Human screams possess a unique acoustic signature that activates the brain's fear centre and is used exclusively to signal danger or distress, a new study has found.
"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," said study senior author David Poeppel, who heads a 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," said Poeppel.
Humans make a variety of meaningful noises. Part of what makes us human is how our ears can distinguish speech patterns made from vowels and consonants, which is a step above being able to identify whether a sound is made by a male or female, or by our species or another species.
Where in the brain we process this information is known, but there was one area that scientists assumed didn't have much to do with human communication. This is where screams come in.
Poeppel's post doc Luc Arnal, now at the University of Geneva, led a series of studies to analyse the properties of screams.
Because there is no repository of human screams, the researchers used recordings taken from YouTube videos, popular films, and volunteer screamers, who were asked to give their all in the lab's sound booth.
They 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 hadn't considered to be important for communication.
"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," said Poeppel.
"In a series of experiments, 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," said Poeppel.
What sets screams and alarms apart from other sounds is that they have a property called roughness, which refers to how fast a 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).
When Arnal and his team asked people to judge screams on how frightening they were, those with the highest roughness came across as the most terrifying.
Modifying the sound wave of a non-scream to be rougher can also make it scream-like. The researchers then confirmed that increases in roughness correspond to more activation of the fear response in the human amygdala.
The study was published in the journal Current Biology.
