We know human screams are jarring. They’re loud, occasionally shrill, and tend to make us feel stressed, or even fearful. What’s unclear is why they elicit anxiety. But a new study suggests this response may have something to do with the acoustic quality of human screams, and how they trigger the brain’s fear response.

According to a new study headed by David Poeppel from New York University, and his postdoc Luc Arnal, now at the University of Geneva, this has something to do with a unique property of sound, called roughness, that activates the brain’s fear circuitry within the amygdala. The details of their work now appear at Current Biology.

Rough Sounds

“Roughness refers to fast sound changes in loudness,” Arnal told io9. “Normal speech for instance only has slow differences in loudness—between 4 and 5 Hz—which is not rough and basically corresponds to the syllabic rate. Screams, on the other hand, modulate very fast—between 30 and 150 Hz—which is rough.”

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Arnal adds that the strength (low vs high) of roughness corresponds to the amplitude, or volume, of these fast changes. Low roughness corresponds to weak loudness changes whereas high roughness corresponds to high loudness changes.

(Credit: Luc Arnal)

“This kind of sound could be compared to a strobe light in the auditory domain,” says Arnal. “Everyone is familiar with those lights that flash super fast in clubs for instance. Screams could be defined as strobophones, since they are modulating super fast in an analogous way in the auditory domain.”

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As their fascinating experiment shows, these rough, strobe-like sounds appear to have a curious, and possibly adaptive, effect on the human psyche.

An Evolved Response

Poeppel and Arnal used recordings taken from YouTube videos, popular films, and volunteer screamers who were recorded in the lab’s sound booth. Then, in a series of experiments involving fMRI scanners, 16 participants listened to sounds of various degrees of roughness. The researchers used three different categories of sounds that were either neutral or unpleasant, namely: human vocalizations (normal voices and screams); artificial sounds (like instruments and alarms); and musical intervals (both consonant and dissonant sounds). The researchers then identified brain regions involved in processing unpleasantness by comparing responses to unpleasant sounds against responses to neutral sounds.

Results showed that unpleasant sounds induced larger hemodynamic responses, i.e. the rate of blood flow, in the bilateral anterior amygdala and primary auditory cortices. The amygdala is a brain structure crucial for regulating emotions.

fMRI measurement of roughness and screams (Credit: Arnal et al., 2015)

“The rougher the sound was, and the more scary it was rated, the more effectively it activated the amygdala,” Poeppel explained to io9.

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Fascinatingly, the researchers found that the amygdala, and not the auditory cortex, is sensitive to temporal modulations in the roughness range.

Their results suggest that rough sounds specifically target neural circuits involved in fear/danger processing. This is the first direct evidence in support of the idea that roughness is an acoustic attribute that triggers adapted reactions to danger. The researchers speculate that this behavioral feature confers an evolutionary advantage, and that rough vocalization, which recruit dedicated neural processes “that prioritize fast reaction to danger over detailed contextual evaluation”—in other words, that a rough sound can trigger your fear response more directly, and therefore faster, than something you, say, witness with your eyes and process in your mind.

I asked both researchers how they were certain that other aspects of the sounds weren’t triggering the fear response, such as spoken words, or some other factors, like context.

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“We are very pedantic researchers,” replied Poeppel. “We matched all the other sounds, in fact all sounds for duration, for loudness, for many of the other features we can control. We try our damnedest to make sure that the one remaining factor is in fact roughness.”

To which Arnal added: “There was no word spoken. Only syllables and artificial sounds were used in that study. We also controlled for other aspects (pitch frequency, valence of the sound) when analyzing the data and found that the amygdala specifically responded to roughness.”

Interestingly, the researchers discovered that rough sounds don’t necessarily have to be uttered by humans to elicit the response. The participants exhibited similar responses to alarm signals, such as car alarms and house alarms.

When Babies Cry

Though outside the scope of this study, the work of Poeppel and Arnal suggests that other human sounds might trigger similar responses, including the sounds made by crying babies. And in fact, that’s exactly what the researchers would like to focus on next.

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“We are planning to continue our research on human screams in the lab, particularly those of infants, to see if their screams are particularly rough,” Arnal told io9. “This is a particularly exciting project because we are focusing on a kind of vocalization that seems to be innate. It is one of the earliest sounds that everyone makes—it’s found across cultures and ages—so we thought maybe this is a way to gain some interesting insights as to what brains have in common with respect to vocalization.”

Read the entire study at Current Biology: “Human Screams Occupy a Privileged Niche in the Communication Soundscape”.


Contact the author at george@io9.com and @dvorsky. Top image by Jim Cooke.