Study Unravels Mystery of Dyslexia

Nov. 11, 2009 -- New research may provide an answer as to why children with
dyslexia often have difficulty hearing someone talk in a noisy room.
Dyslexia is a common, language-based learning disability that makes it
difficult to read, spell, and write. It is unrelated to a person's
intelligence. Studies have also shown that patients with dyslexia can have a
hard time hearing when there is a lot of background noise, but the reasons for
this haven't been exactly clear.
Now, scientists at Northwestern University say that in dyslexia, the part of
the brain that helps perceive speech in a noisy environment is unable to
fine-tune or sharpen the incoming signals.
"The ability to sharpen or fine-tune repeating elements is crucial to
hearing speech in noise because it allows for superior 'tagging' of voice
pitch, an important cue in picking out a particular voice within background
noise," Nina Kraus, director of Northwestern University's Auditory Neuroscience
Laboratory, says in a news release.
The brainstem is the first place in the brain to receive and process
auditory (hearing) signals. It is supposed to automatically focus on the
information, such as repeated bits of speech, and sharpen it so you can discern
someone's voice from, say, the noise of a chaotic classroom. The new study,
however, provides the first biological evidence that children with dyslexia
have a deficit in this auditory process. As a result, the brainstem cannot
focus on relevant, predictable, and repeating sounds.
The new evidence is based on a brain activity study of children with both
good and poor reading skills. The children wore earphones that repeated the
sound "da" in different intervals while watching an unrelated video. The first
time, "da" repeated over and over again in a repetitive manner. In a second
session, the sound "da" occurred randomly along with other speech sounds, in a
variable manner. Electrodes taped to each child's scalp recorded the brain's
response to the sounds.
The children also underwent standard reading and spelling tests and were
asked to repeat sentences provided to them amid different noise levels.
"Even though the children's attention was focused on a movie, the auditory
system of the good readers 'tuned in' to the repeatedly presented speech sound
context and sharpened the sound's encoding. In contrast, poor readers did not
show an improvement in encoding with repetition," Bharath Chandrasekaran, one
of the study's authors, says in a statement.
The tests also revealed that children without dyslexia were better able to
repeat sentences they had heard in noisy environments. However, the researchers
noted enhanced brain activity of the children with dyslexia during the session
when the "da" sound was variably played.
"The study brings us closer to understanding sensory processing in children
who experience difficulty excluding irrelevant noise. It provides an objective
index that can help in the assessment of children with reading problems," Kraus
says.
The findings, which appear in this week's issue of Neuron, may also
help teachers and caregivers devise better strategies for teaching children
with dyslexia. For example, the study authors say children with dyslexia who
have trouble sorting out voices in noisy classrooms may benefit simply by
sitting closer to the teacher.
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