Screams can also be key to survival.
“Everybody screams and everybody has an intuition about what constitutes screams – that they are loud and high-pitched”, says David Poeppel, the paper’s senior author and a professor in NYU’s Department of Psychology and Center for Neural Science. For example, incorporating more roughness into a burglar alarm could make it more attention-grabbing.
Researches in the study compiled screams from YouTube videos, popular films and volunteers and looked at the firing of brain cells that process sound. “The recording part was a lot of fun”, Arnal recalled. The scientists were looking for a quality in screams and screamed phrases that sets them apart from other loud or high-pitched noises. This space is well distinguished from normal speech (4 and 5 Hz) across languages, thereby making it particularly hard to ignore or confuse with other communication signals. The researchers measure a low level of roughness in English, French and Mandarin sentences that were spoken normally, and also in a cappella singing. Where speakers had slow, moderate modulations, screamers modulations measured somewhere between 30 and 150 hertz.
“We know that the feature of roughness in sounds is annoying, and it has been associated with musical dissonance”, said Oxenham, who wasn’t involved with the new research. (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.). If that isn’t clear, he added, then think of roughness like a strobe light. Normal speech isn’t very rough, but vehicle and house alarms are, the researchers found. Smoothing out the rough signals in screams led people to say the screams weren’t as scary, the researchers found. For example, alarms and movie shrieks get our attention in the same way that human screams do. In addition, notoriously dissonant pairs of notes such as the so-called “devil’s interval” also contained screamlike rough frequencies. 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.
Fear or the feeling of being alarmed is triggered in the amygdala by this roughness, and study subjects responded more to screams than to loud buzzers or alarms.
As for why humans scream in the first place, Poeppel says the behavior probably started with babies; their cries’ ability to activate their parents’ amygdala likely helped the infants survive, notes NPR. “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 vocalisation”.
In a series of several studies, researchers discovered a special acoustic trait only exhibited by screams. “How does roughness in human vocalizations compare to those of other animals?”
The fact that screams trigger a reaction in the brain’s fear centre suggests that the scream itself has evolved to match the brain’s structure. Unlike other vocalizations, screams contain an acoustical signature that alerts the brain to danger, scientists report July 16 in Current Biology.