It's almost impossible to ignore a screaming baby. (Click here if you doubt that.) And now scientists think they know why.
"Screams occupy their own little patch of the soundscape that doesn't seem to be used for other things," says David Poeppel, a professor of psychology and neuroscience at New York University and director of the Department of Neuroscience at the Max Planck Institute in Frankfurt.
And when people hear the unique sound characteristics of a scream — from a baby or anyone else — it triggers fear circuits in the brain, Poeppel and a team of researchers report in Cell Biology. The team also found that certain artificial sounds, like alarms, trigger the same circuits. "That's why you want to throw your alarm clock on the floor," Poeppel says.
The researchers in Poeppel's lab decided to study screams in part because they are a primal form of communication found in every culture. And there was another reason.
"Many of the postdocs in my lab are in the middle of having kids and, of course, screams are very much on their mind," Poeppel says. "So it made perfect sense for them to be obsessed with this topic."
The team started by trying to figure out "what makes a scream a scream," Poeppel says. Answering that question required creating a large database of recorded screams — from movies, from the Internet and from volunteers who agreed to step into a sound booth.
A careful analysis of these screams found that they're not like any other sound that people make, including other loud, high-pitched vocalizations. The difference is something called the amplitude modulation rate, which is how often the loudness of a sound changes.
When someone is talking, the modulation rate is about four or five changes a second. But when someone is screaming, it can jump to more than 100 changes a second.
That gives the sound an acoustic quality called roughness.
So we hear the audio of the "rougher voice" clip you see below, instead of the high, loud, "smoother voice" clip below that one.
Screams are the only human sound with this sort of roughness, Poeppel says. But "car alarms, house alarms, sirens work exactly on the same principle and they have this roughness modulation," he says.
To find out how these sounds produce such an immediate and intense reaction, the team used functional MRI to monitor the brains of people as they listened to a variety of sounds, including screams.
The experiment showed that screams had a unique ability to cause activity in the amygdala, "the circuitry associated with generating fear responses," Poeppel says. And screams that had been rated as especially scary by listeners produced the highest levels of activation.
So why have humans have evolved to scream? Part of the answer probably involves babies, Poeppel says. He suspects that babies may be more likely to survive if they can instantly activate the amygdala of a parent or caregiver.