Lisa Potts, Ph.D., research instructor in the Department of Otolaryngology and Instructor for the Program in Audiology and Communication Sciences at Washington University School of Medicine, has been studying how sound is processed by the hearing nerve. She recently completed her 2009 AHRF research project titled, “Speech-evoked auditory brainstem responses in normal-hearing and hearing-impaired adults with and without amplification.”
Potts explains that before we detect hearing any sound, it is already causing neurons to fire and electrical signals are being carried to the brain. Scientists can measure these electrical signals using a process called auditory brainstem response (ABR). This process measures the transmission of sound signals from the auditory nerve exiting the inner ear to the brain, where these signals are translated and perceived as sound.
When this test is done clinically, patients listen to clicks. But now, researchers are exploring the use of more lifelike sounds, including speech sounds such as “da” as the stimulus. This new technique is called speech-evoked ABR, and it was developed by Nina Kraus, Ph.D., Professor of Speech, Communication Sciences and Disorders; Neurobiology and Physiology at Northwestern University, and a member of the AHRF’s research committee. The software associated with speech-evoked ABR is called BioMARK (Biological Marker of Auditory Processing). BioMark has been used primarily in normal-hearing people, most often children with learning impairments. Potts and colleagues used BioMARK to evaluate 30 adult patients with and without hearing loss.
Potts found that louder intensities of sound stimulus produced more distinct responses, for both normal-hearing and hearing-impaired individuals.
Potts also found that age may have an effect on the response, with older individuals having less robust responses. People taking part in the study were aged 42 to 73 years old.
Potts also measured the responses of hearing-impaired individuals who use hearing aids. These measures showed variability between the aided and unaided responses. This difference is reflective of the change in the stimulus caused by amplification from the hearing aid. This finding is important because Kraus has shown that the neural response mimics the stimulus. Therefore a change in the stimulus loudness level due to amplification with a hearing aid would change the neural response, as Potts’ study confirmed.