Researchers from Switzerland and Germany have just published a paper in which they describe using brain imaging and a cool way of looking at sound, called the modulation power spectrum (MPS) to understand just why screams are so alarming. Rather than looking at the amplitude and frequency of sounds over time, the MPS plots the modulation frequency against the number of cycles per octave, shown as a kind of heat map. On this kind of spectrum, there is a clear zone that gives clues to the gender of the speaker, and another distinct zone that gives information about meaning. But there is also a zone that until now hadn’t been associated with any function. In fact, it has been thought to be irrelevant to human communication. This region corresponds to a perception of sound called roughness, which is thought to be unpleasant.
Figure taken from Arnal et al.
The researchers took 19 people and recorded them either screaming or saying a sentence and then analysed the sounds using the MPS technique. They found the screamed versions occupied this rough space, suggesting this zone is used to convey danger. They then analysed musical instruments and artificial alarm signals like buzzers and horns, and found the alarm signals used used this rough zone.
Next they asked volunteers to rate the fear induced by different screams and vocalisations after either adding or taking away roughness. Filtering roughness from screams made them less fearful, and adding roughness to vocalisations made them more fearful. Similarly they asked volunteers to rate the levels of alarm induced by artificial noises after either adding or taking away roughness. Filtering roughness made sounds less alarming, and adding roughness made them more alarming.
And here’s something else interestingly. The rougher the sound, the quicker the participants reacted. The researchers also found that people could determine where the rough sounds were coming from quicker and more accurately too. Imaging people’s brains while they were listening to the different sounds, they also found that instead of stimulating the auditory cortex, rough sounds stimulated the parts of the brain involved in processing fear and danger – the amygdala.
So in summary, screams occupy a special auditory space which cuts straight to the fear/danger processing parts of the brain, making us respond to them faster and more accurately. Clever.
I talked about this story, and about untrustworthy faces and prison sentences with Kathryn Ryan on Radio NZ’s Nine to Noon programme. You can listen here.
Arnal LH, Flinker A, Kleinschmidt A, Giraud A & Poeppel D (2015). Human Screams Occupy a Privileged Niche in the Communication Soundscape. Current Biology. DOI: http://dx.doi.org/10.1016/j.cub.2015.06.043