EVANSTON, Ill. -- A child’s brain has to work overtime in a noisy classroom to do its typical but very important job of distinguishing sounds whose subtle differences are key to success with language and reading.

But that simply 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, according to a new Northwestern University study.

“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.

In other words, 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 stressed.

The Northwestern study is the first to demonstrate an unambiguous relationship between reading ability and neural encoding of speech sounds that previous work has shown present phonological challenges for poor readers.

The research offers an unparalleled look at how noise affects the nervous system’s transcription of three little sounds that mean so much to literacy.

The online version of the study was published July 13 by the Proceedings of the National Academy of Sciences (PNAS) (http://www.pnas.org/papbyrecent.shtml).

The new Northwestern study as well as much of the research that comes out of the Kraus lab 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.

As such, much of the earlier research relating brain transcription errors to poor reading has focused on the cortex -- associated with high-level functions and cognitive processing.

Focusing earlier in the sensory system, the study demonstrates that the technology developed during the last decade in the Kraus lab now offers a neural metric that is 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. Importantly, 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,” said Kraus, the senior author of the study. “Auditory training and reducing background noise in classrooms, our research suggests, may provide significant benefit to poor readers.”