurons in the
auditory system from the cochlea to the cortex where information has to
be passed along," he said. "And in addition to environmental
noise, a number of acquired or inherited disorders could potentially degrade
the signal at any of these points, masking the sensory input. From these
findings, we theorize that disorders, for example, such as focal epilepsies
or defects in myelination, might affect the fidelity of this signal, disrupting
normal development of the auditory cortex. A combination of external and
internal elements would be highly detrimental"
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- Chang's future studies will address whether humans with
developmental disorders have higher levels of noise in their auditory systems.
Such studies, he said, could lead to diagnostic and predictive tests.
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- "If we knew that a child had a susceptibility to
noise, we could intervene to enrich the child's acoustic experience to
foster more normal auditory and language development," said Chang.
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- Editor's Note: The original news release can be found
at:
- http://www.hhmi.org/news/chang.html
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- Note: This story has been adapted from a news release
issued for journalists and other members of the public. If you wish to
quote any part of this story, please credit Howard Hughes Medical Institute
as the original source. You may also wish to include the following link
in any citation:
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- http://www.sciencedaily.com/releases/2003/04/030418081607.htm
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