A really good scream is a beautiful thing, evolutionarily speaking. Fear can set off what's called a defense cascade - a set of behaviors that cause you to run or defend yourself, freeze because you're so scared, or keep you from shouting or moving at all! Sometimes it can cause you to fall over or even faint to protect yourself! Seriously, fear is really powerful.
The source of this emotional response is buried deep in your brain; in a set of neurons called the amygdala. The fear can be emotional like abandonment, or from physical danger like an attack. But it can also be triggered by the sound of screams. And, new research has found just what about a scream triggers your amygdala the best (or worst)? Screams, according to the paper, are a "unique signal" and exploit distinct features not used by regular speech!
When you hear voices, your brain processes them -- "Do I know that man or woman?" "How old are they?" "Are they emotional?" Screams? Nuh-uh. They go right from ear to amygdala. For the first time, the study characterized the scariness of screams, by their roughness.
Roughness, is a measure of modulation. Think of alarm sounds, rougher ones are scarier, like klaxons or alarm sounds. If someone screams, you can hear that modulation. It's like a vocal strobe light! You can't ignore it! The "acoustic roughness," they found, "selectively activates the amygdala" triggering fear and danger processing.
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