A baby wails upon an airplane’s liftoff, a person shrieks when he stumbles upon something shocking, a kid throws a tantrum because she wants to get her way—people scream in reaction to all kinds of situations.
But exactly why we scream has remained a mystery. Now, new research published in the journal Current Biology suggests that hearing a scream may activate the brain’s fear circuitry, acting as a cautionary signal.
Scream science is a new area of study, so David Poeppel, a professor of psychology and neural science at New York University, and his co-authors collected an array of screams from YouTube, films and 19 volunteer screamers who screamed in a lab sound booth. (This last collection method, by the way, was a highlight for Poeppel, who said he found listening to and judging screams an amusing break from the monotony of lab work.)
The researchers first measured the sound properties of screams versus normal conversation. They measured the scream’s volume and looked at how volunteers responded behaviorally to screams. They then looked at brain images of people listening to screams and saw something they found fascinating—screams weren’t being interpreted by the brain the way normal sounds were.
Normally, your brain takes a sound you hear and delivers it to a section of your brain dedicated to making sense of these sounds: What is the gender of the speaker? Their age? Their tone?
Screams, however, don’t seem to follow that route. Instead, the team discovered that screams are sent from the ear to the amygdala, the brain’s fear processing warehouse, says Poeppel.
“In brain imaging parts of the experiment, screams activate the fear circuitry of the brain,” he says. “The amygdala is a nucleus in the brain especially sensitive to information about fear.” That means screams are inherently considered not just sound but a trigger for heightened awareness.
From these screams, Poeppel and his team mapped “roughness,” an acoustic description for how fast a sound changes in loudness. While normal speech modulates between 4 and 5 Hz in sound variation, screams spike between 30 and 150 Hz. The higher the sound variation, the more terrifying the scream is perceived.
Poeppel and his team had volunteers listen to different alarm sounds and found people responded to alarms with similar variations: The more the alarms varied at higher rates, the more terrifying they were judged to be.
That huge variation in scream roughness is a clue to how our brains process danger sounds, Poeppel says. Screaming serves not only to convey danger but also to induce fear in the listener and heighten awareness for both screamer and listener to respond to their environment.