By Will Dunham
WASHINGTON (Reuters) - If you feel like a human scream jolts the deep recesses of your brain, there's a good reason for it. That is precisely what is happening, scientists say.
Researchers who explored how the brain handles a scream said on Thursday the loud, high-pitched sound targets a deep brain structure called the amygdala that plays a major role in danger processing and fear learning.
"We knew pretty well what frequencies are used by speech signals and the brain regions involved in speech processing: the auditory cortex and higher order regions such as Broca's area, for instance," said University of Geneva neuroscientist Luc Arnal, whose research appears in the journal Current Biology.
"But what makes screams so special and unpleasant and how the brain processes these sounds was not clear," Arnal said.
The researchers said an acoustic quality called "roughness," the quick change in sound loudness, sets screams apart from other sounds.
"Normal speech patterns only have slight differences in loudness, between 4 and 5 Hertz (sound wave cycles per second), but screams can modulate very fast, varying between 30 and 150 Hertz," Arnal said, explaining the "roughness" of screams.
As part of the study, the researchers played recordings of screams from horror movies, YouTube videos and those made by volunteer screamers in a laboratory, and asked people to judge how frightening these were. Those with the highest "roughness" were found to be the most terrifying.
To learn how these sounds were processed, the researchers monitored brain activity using a neuroimaging method called functional magnetic resonance while the study's subjects listened to screams.
They found that the screams increased the activation of the fear response in the amygdala, an almond-shaped structure situated deep inside the brain's medial temporal lobe.
"In terms of potential applications, our findings could be used to improve the way we design alarm sounds. The same way a bad smell is added to natural gas to make it easily detectable, adding roughness to alarm sounds may improve and accelerate their processing," Arnal said.
Arnal said he is planning future research on infant screams to see if those have extra roughness.
"I started being interested in screams when a friend of mine told me that the sound of his newborn's screams was literally hijacking his brain, and I wondered what makes screams so efficient as an alarm signal," Arnal said.