The attention-getting quality of a scream explains why it's helped us survive, as infants and in our early evolution, according to a new study.
Neuroscientist David Poeppel at New York University says the research was partly inspired by babies. His colleagues Luc Arnal and Adeen Flinker each had newborns and friends with newborns, and Poepell says, "they kept talking about how their brains were being hijacked by screams, so they started looking into this with a personal fascination."
More well-defined scream
Aside from this personal fascination, the scientists wanted to learn what sets screams apart from other sounds we make, other than being louder and higher-pitched. So they used a relatively new approach called the "Modulation Power Spectrum," which defines sounds based on their rate of change in volume instead.
While human language changes in volume at a low rate of 4 to 5 Hertz or cycles per second, they found that screams have a more rapid modulation rate ranging from 30 up to 150 Hertz. Researchers considered this acoustic range a vast desert, useless for bearing any fruits of human communication. But, Poeppel says, it actually turns out to be a special space that screams occupy for getting others' attention "specifically, efficiently, and rapidly."
The higher the rate of change in a sound, the greater its so-called “roughness” quality. Poeppel compares roughness to "a stroboscopic effect - like when you're at a club and the lights are flashing at very high rates” to the point of becoming unpleasant.
His team also found that manufactured alarms have this same quality, which explains for the first time why they are so good at getting us to turn our heads or wake up in the morning. The more roughness there is in a sound, the scarier it is.
When the researchers looked at people's brain activity as they listened to screams and alarms in an MRI scanner, they saw that the amygdala, an area deep in the brain which typically responds to fear, was being "hijacked" by these rough sounds. That is, it became more active for sounds with more roughness.
Screams can say even more
By identifying this roughness quality in screams and calling attention to how they affect our brains, Poeppel and colleagues have unleashed a new way of thinking about these jarring sounds.
Professor Harold Gouzoules studies screams at Emory University and suggests their paper, published in Current Biology, helps explain the role screams have played in our evolution.
"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.'"
As Poeppel explains, if we were screaming at each other all the time, like patients who express "verbal disruptive behaviors," these alarm signals would lose all meaning. He asks, “what if you then actually need to use the scream for a real alarm signal?”
The answer is, we would be in big trouble.