When it comes to making one immediately vigilant, there are few sounds more effective than a bloodcurdling human scream. We know that a scream can only mean a few things, none of which are very pleasant. Someone was just jumped by a mugger. There’s a grizzly bear approaching camp. A fire is ripping through a building. All of these things instinctively cause fear, as they all pose a basic threat to our existence. The sound of a scream tells us that something dangerous or frightening is happening to one of our own, and we all have the ability to produce this primitive alarm. Why, though, are we able to so quickly distinguish the meaning of a human scream? How does the brain process the sound, and why do we know it’s scary?

Scream If You Have To — A Primitive Alarm

According to recent research, the answer to the first question lies in the acoustic properties of the scream itself. An international team of neuroscientists from the Max Planck Institute for Empirical Aesthetics, New York University, and the University of Geneva has proven that screams possess unique auditory properties. This is the reason we can recognize screams as the specific vocal expression produced only in stressful or dangerous situations.

"Everybody screams and everybody has an intuition about what constitutes screams — that they are loud and high-pitched," said David Poeppel, the paper's senior author and a professor in NYU's Department of Psychology and Center for Neural Science in a press release. "But neither turns out to be quite correct. In fact, screams have their own acoustic niche separate from other sounds. While, like some sounds, they may be high-pitched and loud, screams are modulated in a particular way that sets them apart from the rest."

In a series of several studies, researchers discovered a special acoustic trait only exhibited by screams.

"Screams have a trait called 'roughness,'" Poeppel said. "Roughness occurs when screams obtain a temporal structure due to change of amplitude or frequency. If these changes happen very quickly, the ear is no longer able to 'break down' these temporal changes — such sounds are then perceived as rough and unpleasant. Normal speech has a modulation rate of around 4 to 5 Hz, but for roughness the rate is between 30 and 150 Hz — the temporal changes are therefore significantly faster."

In one experiment, a group of men and women recorded various vocal sounds including screams, screamed sentences (“Look out!”), meaningless vocalizations, and normally spoken sentences. Researchers found that both wordless screams and screamed sentences occupied the “roughness” domain that the other vocalizations did not.

In a different study, screams were compared to artificial alarms — a device whose very purpose is to alert humans to a dangerous situation. They found that both the human scream and alarm (a sort of artificial scream) fell into the 30-150 Hz roughness range, a finding that suggests alarm manufacturers have effectively captured the modulation of a human scream.

Finally, the researchers wished to know how these sounds were processed in the brain. Using functional magnetic resonance imagery, they monitored the study’s participants as they listened to these sounds. For both screams and the alarm sounds, the brain showed increased activity in the amygdala, which is the region of the brain used for processing and remembering fear.

So You Hear A Scream...Now What?

Hearing a scream is only the first step in the brain’s processing of fear. A unique emotion, fear is hard-wired into humans and animals, as it is necessary for survival. Danger requires an immediate response, and our brains have developed a pathway to process fear that is so efficient, it allows possibly dangerous stimuli to bypass parts of the brain that are normally involved in processing sensory input — a sort of brain highway designed to keep us safe rather than sorry.

In the processing of normal stimuli, sensory data is sent from sensing organs (eyes, ears, etc.) to the thalamus, which acts as a sort of switchboard operator, taking the data and connecting it to the appropriate part of the sensory cortex for processing. Say the stimulus is a sound, the thalamus routes the information to the auditory cortex, where it is then interpreted for meaning.

Hearing a frightening sound, however, may prompt the alternative, faster pathway. When a frightening stimulus is sent to the thalamus, the brain structure does not know right away if the stimulus is going to be dangerous or not. Instead of taking chances and routing the data to the appropriate cortical section for conscious processing, the thalamus fast tracks the information to the amygdala.

Upon receiving the neural impulses, the amygdala takes immediate action to protect us; it sends a message to the hypothalamus to trigger the fight-or-flight response that could save our life in a dangerous situation.

Of course, this immediate response can be fooled. This is the reason we can get scared by something that turns out to be harmless, like the wind slamming a door shut or a suspicious shadow outside the window. The fear highway, also known as the “low road,” is meant to be super vigilant, even if that means occasionally sending some false alarms. While the low road is causing an immediate fear response just in case, the usual pathway (predictably named the “high road”) is operating as well, figuring out if the stimulus really is something to worry about.

In the case of a false alarm, the cortex determines that there is more than one possible reason for the stimulus and sends the data along to the hippocampus, part of the brain highly involved in memory. The hippocampus then considers if we’ve seen the particular stimulus before, and if we have, what it meant the last time we encountered it. Upon deciding that the stimulus is not a cause for concern, the hippocampus sends a message to the amygdala telling it there is no danger. The amygdala would then, in turn, tell the hypothalamus to shut off the fight-or-flight response. This type of processing is much more advanced than the primal low road, a conscious analysis of the stimulus and what it means.

Both pathways are important to us, but it is because of the low road that we have the ability to quickly identify potential threats. Human screams, with their unique acoustic quality, are stimuli that will forever take the low road in our brain because as Poeppel observes: “We only use them when we need them.”

Source: Arnal L, Flinker A, Kleinschmidt A, Giraud A, Poeppel D. Human Screams Occupy A Privileged Niche In The Communication Soundscape. Current Biology. 2015.