How noise affects nervous system’s ability to transcribe sounds key to reading skills

noise-induced hearing lossWashington, July 14 : A new study conducted by Northwestern University researchers offers an unparalleled look at how noise affects the nervous system''s ability to transcribe sounds whose subtle differences are key to success with language and reading.

The study suggests that distinguishing such sounds is too much to ask of the nervous system of a subset of poor readers whose hearing is fine, but whose brains have trouble differentiating the "ba," "da" and "ga" sounds in a noisy environment.

"The ''b,'' ''d'' and ''g'' consonants have rapidly changing acoustic information that the nervous system has to resolve to eventually match up sounds with letters on the page," said Nina Kraus, Hugh Knowles Professor of Communication Sciences and Neurobiology and director of Northwestern''s Auditory Neuroscience Laboratory, where the work was performed.

According to the researcher, the brain''s unconscious faulty interpretation of sounds makes a big difference in how words ultimately will be read.

"What your ear hears and what your brain interprets are not the same thing," Kraus said.

This is the first time that any study has shown an unambiguous relationship between reading ability and neural encoding of speech sounds, which previous work has shown present phonological challenges for poor readers.

Published in the online edition of the Proceedings of the National Academy of Sciences (PNAS), the study focuses on what is happening in the brainstem, an evolutionarily ancient part of the brain that scientists in the not too distant past believed simply relayed sensory information from the ear to the cortex.

The researchers focused on the sensory system, and demonstrated that the technology developed during the last decade in the Kraus lab offered a neural metric sensitive enough to pick up how the nervous system represents differences in acoustic sounds in individual subjects, rather than, as in cortical-response studies, in groups of people.

What is significant to note is the fact that this metric reflects the negative influence of background noise on sound encoding in the brain.

"There are numerous reasons for reading problems or for difficulty hearing speech in noisy situations, and we now have a metric that is practically applicable for measuring sound transcription deficits in individual children. Auditory training and reducing background noise in classrooms, our research suggests, may provide significant benefit to poor readers," said Kraus, the senior author of the study.

During the study, the researchers attached electrodes to the scalps of children with good and poor speech-in-noise perception skills, and delivered sounds through earphones to measure the nervous system''s ability to distinguish between "ba," "da" and "ga".

In another part of the study, sentences were presented in increasingly noisy environments, and children were asked to repeat what they heard.

"In essence, the kids were called upon to do what they would do in a classroom, which is to try to understand what the kid next to them is saying while there is a cacophony of sounds, a rustling of papers, a scraping of chairs," Kraus said.

The researcher says that in a typical neural system, there is a clear distinction in how "ba," "da" and "ga" are represented, and the information is more accurately transcribed in good readers and children who are good at extracting speech presented in background noise.

"So if a poor reader is having difficulty making sound-to-meaning associations with the ''ba,'' ''da'' and ''ga'' speech sounds, it will show up in the objective measure we used in our study," Kraus said.

Reflecting the interaction of cognitive and sensory processes, the brainstem response is not voluntary.

"The brainstem response is just what the brain does based on our auditory experience throughout our lives, but especially during development. The way the brain responds to sound will reflect what language you speak, whether you''ve had musical experience and how you have used sounds," Kraus said. (ANI)