- The Washington Times - Wednesday, January 1, 2003

SEATTLE, Jan. 1 (UPI) — A mysterious characteristic of the brain that causes nerve signals to flutter and weaken in strength has been linked to animals' ability to pinpoint where sounds come from in space, researchers reported Wednesday.

Researchers said the characteristic is found all over the brain and might be involved in intellect, memory and other higher mental functions.

"This could have something to do with how the brain codes rapid calculations (such as) how we see colors, how we talk," researcher Bill Spain, director of neurology at the Seattle Veteran Affairs Medical Center, told United Press International.

"This is very basic to the way the brain functions," Spain said. "The more we understand about how brain cells communicate with each other, the more … we can … recognize what goes wrong in diseases and how to intervene to make them right."

Nerve cells are connected to one another at complicated, specialized junction points called synapses. All synapses exhibit a characteristic called depression, which on the cellular level refers not to the blues but to a drop in the strength of a neural connection after repeated use.

Although this signal weakening might seem like a design flaw that prevents synapses from keeping up with the demands placed on them, "if the nervous system went and designed synapses to work like this, it must be good for something," Spain explained.

As reported in the Jan. 2 issue of the British journal Nature, Spain and colleagues studied synaptic depression along a very well-studied brain circuit, the one used to localize sounds in space.

"When you hear a series of noises whose source you cannot locate — a bird chirping, for example — you automatically turn your head as you listen. This is because you determine the location of the sound by using the tiny differences in the time it takes for the sounds to reach your two ears," neurobiologist Chuck Stevens of the Salk Institute in La Jolla, Calif., explained in an accompanying commentary.

"You turn your head to maximize this difference," Stevens added. "If you are facing the source of a sound, the signal arrives simultaneously at both ears, but if the sound originates from, say, your left side, the sound pressure wave arrives a little earlier at your left ear than at your right."

The maximum difference in the time it takes for each ear to pick up the same sound is less than half a millisecond in humans. Still, nerve cells exist that can detect such slight differences, firing off impulses only if signals arrive at both ears at once. The louder a sound is, however, the more nerves fire, a fact that can overwhelm the sound-pinpointing brain circuitry.

Spain and his team confirmed that synaptic depression helps animals locate where sounds are originating. However, they also discovered the phenomenon operates successfully regardless of how loud the sounds are. The researchers took brain cells from chicken embryos and "stuck little electrodes into the cells," to stimulate them, Spain said.

A computer model that analyzed the cells' responses found synaptic depression helped dampen the overwhelming effect of loud sounds. Without depression, sound localization worsened dramatically.

"People have never really understood what this was good for. Other people have speculated over its uses, but this is the first actual case this was related to something in behavior," Stevens told UPI.

The investigators now hope to see whether synaptic depression plays a similar role in other parts of the brain. They said they also plan to see whether its effects can be measured in living animals.

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(Reported by Charles Choi, UPI Science News, in New York)



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