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Sounds abounding
ENG professor explores the neural connections that make hearing possible

By Taylor McNeil

How do we translate vibrations in the air into not only words with meaning, music with emotional content, and sounds signifying danger, but even a sense of location?

Herb Voigt, an ENG professor and associate chairman for the ENG biomedical engineering undergraduate program. Photo by Kalman Zabarsky

Scientific talk about hearing usually focuses on the cochlea, the snail-shaped device in the inner ear that turns vibrations in the air into signals sent to the brain. But the back-office job of hearing -- when the processing really begins -- occurs as the signals sent down the auditory fiber reach the brain.
That has fascinated Herb Voigt, an ENG professor and associate chairman for the ENG biomedical engineering undergraduate program, since he was a graduate student, and it's been the focus of his research at BU for the past 20 years. Voigt and his colleagues -- and that often means his graduate students, he's quick to point out -- have slowly built up, often one neuron at a time, their knowledge of the dorsal cochlear nucleus, the part of the brain that first receives input from auditory nerve fibers.

Despite years of work by many researchers, the unknowns about human and animal hearing still outnumber the knowns. That was made clear in a presentation on the mechanics of the cochlear nucleus that Voigt gave early in the fall semester to entering graduate students. One slide after another highlighted areas of inquiry, posing more questions than answers.

How it seems to work
Auditory nerve fibers terminate within the cochlear nucleus and two other subnuclei in the brain. All three subnuclei apparently process the same information, says Voigt, but in different ways. But it's the dorsal cochlear nucleus that he has focused on -- "the most complicated of the three subnuclei," he says.

To study it, Voigt chose gerbils, primarily because they have very good low-frequency hearing. "Like humans, they can hear down to about 100 Hertz and lower, which is very unusual for a rodent of this size. They live in very quiet places, such as the Mongolian desert, thought to be one of the quietest places on earth. So they have evolved a very exquisite sensitivity to sounds, probably to avoid predation."

Using painless methods, the researchers insert a probe with a micron-sized tip into a gerbil's dorsal cochlear nucleus, directly into an individual neuron. They record the effects of different sounds on an individual neuron, then stain the neuron in question. The stained neuron is later recovered during dissection and its structure mapped and linked with its auditory response.

The undertaking is an integral part of a research project funded by a five-year, $1.1 million National Institutes of Health grant, with Voigt as principal investigator, to identify the relationship between structure and function in cochlear nucleus neurons.

Already some new conclusions are in. "One major hypothesis had been that a specific shape of a neuron would dictate the function of that neuron," Voigt says. But conventional wisdom was wrong; a single cell type seems to be capable of different responses. "That was a surprise to me," Voigt says. "But it may be that we're just not discriminating well enough in terms of describing the cells that we have. There may be subtle differences in the physiology we record. It's been very interesting, very tricky."

One area Voigt focuses on is the role of the dorsal cochlear nucleus in sound localization -- that is, not the localization of sound on a horizontal plane (whether the sound is coming from the left or right), but the location of sound in the medial plane. To illustrate his point, he snaps his fingers directly in front of his head. "The sound reaches the ears at the same time, and because of the symmetry of the head, the sound levels are more or less identical, so you don't have those two cues for localizing sounds. Yet, we're able to localize sounds." There seems to be a spectral notch in sounds, a frequency where energy is pulled out of the signal, highlighting information, as it were, that is created by the pinnae, or external ear. "The hypothesis is that the spectral notch is telling us where the sound source is," Voigt says. "We may have to learn to associate specific spectral notches with position as we grow up, because a priori, how would we know what the notch refers to?"

The research is often painstaking. Largely because the experiments are so difficult, ENG's Auditory Physiology Laboratory, with Voigt at the helm, is one of a handful of laboratories working on in vivo auditory intracellular recordings experiments. But that hasn't deterred Voigt, whose former Ph.D. students Kenneth Hancock (ENG'92,'01), now a postdoctoral fellow at Harvard, and Jiang Ding (ENG'97), a principal scientist at Guidant Corporation, have become experts in the technique. The goal -- to develop a model of the human auditory system -- requires the effort.

Voigt cites cochlear implants as an example of where basic research can lead. "Cochlear implants are arguably the most successful neural prosthetic device developed -- there are now 40,000 people in the world with them," he says. In an introduction to his engineering class The Auditory System and Hearing Prosthetics, Voigt has students read Wired for Sound: A Journey into Hearing by Beverly Biderman, who as a teenager gradually became profoundly deaf, and 30 years later got a cochlear implant. "She found out
I was using the book and e-mailed me. I didn't respond for 24 hours, so she called me," he says. "I'm speaking with her through her cochlear implant -- it was amazing for me, to know that the technology was really working for this individual."

The next step is to perfect a cochlear nucleus implant. "There are probably a couple of dozen people in the United States right now with these implants, which are fairly primitive, with a limited number of channels," Voigt says. For people whose auditory nerves are damaged by tumors, the implant provides a way to process sound while bypassing the cochlea. Although rough and rudimentary, it too may someday be developed to the point where it becomes more commonplace.

And that, Voigt says, is what his work is about: doing basic research that can be applied to human lives.

25 January 2002
Boston University
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