Human screams tap into a unique set of reserved frequencies that cause the brain to interpret them as urgent, chilling and unpleasant, a new study has shown.
Screams reproducibly get our attention. Regardless of age, gender or culture, they trigger eruptions of goose-pimples and send shivers down our spines. Now we know why.
Writing in Current Biology, New York University's David Poeppel and his colleagues show that, when we scream, the sounds are modulated - in other words increase and decrease in intensity, at between 30 and 150 times per second.
This modulation is exclusively applied to screams and is absent from normally spoken English, French or Mandarin.
In this way, it serves as a special signal to which the brain is tuned, which provokes the alarm reaction we're all familiar with whenever we hear a blood curdling cry.
Critically, it's not confused with normal speech patterns, which occupy a much lower range of modulations.
The researchers made the discovery by asking volunteers to utter sentences both in normal speech and while screaming.
Using a computer, graphical representations of the sounds, called modulation power spectra, were produced.
These showed the screamers were applying this additional modulation signal on top of the sounds they made. Further tests on volunteers confirmed that sounds containing a greater proprtion of these alarm signal modulations were rated by the subjects as more frightening than sounds from which they were absent or from which they had been electronically filtered out.
The scream signals, which are perceived as rough and unpleasant to raters, also appear to aid in localising the source of a sound. Subjects responded more quickly and accurately to a sound containing the alarm signals compared with other noises.
The team also analysed a series of artificial alarms, such as those used to deter burglars or to signal a fire. The same frequency modulations were being used in these man-made devices, accounting for their success as attention-grabbing devices.
These sounds appear to exert an effect on the brain's fear centre, a structure called the amygdala, which is tuned specifically to pick up on the signal.
The researchers brain-scanned sixteen subjects as they experienced sounds that did and did not contain added alarm "roughness". Critically, the amygdala, they found, lit up when screams replete with the alarm signal were played to the volunteers.
This, the scientists say, "supports the view that roughness, as featured in screams, improves the efficiency of warning signals, possibly by targeting sub-cortical neural circuits that promote the survival of the individual and speed up reaction to danger..."
One wonders whether Hollywood are already onto this...?