Screaming is primal. It warns other people about hazards, and it keeps parents alert to their babies’ needs. In a noisy world of thunder, dance parties, and police sirens, our shrieks and cries need to stand out for us to survive. And they do. According to a new Current Biology study, human screams activate not only the auditory parts of our brain, but also its fear circuitry.
"If you ask a person on the street what's special about screams, they'll say that they're loud or have a higher pitch," New York University’s David Poeppel says in a statement. "But there's lots of stuff that's loud and there's lots of stuff that's high pitched, so you'd want a scream to be genuinely useful in a communicative context."
Poeppel and colleagues studied recordings taken from YouTube, popular movies, and a few volunteer screamers in a sound booth. When they plotted the sound waves to reflect the firing of auditory neurons, they noticed that screams activate a range of acoustic information that weren’t thought to be important for communication. Singing and speaking in various languages didn’t seem to occupy this chunk of the auditory spectrum. In fact, car alarms and similar signals are the only things that seem to activate this range that’s apparently reserved for screams.
What’s special and so fear-inducing about screams (and alarms) is a property known as roughness – or how fast a sound changes in loudness. Screams modulate insanely quickly, varying between 30 and 150 Hz. The volume difference of normal talking, for comparison, is between 4 and 5 Hz.
Screams with the highest roughness terrified volunteers the most, they said. Sure enough, when the team imaged the brains of participants listening to various sounds, they saw that increases in roughness engaged the amygdala – an almond-shaped brain structure that’s essential to our ability to feel certain emotions and rapidly appraise danger.
A property called roughness gives human screams their fear-inducing quality. Luc Arnal