There's a good reason the soundtrack to all those scary movies includes a good lungful of screaming. Our response to what we hear stems from something far more than a conscious signal of danger. The instinct goes much deeper. Now an international team of researchers has looked at exactly what it is about the scream that has us on the edge of our seats.
"Screaming is arguably one of the most relevant communication signals for survival in humans," say the authors of a paper published this month by the online journal Current Biology. The neuroscientists from the Max Planck Institute for Empirical Aesthetics, the University of Geneva and NYU have outlined the unique value of screams for the first time. They discovered that, rather than being just a high-pitched bawl, a scream possesses very specific acoustic properties.
"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. "But neither turns out to be quite correct. In fact, screams have their own acoustic niche separate from other sounds. While, like some sounds, they may be high-pitched and loud, screams are modulated in a particular way that sets them apart from the rest."
Poeppel and his colleagues discovered a special acoustic trait — called "roughness" — only present in screams. Roughness, they say, is when a certain change of amplitude or frequency is reached. If variations in pitch and/or volume happen very quickly, the ear is no longer able to correctly process the changes — the sounds become unpleasant. "Normal speech has a modulation rate of around 4 to 5 Hz," says Poeppel — that's a cycle of 5 times a second — "but for roughness the rate is between 30 and 150 Hz — the temporal changes are therefore significantly faster."
The researchers created a bank of natural and artificial sounds — this included normal speech, screaming and the sound of alarms. They noticed that both screams and artificial alarm sounds were within the 30-150 Hz range of roughness. Effective alarms rely on the same neurological response caused by a human scream. When the scientists asked a group of subjects to record their own sounds — screams, screamed sentences, normal speech and unscreamed wordless vocalizations — it was found that both screams and screamed sentences occupied the "roughness domain" while the other sounds did not. Another group of subjects were asked to listen to the sounds and rate them for how "alarming" they were. Unsurprisingly, those with the most roughness received the highest ratings.
The researchers then used an advanced kind of MRI scan — fMRI measures neural activity by blood flow — to map how the sounds affect the subjects' brains. Both the screams and the alarm sounds caused increased activity in the amygdala — which, according to neuroscientist Joseph LeDoux is "most commonly associated with fear." Any roughness of the sounds we hear, therefore, results in an increase in an instinctive reaction of fear or alarm. It's not so much the volume of your alarm clock that wakes you up but the particular patterns of volume and pitch that it puts out.
The research appears to show that a human scream is a specific type of vocal expression that is only used in stressful and dangerous situations. Poeppel says that the particular acoustic character of screams "ensures their biological and ultimately social efficiency — we use them only when we need them." Even if that's the need to sell you more popcorn.