November 11, 2009
Neural Mechanism Reveals Why Dyslexic Brain Has Trouble Distinguishing Speech From Noise
New research reveals that children with developmental dyslexia have a deficit in a brain mechanism involved in the perception of speech in a noisy environment. The study, published by Cell Press in the November 12 issue of the journal Neuron, provides the first direct evidence that the human auditory brainstem exhibits remarkable moment-to-moment plasticity and undergoes a fine tuning that is strongly associated with noise exclusion.
Most people have little trouble carrying on a conversation with a friend in a noisy restaurant thanks to the highly adaptive auditory system which manages to focus in on the predictable, repeating pitch of the friend's voice and effectively tune out the random, fluctuating background noise. Although it may be a routine occurrence, exactly how the nervous system manages to accomplish this feat is still a mystery.
"Understanding the relationship between the adaptive auditory system and perception of speech in noise is clinically relevant because recent studies have demonstrated that the 5%Ã³ËÂ¶% of school-age children who are diagnosed with developmental dyslexia can be particularly vulnerable to the deleterious effects of background noise," explains senior study author Dr. Nina Kraus, who directs the Auditory Neuroscience Laboratory at Northwestern University.
Dr. Kraus and colleagues measured auditory brainstem responses to a speech syllable presented in a repetitive or variable context in children with and without developmental dyslexia. The brainstem is the first part of the brain that processes auditory information and relays that information to higher brain centers. Children without dyslexia showed enhanced brainstem representation of features related to voice pitch in the repetitive context, relative to the variable context. In contrast, brainstem encoding in the children with dyslexia did not adapt well to the repeating auditory signal.
The researchers went on to show that the extent of context-dependent encoding in the auditory brainstem was positively correlated with the successful perception of speech in noise. "The ability to sharpen representation of repeating elements is crucial to speech perception in noise, since it allows superior tagging of voice pitch, an important cue for segregating sound streams in background noise," offers Dr. Kraus. "The disruption of this mechanism contributes to a critical deficit in noise exclusion, a common symptom in developmental dyslexia."
Interestingly, Dr. Kraus's team also observed that, when compared with the children that did not have dyslexia, the dyslexic children exhibited enhanced brain activity during the variable condition. "This may enable dyslexic children to represent their sensory environment in a broader and arguably more creative manner, although at the cost of the ability to exclude irrelevant signals," speculates Dr. Kraus.
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