I scream, you scream…

Posted by Susan Lubejko on October 31, 2019 in Neuroscience | Leave a comment

You’re walking down a dark, cobweb-swathed hallway with only the light from a few candles to guide your way. You note the torn and dirty oriental rug beneath your feet and the strange shadows that dart across the ceiling overhead. On the walls, you pass dusty, old-fashioned portraits of stiffly-posed families whose eyes seem to follow you as you pass. On your right, you hear the angry wind howling through a broken window, and on the wall to your left, you see… is that a blood stain? Suddenly, a masked man with a knife leaps from behind a door in front of you! What do you do?

While scenarios like these are hopefully restricted to Halloween season visits to haunted houses, the very first reaction most people have in this situation, real or dramatized, is to let out an instinctive shriek. Screaming as a behavioral response to something scary, surprising, or unpleasant is a well-known phenomenon among humans and other primates in the animal kingdom. Like laughter, screaming is a type of non-linguistic communication that appears to be universal among speakers of different languages. It is also hardwired, as babies exhibit a variety of screaming cries before learning language [1]. However, until recently, it remained unclear what actually makes a scream such a distinctive vocal signal and why screaming in the face of a threat might be advantageous.

 

What makes a scream a scream?

Non-verbal communication in humans has long been studied in the context of infant distress signals (crying, screaming, etc.) and their comparison with similar vocalizations from the animal kingdom [1]. However, we’re only starting to understand how screams, like your reaction to the masked man in the haunted house, are produced and perceived. Some key questions that remain include: 1) how do we define a scream acoustically, and 2) how does a scream compare to other types of sounds that are processed by our brain?

Researchers from New York University and Geneva, Switzerland [2] teamed up in 2015 to determine what acoustic properties define screams and make them so fundamentally unpleasant to listen to. Using the modulation power spectrum (MPS) that tracks how the frequency of a sound wave changes over time to represent sound data visually, the researchers compared normally spoken and screamed sentences (like, “oh my god help me!”). They found that while spoken words are represented in the MPS in areas that would contribute to our perception of the gender of the speaker and the meaning of the sentence, screaming fills the parts of the spectrum corresponding to higher frequency, faster fluctuating sounds. How much a sound fills that part of the spectrum is known as its “roughness,” and it is thought that screams are the only natural communication signals that are represented in the roughness part of the MPS.

Interestingly, when man-made alarm signals, like a buzzer or alarm, were put to the test on the MPS spectrum, they also exhibited increased roughness when compared to notes from musical instruments. This might mean that a sound’s roughness could be what makes it so alarming to the listener, and that screams are simply our natural version of an alarm signal.

 

Why are screams so alarming?

Now that we have defined screams as nature’s vocal alarm, we can try to understand what makes us perceive screams as so unnerving. Building upon their findings regarding roughness of screamed phrases, the NYU scientists wanted to know if the roughness of sounds affected how the listener reacted to them. Twenty participants were asked to rate their emotional reaction to screams and other vocalizations by how fearful they felt upon hearing them. As you might expect based on your own experiences in that haunted house, the participants found that they were more fearful of screams than other spoken words. Next, the researchers filtered the screams so that the parts of the sound that represent roughness were removed and added roughness to the neutral spoken words. The participants were less fearful of the filtered screams than the original scream and more fearful of the words with added roughness as compared to normal words, indicating that the “roughness” really is the alarming aspect of the scream [2].

While these results demonstrate that there is a correlation between the roughness of screams and a listener’s fearful response to them, it doesn’t quite explain why screams make our skin crawl. To begin to answer this question, the NYU researchers looked to brain imaging using fMRI to measure blood flow to different areas of the brain when participants heard screams versus neutrally spoken words and alarms versus neutral sounds. Intriguingly, while neutral speech is processed through the typical sound processing pathways that start in the inner ear and make their way to the auditory cortex, screams and alarm sounds with increased roughness cause an increase in blood flow to the amygdala, a known emotional processing center in the brain [2]. The amygdala has long been implicated in humans and animals in a variety of behaviors, but is best known for its role in fear processing and fear learning [3]. The fact that the amygdala is active while listening to screams and not other vocal sounds suggests that’s the place in the brain where the perception of a shriek and the resulting unsettling fear may come together.

 

The advantages of screaming

Using recent research, we have defined screams and looked for reasons that these vocalizations might make listeners fearful. What might be some advantages to communicating through screaming? First, like other types of verbal communication, screams can be used to recognize far-away individuals. Though it has been historically controversial whether incredibly dynamic and often-changing screams can be identified by others as specific to an individual, more recent studies suggest that screams can be used for individual identification. Study participants are better than simply guessing at determining if two screams came from the same recorded monkey or human [4,5]. Therefore, screams may aid in individual recognition, perhaps over longer distances.

Additionally, in their final experiment, the NYU researchers directly tested the question of whether hearing screams gives some behavioral advantage. Participants were played recorded real screams, synthetic screams, and other vocalizations with varying levels of roughness and asked from where in their environment the sound came. The participants were more efficient (they were more accurate in the direction they chose and had shorter response times) when they heard real or synthetic screams as compared to other vocalizations that lacked roughness [2]. This result indicates that we are better at figuring out where screams are coming from in a shorter time than other types of communication signals. Since screams are used to communicate impending danger, our survival just might depend on our ability to locate and react to the threat.

 

Imagine that you are now back in that haunted house facing down the masked man with the knife. You’ve just let out your instinctive non-linguistic vocal alarm signal with increased roughness. While there is still much to be learned about why and how you are screaming, at least your shriek has acted to warn those in the creepy hallway behind you or outside the broken window by activating their brains’ fear centers and telling them exactly where the threat is. Hopefully this masked man is just a haunted house actor, and when he returns to his hiding place, you giggle with your friends about your extreme reaction. But through your snickering, you hear… what was that? The cackle of a witch?

 

 

 

 

References

1. Lingle S, Wyman MT, Kotrba R, Teichroeb LJ, Romanow CA (2012) What makes a cry a cry? A review of infant distress vocalizations. Current Zoology, 58(5):698-726
2. Arnal LH, Flinker A, Kleinschmidt A, Giraud AL, Poeppel D (2015) Human screams occupy a privileged niche in the communication soundscape. Current Biology, 25, 2051-2056
3. LeDoux J (2007) The amygdala. Current Biology, 17(20), R868-874.
4. Fugate JMB, Gouzoules H, Nygaard LC (2008) Recognition of rhesus macaque (macca mulatta) noisy screams: evidence from conspecifics and human listeners. American Journal of Primatology, 70:594-604
5. Engelberg JWM, Schwartz JW, Gouzoules H (2019) Do human screams permit individual recognition? PeerJ, 7:e7087

 

Image Sources

1. Cover image: https://www.history.com/news/psycho-shower-scene-hitchcock-tricks-fooled-censors
2. Haunted House: https://www.flickr.com/photos/shanegorski/2324747282
3. “The Scream” by Edvard Munch: Wikimedia Commons