University at Buffalo - The State University of New York

Published: October 24, 2002

By LOIS BAKER
Contributing Editor

Most hearing research and treatment to date has concentrated on the transmitting of auditory signals to the brain—the "sending" end. It is possible, however, for people to have trouble hearing even when the sending mechanism is in fine shape. One new and promising field of hearing research focuses on how and where the brain receives and deciphers certain signals from the auditory nerve.

David Eddins and Ann Clock Eddins, both associate professors in the Department of Communicative Disorders and Sciences—and also husband and wife—are conducting frontline research in this area within the UB Center for Hearing and Deafness.

They work in the field of sensory physics, which is the study of how sound, taste, smell, touch and vision are perceived, and within this larger field, in the subspecialty of psychoacoustics.

Ann Eddins, a specialist in auditory physiology, studies the brain's temporal processing of sound, or how sound varies over milliseconds of time. "There is a lot we don't know about how the auditory system codes time," she says. "Most people are able to process temporal variations in sound, but in hearing loss we think people lose some of this processing ability."

This results in sounds being smeared together, especially if the person is listening in an environment with background noise, Eddins says, which probably contributes to poor understanding of speech in the hearing impaired.

Eddins is studying the question of how the temporal aspects of sound are processed in the brain using several approaches, working with an animal model. On a "global" level, she measures the electrical action created by groups of cells in the brain, called evoked potentials, during sound. This identifies the parts of the brain that are activated. She then measures responses of single neurons in the regions activated to determine which cells respond to sound duration, or to high or low frequency.

One of the theories she is following is that during hearing loss, these cells may lose their sensitivity due to lack of stimulation. "This leads us to address a number of questions," she notes. "How plastic is the brain? Are the cells in the brain being damaged? If we can provide some other type of stimulation, can they recover? Can we stimulate them in a way that will help them respond better?

In another approach to studying the temporal quality of sound, Eddins is conducting Positron Emission Tomography (PET) studies on human volunteers to observe which parts of the brain are active when exposed to auditory signals and what features of sound prompt the brain to shift focus from one part to another.

"We are trying to understand why hearing comes easily when listening to certain aspects of sound, while other aspects are more difficult," she says. "We've found that processing shifts from one side of the brain to the other, depending on whether you are listening globally, such as to general conversation, or locally, such as to a teacher's instructions."

This work may help to explain why a problem student "can't hear," even though a hearing test finds no deficit—there may be a glitch in the central processing.

David Eddins, trained in clinical audiology and experimental psychology, studies how the intensity of a sound varies across different frequencies, a concept called spectral processing.

"The ability of the ear to identify peaks and valleys of sound is very important in identifying the characteristics of sound," he says. "Every sound has a characteristic spectral pattern, which helps in identifying the source of the sound and in telling the difference between sounds, but we don't know how the brain processes this information."

Eddins bases his research on the earlier work of vision researchers. These scientists had shown that the brain breaks down an image into many different parts, then assigns the parts to specific places in the brain's visual center, where specialized cells tuned to certain spatial frequencies become excited and create a neural representation of the image.

"This discovery brought a revolution in visual science," Eddins says. "We think there may be a general mechanism for processing features of all stimuli. Are cells in the brain tuned to certain spatial frequencies for hearing? We have found strong evidence of "tuning," and we think tuning can be explained by the presence of channels—groups of cells devoted to different spatial frequencies of sound. This provides us with a basic understanding of how sound is interpreted in the brain and how this tuning changes with hearing loss.

"We suspect that the evidence we find in this research will completely change the way we think about how the central-auditory system works."