As audiologists, we understand the important role hearing sensitivity plays in the development of reading skills. What we don't always consider, however, is the effect of central auditory processing.
The accurate representation of speech sounds is an important aspect of learning to link sounds with orthographic symbols, a fundamental building block of reading (Proc Nat Acad Sci U S A 2010;107:7939-7944 ).
Furthermore, if a child is not making sound-to-meaning connections in language, then efficient, automatic representation of sound by the auditory system will not develop. It stands to reason that the evaluation of auditory-processing abilities, in addition to basic hearing thresholds, is an important part of the assessment of children who are having difficulty reading.
Inpairments in frequency modulation, speech perception, and phonological awareness in kindergarteners and first graders predict a dyslexia diagnosis in third grade (Res Dev Disabil 2011;32:560-570 ). Speech perception in children with dyslexia is particularly problematic in noise (Dev Sci 2009;12:732-745 ), and temporal-processing deficits appear to underlie at least some types of dyslexia, with training leading to improvements (Science 1996;271:77-81 ).
One of the mechanisms influencing the development of auditory skills and the quality of sound representation is the trial-to-trial consistency of neural responses. If there is jitter or asynchrony, the response will be degraded, resulting in an absent auditory brainstem response (ABR).
Asynchronous firing may degrade the response to the extent that it interferes with reading. In fact, this effect was demonstrated in a rat model of dyslexia (Cereb Cort 2013; doi:10.1093/cercor/bht028 ).
Rats were genetically engineered to have reduced expression of KIAA0319, since mutations of this gene are associated with dyslexia. The rats had lower trial-to-trial consistency and reduced discrimination of speech sounds, providing evidence that decreased expression of the KIAA0319 gene can lead to impaired phoneme processing in the cerebral cortex.
The importance of neural response consistency was also recently demonstrated in children who had a wide range of reading abilities. Those with good reading scores had higher trial-to-trial consistency in the auditory brainstem response to a speech syllable than did children with poor reading scores (J Neurosci 2013;33:3500-3504 ). Overall, these results provide convincing objective evidence for underlying auditory-processing deficits in at least some children with reading disorders.
This study has important implications. Response consistency—an objective measure of auditory processing that reflects neural synchrony—appears to be an important factor in the development of successful reading. Because response consistency is an ABR measure, it would be feasible to incorporate it into the audiological battery.
But let's take this a step further. Are there efficacious treatments for individuals with biological evidence of auditory-processing disorders?
There is now support for the benefits of using FM systems for improving both reading performance and neural response consistency (Proc Natl Acad Sci U S A 2012;109:16731-16736 ). This and other studies evaluating treatment strategies for children with language-based learning impairments will be discussed in the November issue of The Hearing Journal.