Hearing: It's a more complex process than we realize

Ellen Covey

Hearing is a far more complicated process than once imagined. But neuro-scientists are beginning to unravel the ways individual brain cells continually perform complex computational tasks to help creatures as diverse as humans, gerbils, bats and birds distinguish what a sound is and where it is coming from.

Individual neurons, or brain cells, do not just relay information from one point to another, according to a group of researchers from across the United States who discussed new insights into the process of hearing at the recent Society for Neuroscience's annual meeting in New Orleans. Instead, they said, each neuron could be compared to a tiny computer that compiles information from many sources and makes a decision based on that information

"In hearing, the brain does not function as one big computer, but rather as a series of small computers working in series and in parallel. Now, for the first time, we are getting a good idea of how individual neurons work as computers," said Ellen Covey, an assistant professor of psychology at the University of Washington and organizer of the symposium.

Other members of the panel were Dan Sanes, associate professor of neural science and biology at New York University; George Pollak, professor of zoology at the University of Texas, and William Spain, associate professor of neurology at the University of Washington.

In New Orleans, the researchers reported on new techniques that for the first time permit them to record and monitor low-level electrical activity in single neurons of awake animals. They also discussed a number of findings showing how neurons analyze and integrate information from different sources.

Understanding the mechanisms of sound recognition in the brain and in single neurons is basic neuroscience that Covey said may permit researchers to design better processors used in hearing aids for the hearing impaired and the totally deaf. The research also has implications for improving sonar devices and creating speech recognition systems for computers.

Here are highlights of what the UW panelists discussed:

While the bat's awake: Covey works with the widely distributed North American big brown bat (Eptesicus fuscus) and reported on the first successful use of a technique utilizing tiny glass electrodes one micron in diameter to record very low-level, sound-evoked electrical activity in single neurons in awake bats.

The auditory system in mammals and birds initially is divided into parallel pathways so different types of information can be extracted from a complex signal, Covey explained. To fully analyze a signal or set of simultaneous signals, the results of the calculations in the different pathways must be integrated. An important center for this activity is a portion of the midbrain called the inferior colliculus, where many auditory pathways converge.

The outputs from some pathways excite the cells on which they terminate, making the cells more likely to respond to a signal. Other signals inhibit cells, making them less likely to respond to a signal. Covey said it is computations that result from the interaction between the excitatory and inhibitory inputs that ultimately tells an animal not only where a sound is coming from but also what the sound is.

Big brown bats echolocate by emitting calls and listening to the echoes reflected from objects in their environment. Echolocation calls, while higher in frequency, possess many of the characteristics of human speech. The bats' auditory pathways are similar to those of humans. Because of these similarities, it is possible that some of the same mechanisms used by bats to process echolocation sounds also are used by humans to process speech signals, she added.

Telling the time of sound: Spain's research involves using chicken embryo cells to study how cells in the brain stem can calculate the spatial location of a sound source based on signals from an animal's two ears that are received microseconds apart.

In order to detect the very small delay in the time of arrival of a sound at both ears, neurons must be able to sense differences in arrival times within 1/2000th of a second, Spain said. Sound detected independently by each ear is turned into an electrical signal and the timing of the electrical signals are checked for coincidence. Spain and his colleagues have begun to investigate how coincidence detection is accomplished inside individual cells. ¶

Joel Schwarz, News and Information