Brainstem Response Testing Lends Insights Into Brain's Speech Processing

Newswise — A synchronized response of brainstem neurons to sounds seems to play a critical role in the brain's ability to "hear" speech, suggests a report in the September/October issue of Ear and Hearing, the Official journal of the American Auditory Society.

Assembling more than a decade's worth of research by the Auditory Neuroscience Laboratory at Northwestern University, Evanston, Ill., the article discusses a direct link between the science of speech acoustics and the way speech sounds are processed by the brain. In addition, a test showing the absence of the normal synchronized brainstem response to speech could prove useful in identifying children with learning problems.

Under the direction of Dr. Nina Kraus, the researchers study the way the brainstem, or "primitive brain," responds to specific types of sounds. Recording the brainstem's response to sounds has long been used to assess transmission of acoustic stimuli through the nervous system. Traditionally, auditory brainstem responses are generated, or "evoked," using simple tones or clicks.

Dr. Kraus and colleagues have developed a technique that measures responses to speech-like sounds—specifically, the beginning part of the syllable "da." The results show that the neural responses mirror the acoustic characteristics of speech-like sounds with "remarkable fidelity." This highly synchronized pattern of responses plays a critical role in the individual's ability to understand speech content.

Studies also suggest that the brainstem separately encodes the "source" and "filter" characteristics of the acoustic signal, which largely reflect the "paralinguistic" and "linguistic" information conveyed in speech. When the acoustic signals are presented at faster rates, or in background noise, there is an increasingly large gap between the two types of responses.

What's more, studies show a "selective deficiency" in the way the brainstem processes the "filter" characteristics of speech in children with certain types of learning problems—many of whom have abnormal processing of rapid speech-like sounds. The same children have no abnormality in processing of the "source" characteristics of speech. This strengthens the idea that the two processes are separate and that abnormalities in "filter" processing are linked to learning problems.

Growing up in a musical and bilingual home, Professor Kraus was fascinated by the neurobiological foundations of speech and music. Together with Lab Manager Trent Nicol, Dr. Kraus founded the Auditory Neuroscience Laboratory in 1990. Along with Dr. Steve Zecker, Associate Professor of Learning Disabilities, and Dr. Ann Bradlow, Associate Professor of Linguistics, they have investigated speech sound processing by the brain in both normal and learning-impaired children.

Doctoral student Krista Johnson is lead author of the new article, which summarizes a decade of research. The investigators communicate their model for speech sound encoding in the brainstem, presenting the supporting data in its entirety for the first time.

"The new article summarizes a unique body of research data, with clear clinical applications," comments Dr. Mario A. Svirsky, Ph.D., Editor-in-Chief of Ear and Hearing. "The test developed by Dr. Kraus and her group not only offers new understanding of the way the brain processes speech, but may help to identify children at risk of learning problems."

About the Auditory Neuroscience LaboratoryThe Auditory Neuroscience Laboratory at Northwestern University was founded in 1990 by Nina Kraus, Hugh Knowles Professor. Together with her colleagues, staff and graduate students, Kraus investigates the neural encoding of complex sounds such as speech and music as well as learning-associated neural plasticity in normal listeners and a variety of clinical populations. For information on the Auditory Neuroscience Laboratory, visit the Web site at http://www.communication.northwestern.edu/brainvolts/ . A list of publications related to the BioMAP (a clinical technology resulting from the work discussed in this article) can be found through the link to "Clinical Technologies."

About Ear and HearingFrom the basic science of hearing to auditory electrophysiology to amplification and the psychological factors of hearing loss, Ear and Hearing covers all aspects of auditory disorders. Consolidating the various factors that contribute to identification, remediation, and audiologic rehabilitation, it is the one journal that serves the diverse interest of all members of this professional community—otologists, educators, and those involved in the design, manufacture, and distribution of amplification systems. The original articles published in the journal focus on assessment, diagnosis, and management of auditory disorders. Ear and Hearing is The Official Journal of the American Auditory Society.