VOA慢速英語：尖叫聲在大腦中會有特殊的位置

From VOA Learning English, this is the Health &Lifestyle report.

Have you ever thought about screaming?

A blood-curdling scream is a natural reaction to sudden, extreme fright. When people hear a truly disturbing scream they usually respond … quickly. But why are screams so useful in warning us of danger?

New research helps to explain why screaming is disturbing and useful.

Screams, like those we hear in horror movies, have a special quality that separates them from other noises we make and hear. These screams are recognized by people all over the world. People of all cultures and languages hear the same thing in a scream: fear.

This is a vintage American horror scene. Girls scream at a "haunted mansion" in suburban Sacramento, California, when a Halloween "monster" scares them, October 30, 1979. (AP Photo/RCP)

"Every kid in every culture screams. Every adult in the context of a true fear response screams. So it's genuinely a feature of the human mind and brain."

David Poeppel is a neuroscientist at New York University. He wondered why screams are recognized the same way by people all around the world. So, he and his colleagues set up an experiment.

They recorded screams from movies and from volunteers who took part in the research. The scientists, however, did not measure the screams for loudness or pitch. Instead, they measure how quickly the sounds in the scream changed in volume.

It was in this area -- the change in volume -- where screams stand apart from other sounds.

The researchers found that screams occupy a special part of the acoustic, or sound, spectrum that other sounds do not. Professor Poeppel says screams have a reserved place, a niche.

"When we analyzed all the screams using these more novel techniques, it turned out that screams occupy this unique niche in the acoustic spectrum that's actually not used by other sounds for any other communicative function. So they really have a reserved place."

How we measure sound

Sounds are described in terms of their frequency. Frequency is measured in hertz (Hz.) A person who has normal hearing can hear sounds that have frequencies between 20 and 20,000 Hz. The most important sounds we hear every day are in the 250 to 6,000 Hz range.

Normal speech changes in volume at a low rate – about 4 to 5 Hertz or cycles per second. Screams, however, change in volume very quickly and very widely -- from 30 Hz up to 150 Hz.

When the volume of a sound changes that quickly it has a quality called “roughness.” The more roughness a sound has, the more worrying it is.

David Poeppel and his team found that caralarms, sirens, and alarm clocks also have this quality, this "roughness.”

"Car alarms, sirens, annoying alarm clocks, and so on. Sound designers happened to come across exactly this design feature without actually even trying."

The scientists then studied how this "roughness" changed brain activity. They asked the volunteers to listen to different types of screams and alarms in an MRI scanner. The researchers found that the greater "roughness" of a sound, the more it activates the amygdala. The amygdala is an area deep in the brain that answers to fear.

"The amygdala acts like a gauge that says 'wow, this sound has a lot of roughness in it, that's particularly alarming and scary.'"

Screams, it turns out, are a direct link to the part of our brain that tells us if we should be afraid or not. People who hear these rough sounds are also more likely to react to them very quickly.

Professor Harold Gouzoules teaches psychology at Emory University in the United States. He studies screams in animals and humans. Mr. Gouzoules says screams played a very important evolutionary role in our survival.

"The properties of sounds that evolved to serve in communication are tailored to fit their needs. A scream isn't really so much to announce 'Hey, watch out, there's danger around here.' We have language. We can be far more precise in specifying our danger. A scream is to say 'I'm in trouble, I need help, and I'm over here.'"

Now, we know why a scream …

… gets so much attention, so quickly.

Mr. Poeppel and his team wrote their findings in the journal Current Biology.

And that’s the Health & Lifestyle report for VOA Learning English.

I’m Anna Matteo.

Maia Pujara reported this story from Washington, D.C. Anna Matteo wrote it for Learning English. Caty Weaver edited it.

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Words in This Story

bloodcurdling– adj. causing great horror or fear : this is a very common adjective used to describe screams

nightmare – ­n. a dream that frightens a sleeping person : a very bad dream : a very bad or frightening experience or situation

pitch – ­n. the property of a sound and especially a musical tone that is determined by the frequency of the waves producing it : highness or lownessof sound

vibration – ­n. a continuous slight shaking movement : a series of small, fast movements back and forth or from side to side

frequency– [count] technical the number times that something (such as a sound wave or radio wave) is repeated in a period of time (such as a second)

Hertz– [count] technical a unit used for measuring the frequency of sound waves — abbreviation Hz

evolutionary– adj. of, relating to, or in accordance with a theory of evolution, especially in biology.