Monday February 20, 2017 Q&A

Why do we find screams so alarming?


1 They make our peripheral nerves vibrate

2 They paralyze our inner ear

3 They activate our cerebral amygdala

Question related image
Correct answer: The sound activates the amygdala, the part of the brain that is associated with emotions and our response to fear Screaming is one of our key survival mechanisms – provided it doesn’t paralyze the listener with fear. Researchers at the University of Geneva have been studying this form of vocal communication for the past few years. It’s innate – babies start wailing straight out of the womb – but is it universal too? Do other mammals do it? And how do our brains process screams? The researchers, drawing on functional MRI data, discovered that human screams – as opposed to other vocal sounds – occupy a specific modulation frequency range of the audible spectrum. Sounds in this range trigger our alarm or fear mechanism, while someone singing – even thunderously – does not have the same effect. That’s because screams have a quality that psychophysicists call ‘roughness’. This is what lets us know when someone is crying wolf or not. When we speak normally, our voice is modulated by the movement of our mouth, and spoken sounds generally have a frequency of 5 hertz. When we scream, on the other hand, the modulation rate rises to between 30 and 150 hertz. It is this range of frequencies that corresponds to roughness, a term that evokes the feeling produced when you run your finger along a rough surface. Rough sounds, such as a raspy voice, fall into a certain range along the audible spectrum that immediately triggers an unpleasant feeling. This effect is generally absent when someone talks or sings, thereby avoiding false alarms. Rough sounds immediately activate the amygdala, a region of the brain involved in processing emotions and reacting to danger. It appears that the higher modulation rate overloads the auditory system, sort of like the effect of a strobe light on our eyes. The sensation is so overpowering that it forces us to react immediately. These frequencies also make the sounds easier to locate – we can quickly pinpoint the source of the alert. Rough frequencies have been identified in man-made alarm systems like ambulance sirens. The researchers are now looking at the role that neurons play in routing these signals through our auditory system. One hypothesis is that screams are fast-tracked in the brain, taking a shortcut from the thalamus to the amygdala while skipping over the rational cortex. In other words, our brain sounds the alarm bell without having to think about it. Thanks to Luc Arnal, Department of Basic Neurosciences, University of Geneva Campus Biotech