If there is one sound that bettered our ancestors' chances of survival, it might be the scream. When a baby needs food, it hollers; if a ravenous lion prowls a little too close, a blood-curdling shriek alerts the tribe. Yet from an acoustic standpoint, screams—and how our brain processes the sound—have been largely overlooked by researchers, until now. A study published in July in Current Biology found that screams are sonically unique in a way that perfectly captures our attention.
By analyzing screams culled from YouTube videos, films and volunteer shriekers, researchers led by neuroscientist David Poeppel, who runs a language-processing laboratory at New York University, found that screams occupy a dedicated position on the auditory spectrum. Specifically, what sets them apart from other human vocalizations is how fast they change in loudness. Normal speech has only slight variations in loudness—changing at a rate of just four to five times per second—whereas screams violently clamor through our vocal cords varying in volume 30 to 150 times per second. This rapid, large variation is too fast to be perceived consciously as volume changes; instead it results in what is called roughness, a certain startling discordance in sound that the human brain associates with fear. To this end, Poeppel and his colleagues used functional MRI to show that increases in roughness raise the activation of the amygdala, the brain's fear and emotion center.
Roughness is what allows screams to outcompete other sounds. Plenty of things are loud—jet engines, for example. Yet a wailing five-year-old tends to cut through the roar and grab our attention. The unique spectral qualities of screams are also thought to reduce the incidence of false alarms from other loud but nonrough sounds. Incidentally, it turns out engineers have—knowingly or not—been taking advantage of roughness for decades: as Poeppel's work shows, most sirens and alarms also oscillate in loudness in the same wide, chaotic frequency pattern as screams do.
Poeppel plans on exploring next whether or not infant screams have the same roughness patterns as adult ones. He also hopes to look for correlates in the animal world. “Screams are arguably the oldest vocalizations,” he says, “so understanding more about their properties illuminates fundamental features of the mind and brain.”