Scientists have long recognized that children who can best process various aspects of the sounds of language are more likely to read earlier and develop into better readers and writers than those who cannot. After a decade of research, Northwestern Professor Nina Kraus and her colleagues have discovered a subset of learning disabilities that results from a dysfunction in the way the brainstem encodes certain basic sounds of speech.
In an article in the April "Trends in Neurosciences," Kraus, who is Hugh Knowles Professor of Communication Sciences and Neurobiology, and senior research analyst Trent Nicol for the first time ever have linked the source-filter model of acoustics with the cerebral cortex's "what" and "where" pathways via the auditory brainstem.
The research they present in "Trends" represents the theoretical underpinning for BioMAP, the simple neurophysiological test that can identify children with sound processing disorders. Kraus's laboratory, in partnership with Bio Systems Corp., will soon make the diagnostic tool available in the marketplace.
BioMAP objectively measures whether a child's nervous system can accurately translate a sound wave into a brain wave. If it cannot, the affected individual -- like nearly a third of the language-disordered children Kraus has studied -- demonstrates problems in discriminating speech sounds that interfere with normal learning. Once identified, children with these problems will be able to improve their speech discrimination skills through auditory training.
Early in her work -- because the deficits she was exploring related to the complex processes of reading and writing -- Kraus studied how the cortex, the part of the brain responsible for thinking, encoded sound. She and her colleagues now understand that problems associated with the encoding of sound can also occur earlier and lower in the auditory pathway in the brainstem. After analyzing years of data, they have discovered that, when recorded, the brain waves generated at the brainstem level in non-learning disabled children can look almost identical to the sound wave itself. In contrast, the brain waves of language-impaired children look somewhat different from the sound wave, showing evidence of what Kraus calls a "jitter" in the encoding process. In a perfectly functioning system, a given sound will unfailingly induce a neuron to fire a precise number of milliseconds later. In a disordered system, however, the timing of these firings can vary markedly.
"We record the averaged activity of large numbers of neurons," Kraus explains. "If the neurons are not firing when they should, the response gets blurred." She has found a "jitter" in the brainstem's filter-class response (its response to the linguistic content of a sound wave) while its source-class response (its response to the non-linguistic aspects of speech, such as intonation, emotion, pitch and inflection) appears normal.
"What's compelling is that we can actually see the neural response from the brainstem to a given acoustic signal," says Kraus. And they can see it both in terms of the nonlinguistic aspects and linguistic characteristics of sound waves. In contrast, when she was recording cortical waves, Kraus had to infer that the electrical activity measured was linked to the characteristics of sound. Now she can see what the sound wave looks like compared to the brain wave, separating the filter and source response.
With funding from the National Institutes of Health, Kraus pays her young subjects five dollars an hour -- ample compensation for the 8- to 12-year-old youngsters -- for participating in achievement assessment, listening skill activities and, most important, the brain related research.
For the latter, non-invasive electrodes are placed on the subjects' scalps and an earpiece delivers carefully crafted acoustic sounds in one ear. While her subjects contentedly watch a video, Kraus measures the brains' response to these sounds. Brain activity is recorded by monitoring electricity given off by the nerves in the brainstem at a "pre-attentive" level.
"What makes this translate perfectly into a diagnostic tool is the fact that we don't have to ask our subjects to follow any directions or engage them in specific tasks," says Kraus. "We simply measure an automatic function of the nervous system while a child watches TV."
Kraus expects BioMAP to become part of the arsenal of tools used by specialists in learning disabilities. What's more, by linking two basic principles of neuroscience and sensory systems – the acoustic source-filter model with the 'what' and 'where' cortical pathways – she and her team are providing researchers with a new way to think about how the brain processes speech in general and how, in particular, the diagnosis and remediation of learning disorders can be improved.
Journal
Trends in Neurosciences