Screaming grabs your attention.
The "acoustic roughness" of screaming selectively activates the amygdala, involved in danger processing, concludes the study. A rough sound is not how people typically describe a scream, though.
"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," says study senior author David Poeppel, PhD, head of the speech and language processing lab at New York University.
"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."
Prof. Poeppel, also director of the Frankfurt Max-Planck-Institute Department of Neuroscience in Munich, Germany, adds:
"We found that screams occupy a reserved chunk of the auditory spectrum, but we wanted to go through a whole bunch of sounds to verify that this area is unique to screams."
There was no repository of human screams, the researchers found, so used recordings from YouTube and popular films, as well as volunteer screamers in the lab's sound booth.
The team plotted the sound waves "in a manner that reflects the firing of auditory neurons," and they noticed that screams activate a range of acoustic information that scientists had not considered to be important for communication.
"In a series of experiments," explains Prof. Poeppel about the uniqueness of the sound, "we saw [that] this observation remained true when we compared screaming to singing and speaking, even across different languages.
"The only exception - and what was peculiar and cool - is that alarm signals (car alarms, house alarms, etc.) also activate the range set aside for screams."
"Screaming really works," Prof. Poeppel says. He elaborates:
"It is one of the earliest sounds that everyone makes - it's found across cultures and ages - so we thought maybe this is a way to gain some interesting insights as to what brains have in common with respect to vocalization."
Roughness refers to how fast the sound changes in loudness. Normal speech patterns only have slight differences in loudness (between 4 and 5 Hz), but screams can modulate very fast (varying between 30 and 150 Hz).
Prof. Poeppel's postdoctoral researcher Luc Arnal, PhD, now at the University of Geneva in Switzerland, asked people to judge screams on how frightening they were. Those with the highest roughness came across as the most terrifying.
Modifying sound waves of nonscream sounds to be rougher also made them scream-like, and the researchers confirmed that increases in roughness corresponded with greater activation of the human amygdala - the fear response.
"These findings suggest that the design of alarm signals can be further improved," Dr. Arnal says. "The same way a bad smell is added to natural gas to make it easily detectable; adding roughness to alarm sounds may improve and accelerate their processing."
The researchers now plan to investigate infants' screams in particular, to see if they are particularly rough. The team would also like to apply their work to animal screams.