Whether it comes from your TV or a deserted alleyway, there is no mistaking a scream of terror. Now, scientists have discovered one reason a fearful scream is so effective at making our hair stand on end: It has auditory qualities that no other human vocalization shares. These unique properties may explain how our brains recognize and react to a scream so quickly, and could inform the generation of new alarm systems.
Few people have studied human or animal screams of terror in the past. “There’s been so much attention paid to understanding speech and song that we’ve really ignored this much more innate vocalization until now,” says Tecumseh Fitch, an evolutionary biologist and cognitive scientist at the University of Austria, who was not involved in the work. The results of the new human scream analysis could shed light on how some of the earliest forms of communication evolved, he adds.
To scrutinize the qualities of humanmade noises, from everyday conversation or public speeches to operas, scientists have typically relied on a few conventional ways of visualizing the sounds. They can graph the pressure of soundwaves, or they can plot the frequencies of these soundwaves over time. But the only thing these approaches reveal about screams is that they are louder and higher pitched than normal speech, says neuroscientist David Poeppel of New York University. “And lots of other things are loud and high-pitched.”
Poeppel and his colleagues decided to explore screams in more detail, turning to a third level of analysis that has only recently been applied to sounds. Called a modulation power spectrum, it charts how quickly the volume of a sound changes over tiny amounts of time. The scientists applied the analysis to both normal speech and screams of fear or terror collected from movies, YouTube videos, and volunteers in the lab. Whereas typical speech changes less than 5 hertz per second—meaning it stays around the same volume—the loudness of screams quickly fluctuates by anywhere from 30 to 150 hertz per second, Poeppel’s group discovered. The fluctuations give the sound a quality dubbed “roughness” that isn’t found in any other human speech—whether male, female, or child.
“It turns out that screams occupy a part of the soundscape that had previously been assumed to be irrelevant to human communication,” Poeppel says. So why are screams so scary? The group asked volunteers to listen to a variety of the terror screams and sentences, some modulated to be more or less rough than usual. The rougher the sound—or the larger the variation in volume within a scream—the more fearful-sounding the volunteers ranked it, the scientists report online today in Current Biology. Furthermore, when the team observed the brains of 16 participants with functional MRI as they listened to vocalizations, screams that were rougher more effectively activated the amygdala, where the brain’s fear circuits live. Most other sounds, by comparison, initially activate only the auditory cortex of the brain.
“If you ask someone which [direction] a sound came from, they’ll be faster and much more accurate if it’s a scream,” Poeppel says. “Now, we think that’s because the brain is uniquely tuned to screams.”
Alarmmakers, Poeppel says, may have already discovered this fast path into the brain. House and car alarms, for example, rank high on the roughness scale, he found. “I think they accidentally stumbled on this without knowing it when they asked ‘What sound can we produce that’s super annoying and hard to miss?’” But the new knowledge on roughness could lead to more effective alarms, automated systems that can detect when someone is truly in distress, or better screams in movies, he says.
Fitch says the findings bring up future questions about whether calls of distress and alarm by nonhuman primates and other animals have the same properties, which could shed light on how the brain evolved to link the sound of a scream to fear. “They’ve made a nice start to understanding this and I think it could be a great way going forward to learn more about the neural basis of emotion.”
Poeppel adds that his group would like to study not only other species, but other types of screams—such as screams of passion or rallying cries at sporting events—as well as the tell-tale screams of human infants to find out whether they have the same properties and activate the brain in the same way.