College of Engineering Magazine- Fall 2001

Sounds Abounding

Professor Herb Voigt Delves into the Neural Connections that Make Hearing Possible

by Taylor McNeil

Think about it and you'll understand the challenge: 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.

Scientific talk about hearing usually focuses on the cochlea, that snail-shaped device in the inner ear that turns vibrations in the air into signals to send on 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 Professor Herb Voigt since he was a graduate student, and it's been the focus of his research at BU for the past twenty 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, unknowns about human and animal hearing still outnumber knowns. That was made clear in a presentation Voigt gave early in the fall semester to entering graduate students on the mechanics of the cochlear nucleus. 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 but in different ways, says Voigt, who is a past chairman of the Department of Biomedical Engineering and currently associate chairman for the ENG biomedical engineering undergraduate program. But it's the dorsal cochlear nucleus that he has focused on. "It turns out that it's the most complicated of the three subnuclei," he says.

To study the dorsal cochlear nucleus, Voigt chose gerbils, primarily because they are very good low-frequency hearers. "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, Voigt 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 our pinnae, or external ear. "The hypothesis is that the spectral notch is telling us where the sound source is. 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. 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, largely because the experiments are so difficult. But that hasn't deterred Voigt, whose former Ph.D. students, Kenneth Hancock ('92, '01), now a post-doctoral fellow at Harvard, and Jiang Ding ('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 thirty years later got a cochlear implant. "She found out I was using the book and e-mailed me. I didn't respond for twenty-four hours, so she called me," Voigt says. "I'm speaking with her through her cochlear implant - it was really amazing for me, to really 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." For people whose auditory nerves are damaged by tumors the implant pro- vides a way to process sound, while bypassing the cochlea. Although rough and rudimentary, they too may someday be developed to the point where they become more commonplace. And that, Voigt says, is what his research is about: doing basic research that can be applied to human lives.


Sounds Abounding Professor Herb Voigt Delves into the Neural Connections that Make Hearing Possible by Taylor McNeil Think about it and you'll understand the challenge: 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. Scientific talk about hearing usually focuses on the cochlea, that snail-shaped device in the inner ear that turns vibrations in the air into signals to send on 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 Professor Herb Voigt since he was a graduate student, and it's been the focus of his research at BU for the past twenty 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, unknowns about human and animal hearing still outnumber knowns. That was made clear in a presentation Voigt gave early in the fall semester to entering graduate students on the mechanics of the cochlear nucleus. 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 but in different ways, says Voigt, who is a past chairman of the Department of Biomedical Engineering and currently associate chairman for the ENG biomedical engineering undergraduate program. But it's the dorsal cochlear nucleus that he has focused on. "It turns out that it's the most complicated of the three subnuclei," he says. To study the dorsal cochlear nucleus, Voigt chose gerbils, primarily because they are very good low-frequency hearers. "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, Voigt 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 our pinnae, or external ear. "The hypothesis is that the spectral notch is telling us where the sound source is. 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. 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, largely because the experiments are so difficult. But that hasn't deterred Voigt, whose former Ph.D. students, Kenneth Hancock ('92, '01), now a post-doctoral fellow at Harvard, and Jiang Ding ('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 thirty years later got a cochlear implant. "She found out I was using the book and e-mailed me. I didn't respond for twenty-four hours, so she called me," Voigt says. "I'm speaking with her through her cochlear implant - it was really amazing for me, to really 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." For people whose auditory nerves are damaged by tumors the implant pro- vides a way to process sound, while bypassing the cochlea. Although rough and rudimentary, they too may someday be developed to the point where they become more commonplace. And that, Voigt says, is what his research is about: doing basic research that can be applied to human lives.
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