Chances are strong that the first thing you ever did was scream, and you probably still scream today when fear strikes your heart or anger overtakes your brain. It is, after all, a pretty good way to get attention. Now, a team of scientists think they know why: an audio effect called roughness, which seems to set off alarms in parts of the brain thought to be responsible for assessing danger.
Screaming serves a pretty obvious purpose: alerting others to immediate danger or an extreme emotion. (If you're a baby, you might include hunger, discomfort, or dirty diapers, too.) Yet even figuring out what constitutes a scream remains unclear. Dictionaries describe a scream as a "loud, high-pitched" utterance, while previous research suggests screams are defined more precisely by jitter or shimmer, acoustic properties that correspond to modulations in pitch and volume. (To get a sense of what jitter sounds like, imagine singing at a constant volume, but varying the pitch rapidly between two notes. Shimmer is similar, except it's volume, not pitch, that varies.)
On the other hand, neuroscientists Luc Arnal, David Poeppel, and colleagues write today in Current Biology, "whether such dynamics and parameters correspond to a specific acoustic regime and how such sounds impact receivers' brains remain unclear." It was their goal, then, to figure it out.
"The rougher the screams, the more fearful the induced emotional reaction."
First, the researchers had 19 people record two utterances, either the vowel "a" or the sentence, "Oh my God, help me," in both neutral and screaming tones of voice. Normal speech, the team found, had relatively slow variations in pitch and volume, while the fluctuations in a scream lay in a domain termed "roughness." Imagine varying your voice's pitch or volume 30 to 150 times per second, and you get some sense of why it's called roughness—and why screams sound rough.
Intriguingly, roughness appears to be a universal feature of screams, but it isn't unique to them. A review of sounds culled from YouTube, movies, and recordings of volunteers screaming indicated that screams, but not normal speech or singing, sounded rough. What's more, alarm sounds—from, say, a home security system—and dissonant musical chords showed the same signs of roughness as did screams.
The similarities between screams and artificial alarms led the researchers to conjecture that roughness could be a way of indicating danger. In separate experiments, rougher-sounding screams and alarms struck more fear in the hearts of listeners—"the rougher the screams, the more fearful the induced emotional reaction"—and also helped participants accurately and quickly locate the source of a scream.
Finally, the team looked at brain responses to rough sounds in 16 people who listened to normal speech, screams, and other noises while inside an MRI machine. The experiment revealed that rough sounds activated the amygdala, a part of the brain long known to play a role in processing fear and other emotions.
Taken together, the results suggest a specific role for acoustic roughness—"a privileged acoustic niche," as the researchers put it—as a warning sign for danger.
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