White noise delays auditory organization in the brain
April 22, 2003
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.
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.
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.
"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."
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.
"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."
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.
The mapping technique consisted of recording the responses of auditory
cortex neurons to a variety of sounds presented to anesthetized animals.
"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.
Additional tests on the maturing noise-reared rats showed that their
auditory regions continued to be plastic ž 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.
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."
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.
"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."
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.
"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.
Howard Hughes Medical Institute
|
