It started with the babies.
Multiple postdoctoral students in David Poeppel's speech and language processing lab at New York University recently had become new parents, and they found themselves spending part of their days commiserating about the infant shrieks and cries that were filling their nights.
Soon, those shared sympathies turned into questions about the science behind human screams, which have a way of getting our attention and rattling our emotions in ways other noises simply do not.
"What makes them what they are? Why are they so effective?" said Poeppel, a NYU professor of psychology and neural science and director of the Max Planck Institute for Empirical Aesthetics in Germany. "When you ask someone what a scream is, a person on the street will say they are loud or high-pitched. But there's lots of stuff that's loud or high-pitched. ... It's something that's actually not understood well at all."
To decipher what give screams their fear-inducing quality, researchers analyzed a broad range of human screams from YouTube videos, popular horror films such as Psycho and volunteer screamers who recorded an array of shrieks and shouted sentences (i.e., "It's right behind you!") in a sound booth.
They were surprised to find that human screams occupied a part of the sound landscape previously thought to be irrelevant to communication. Normal speech patterns typically have only slight difference in "modulation rate," between 4 and 5 hertz. Screams, on the other, can fluctuate wildly and rapidly, varying between 30 and 150 hertz. That gives them a trait known as "roughness."
"That's precisely where screams live," said Poeppel, who along with other members of his lab published their findings Thursday in the journal Current Biology. "They occupy a very restricted range of the soundscape."
Researchers tested musical instruments, traffic noises and other sounds to see if anything else displayed similar roughness to human screams. Singers such as Tom Waits and Steven Tyler at times come close. The only other sounds that showed similar modulation were car alarms, house alarms and alarm clocks, whose creators intuitively have figured how to trigger human fear.
To confirm their findings about the unique nature of human screams, the researchers had people listen to a range of sounds while monitoring their brain activity. Human screams and alarms generated increased activity in the amygdala, the region of the brain used for processing and remembering fear. In addition, when researchers asked volunteers to rate screams based on how frightening they were, the ones with the most "roughness" were deemed the most terrifying.
"The rougher a sound is, the scarier people rated it," Poeppel said.
Scientists have long studied the meaning behind primal ways of communicating, particularly the cries of infants. They have developed programs to analyze a baby's cries in efforts to decipher whether they are driven by pain, hunger or some other form of distress. They have studied how parents are able to recognize the cries of their own child in an instant.
Poeppel said understanding more about the precise variations of infant cries eventually might lead to a better understanding about what each one means. But he said the possibilities for further research extend beyond babies. Researchers would like to see whether similar properties exist in animal screams. They would like to study screams associated with positive situations, such as cheering at a sporting event.
In the meantime, he said, Thursday's study helps explain why human screams really work and how they manage to get our attention in a world cluttered with plenty of other loud noises. That emerging science might lead to real-world applications we hadn't yet imagined.
"You could optimize alarm signals. But you can also make scarier movies, scarier soundtracks, scarier YouTube videos," Poeppel said. "You can scare the bejeezus out of everybody."