The Hearing Review Cross-Currents are staff-reviewed articles, features and news items that relate to hearing issues from a variety of sources and disciplines. If you are interested in a particular item, we encourage you to obtain a copy of the cited publication.

Hyperacute directional hearing in a microscale auditory system

By AC Mason, ML Oshinsky & RR Hoy

Information obtained from a tiny fly that uses its hearing to home in on crickets is providing hearing scientists with ideas for improving directionality in hearing instruments. The 1-cm long parasitic fly, Ormia ochracea, uses its binaural hearing to find the 5-kHz chirp of a cricket on which it lays its eggs. Its hearing ability—particularly its directional acuity—is no small feat when considering that the fly’s “ears” are only 0.5 mm apart and its body is so small that there is effectively no interaural intensity difference (analogous to our head-shadow effect) caused by the tiny insect’s body. Therefore, the only physical cue that the bug uses is the interaural time difference (ITD)—the minute time differences of sound arriving at one ear compared to sound arriving at the other ear.

Researchers analyzed the movements of female flies that were placed on a treadmill—essentially a ping-pong ball floating on an air stream. The flies walked in response to cricket chirps broadcasted from stereo speakers, and had their movements analyzed by a computer. The researchers found that the O. Ochracea were capable of reacting to stimuli and discerning at extremely small angles from the 0° azimuth, and in fact, could react to a change in speaker location from as little as 1-2° azimuth, which corresponds to the amazingly small ITD of 25-50 nanoseconds.

How do they do it? Each of the fly’s ears contains about 100 receptor neurons which vary in sensitivity to sound levels and have thresholds at 5 kHz of 55-95 dB SPL. The fly’s ears are able to gauge the phase shift of the sound between the two ears and, using the molecular equivalent of a directional microphone to filter out the background noise, home in on the object. The fly’s auditory receptors use a “spike time code” that responds to a pulse of sound and selectively filters out increases in the stimulus. Additionally, its eardrums vibrate in response to differences in the sound intensity arriving at each of the respective ears, which is similar to the mechanism of using interaural differences in stimulus intensity.

“Our results provide a neuroethological analysis of a remarkable behavioural ability that links both applied and theoretical studies of sensory systems,” say the researchers. “These results have significant implications for efforts to develop artificial acoustic devices based on the design principles of O. Ochracea ears. The acoustic orientation behaviour of O. Ochracea demonstrates that such devices have the potential for very high sensitivy and accuracy.” The researchers go on to state, however, that there is probably more to the fly’s movement than only its acoustic receptors (e.g., neural processing, etc.).

The research was conducted at Cornell Univ. and the Univ. of Toronto (Scarborough), and was also reported in the April 7 issue of New Scientist (“A Fly in Your Ear”, p. 25) and the April 30 issue of Business Week (“Guess What’s the New Buzz in Hearing Aids”, p. 108). The research team is now building what is reported to be the smallest directional microphone—said to be one-third the size of today’s smallest microphone—for use in an ITE hearing instrument.

Nature
V 410, April 5, 2001: 686-690


Superior auditory spatial tuning in conductors

By TF Münte, C Kohlmetz, W Nager & E Altenmüller

Did you ever watch a symphony conductor and wonder how in the world he/she could ever single out the one musician who wasn’t hitting the right note? Four German scientists have recently conducted an experiment to explain how, for example, a conductor can identify a specific musician in a multiplayer section.

Seven conductors, seven pianists and seven non-musicians were tested using brain-potential recordings. The subjects listened to pink-noise bursts within central and peripheral loudspeaker arrays arranged in an arch from 0° azimuth to 90° to the right-ear side of the subject. The subjects were asked to identify from which speaker “deviant” sound bursts of increased bandwidth (16% of the time, ranging from 500-15,000 Hz) were eminating as opposed to the typical stimuli of the loudspeakers (84% of the time, 75 dB, 500-5000 Hz and 80 ms in duration).

Event-related brain potentials (ERPs) indicated that, for all groups there was a gradient response when subjects were presented with the “deviant” sounds from the central locations; most subjects could identify the bursts when occurring in front of them. However, for the peripheral sounds, only the conductor group displayed a gradient. In other words, the conductors were able to monitor peripheral sounds as well as centrally located sounds. Additionally, this group had fewer false-positives relative to identification of peripheral sounds. Pianists and non-musicians did not have these same abilities.

The electrical activity in the conductors’ brains also showed a pattern associated with heightened attention to either the central or peripheral sounds, while the other two groups showed attention-related brain activity only to the central sounds. Scalp topography of this attention effect showed that the conductors probably do not use different neural populations. The researchers conclude that learning and conditioning is more likely to be involved: “Improved learning-induced use of spectral cues generated by the head and outer ears, and analysed by the auditory cortex, might underlie the localization advantage experienced by conductors. Although conductors probably employ other mechanisms such as perceptual grouping to identify single musicians, our findings provide another example of how extensive training can shape cognitive processes and their neural underpinnings.

Nature
February 1, 2001: 580


Tune in, tune out

By D Graham-Rowe

Scientists at the SINTEF Research Lab in Trondheim, Norway, have been working on an earpiece called the Personal Active Radio/Audio Terminal (PARAT) to help military personnel hear speech over the sounds of tanks, planes and artillery. According to the author, the device contains a tiny computer that recognizes particular sounds, including the human voice, and then filters out competing signals.

The internal part of the system is sealed to block out as much sound as possible, while the outside microphone picks up sounds from the environment and relays it to a “signal processing circuit that drives a miniature loudspeaker inside the earpiece.” Essentially, the device looks for voice signals by using temporal and frequency cues, while the internal microphone monitors the person’s own voice and keeps the amplified sound at safe levels.

According to the article, “PARAT is smart enough to block out droning, cyclical sounds with components within the vocal frequency range” and has the “ability to home in on a single voice.” Military trials of the system have been conducted and a spin-off company, called NACRE, has been started in Trondheim to develop a product using the PARAT technology for commercial purposes in 2002.

More information on this study is also reported in the February 3 issue of Science News (v. 159, p. 69).

New Scientist
February 17: 21


A light and a dark side

By T Elbert & S Heim

This intriguing one-page article takes on the subject of cortical reorganization and its implications on such things as tinnitus, phantom limb-pain, and even dyslexia and autism.

Auditory, visual and somatosensory perception depend on the spatial arrangement of sensory receptors (i.e., the cochlea, retina and various nerves on the body). The receptors form a map that is imprinted on the cortical sheet. The representation of the receptors on the map are crucial. For example, two nerves on your index finger need to be distinguished from a nerve on the thumb of your same hand, and all of these nerves need to be distinguishable from the areas on your legs,etc. The body does this by mapping these particular regions on the brain.

The representation of these regions, however, can become enlarged. It has been found that the representation of a particular finger can invade the territory of other fingers’ mapping, or the representation of two fingers can become fused. Right-handed violinists, for example, who play constantly for years may have large, expanded cortical representations of the fingers on their left hand that manipulate the strings, as compared to the fingers that move the violin bow (a relatively easier task). It has also been found that these musicians will have overly large representations in their auditory cortex for their particular instrument; the activity in their brain is much greater when they hear their own instrument as opposed to the tones of other instruments. Elbert and Heim call this “the ‘bright side’ of cortical plasticity.”

The ‘dark side’ of cortical plasticity, the authors explain, can occur when the brain reorganizes itself inappropriately due to some form of trauma. For example, people who have had an arm amputated often feel “phantom limb” pain. Because of the non-stimulation of the cortical region that corresponded to the nerves in the arm, nearby zones of the adjacent face and shoulder often invade the cortical map territory of the missing arm. “There is a close association between this invasion and adverse symptoms such as phantom-limb pain and, in the auditory system, tinnitus,” say the authors. “Because treatment designed to reverse this invasion reduces phantom-limb pain, the adverse response is probably an unwanted consequence of the dynamic brain.”

Elbert and Heim explain that the way we perceive the outside world is largely due to the functional organization of the central nervous system. They point out that there are nearly 10 times more neural fibers running down from the representational cortex as there are fibers running to it. This top-down anatomy results in a highly processed, filtered, interactive and organized system of receiving sensory information. Therefore, lesions to the cortical areas can cause changes in functional organization: “After a brain lesion, there are initial deficits in behaviour, perceptual or cognitive skills, but often spontaneous relief of symptoms. Cortical reorganization may be crucial for such recovery.”

They authors point out that new treatment methods in neurorehabilitation are a “bright side” to the organization of the system. However, the ‘dark side’ is that small changes—whether physical lesions or mental traumas—may end up producing large alterations to the cortical organization, resulting in negative or catastrophic consequences. “Fusion of representational zones is at the core of dystonia (such as musician’s cramp or writer’s cramp); Michael Merzenich’s notion that dyslexia and even autism may be a consequence of inadequate functional organization (of the phonemetopic representation) stirs an exciting new avenue of research. And what about extreme experiences, such as traumatic stress or childhood abuse?” ask the authors.

Nature
May 10, 2001, Vol. 411: 139


Hear no EVIL

By David Fetherston

If that muscle car rumbling down your street sounds strangely pleasurable, it may not be a sign (necessarily) that you’re entering a mid-life crisis; rather, it may be the engineering of its muffler system. This Hot Rod article talks, oddly enough, about the the use of electrophysiological responses to sounds, and research being conducted by the California companies of 3R Company, Sebastopol, CA, and Flowmaster, a Santa Rosa, CA, muffler manufacturer.

The exhaust system of a car affects an engine in many ways, including its horsepower and performance. Having mastered much of the flow dynamics of the exhaust from the engine (i.e., the number of turns and metallurgy of the pipes, pressure aspects, etc.), muffler manufacturers have increasingly gone beyond the attenuation characteristics of their products to the actual quality of the sound. “Back in the ‘60s, I did my first experiment with exhaust tone quality by installing a dual system with glass-pack mufflers of different lengths, an 18-inch on one side and a 24-inch on the other. This created two frequencies and produced a two-toned chord which gave a smooth sound with a cruiser warble,” says Ray Flugger of Flowmaster.

Motor sport racers and engineers have also been interested in producing more pleasing exhaust sounds as they try to optimize concentration and reduce distractions for race car drivers. About two years ago, Flugger and Peter Madill, PhD, of 3R Company started developing research protocols for understanding human reactions to the sound of mufflers. Through a proprietary electrophysiological monitoring system, they are recording body responses to the sound of muffled and unmuffled exhaust. With this information, they plan to evolve and refine Flowmaster mufflers.

Additionally, they are seeking to reduce the harmful acoustic components by involving specific frequencies in their products, “eliminating the damaging sounds through sound spectral analysis on the dynamometer.” They believe their work will have broader implications for the race car industry and the hearing conservation of motorsports enthusiasts in general: “A thundering Pro Street car with Flowmaster mufflers may rumble the ground and grandstands, but it won’t hurt your hearing because of the frequency bandwidth it produces,” predicts the article. (Editor’s Note: Well, we’ll see. It’s a little disconcerting that some pages in this article are labeled, “EVIL.”)

The author, with the help of Madill, provides an interesting look at the body’s natural stress response mechanism in response to noise. They relate how we increasingly encounter an array of social and psychological stressors, even though few of these are actually threatening to our survival. Additionally, discordant sounds from our high-tech world add to the level of stress, and excessive aggravation of the stress response can even suppress the immune system.

Hot Rod
June 2001: 58-61.