The human scream is highly specific. Not even the sound of a plane taking off can stop you from perceiving and recognizing nearby screaming. Screaming is an innate communication signal, shared by everyone, that has the ability to immediately draw attention. It's thanks to this feature that babies can instantly attract attention from parents. How are screams unique and how are they processed by the brain? A new study seeks to answer these questions.

In terms of auditory communication, several characteristics distinguish us as humans: in particular, distinguishing between male and female voices, differentiating human sounds from animal noises, and discriminating between vowels and consonants. And what about screams? Until now, scientists studying human communication have paid little attention to the question. And if we ask you the question: What's special about screaming? What would you say? Most likely you would evoke the loud, shrill nature of a scream. But many sounds are loud and shrill, so there must be something else about screaming that makes it different.

Given this observation, Luc Arnal at the department of fundamental neuroscience and faculty of medicine in Geneva, along with David Poeppel at New York University, examined the acoustic properties of the human scream and its consequences on brain activity. Their results were published in Current Biology. The scientists first compared the frequency modulations of speaking, singing, and screaming. Screaming proved to be the only one to occupy a wide range of frequencies from 30 to 150 hertz. In comparison, speaking registers at frequencies of about 5 hertz. Screaming, with its high frequencies, is associated with disturbing or aggressive sounds. These sounds are defined as being “rough,” and are often found in alarms in order to signify danger.

To study human behavior in response to the frequencies, the researchers next asked volunteers to evaluate how unpleasant and frightening certain sounds were on a scale from 1 to 5. The most frightening and unpleasant sounds corresponded to the roughest frequencies. The volunteers were then asked to identify the source of the different sounds. The rough sounds turned out to be the easiest and fastest to locate. This supports the view that screams improve and speed up reactions to danger.

Finally, the scientists studied the impact of different sounds directly on the brain using an MRI. While standard sounds are processed by the auditory cortex, rough sounds are processed by the amygdala, a subcortical region which is part of the limbic system, involved in quickly evaluating danger. Processing of rough sounds is thus separate from other sounds used in communication, allowing reactions to screams to be adapted to danger without interference from other sounds, like speech.

These studies show that screams have specific acoustic properties that receive special processing in the brain, filtering out other noises from the environment.