By Emma Young
You see a pedestrian about to step out in front of an oncoming car. Is it better to calmly call out a warning, or to scream?
Of course, it’s better to scream — but not just because a scream is loud. Car alarms, police sirens and smoke alarms are all loud, too. But, like screams, they also feature fast but perceptible fluctuations in loudness, usually at frequencies of between 40 and 80 Hz, making them acoustically “rough”. Quite why such sounds should be so attention-grabbing, and even unbearable, hasn’t been clear. Now a team led by Luc Arnal at the University of Geneva has found that this type of sound triggers activity in brain areas related not just to hearing but also to aversion and to pain. This makes them impossible to ignore.
For the new study, published in Nature Communications, 16 participants listened to streams of repetitive clicks played at various frequencies, between 50 to 250 Hz. At frequencies below about 130 Hz, participants could hear discrete clicks. Above this frequency, the clicks were usually perceived as being one continuous sound. While very loud sounds were considered unpleasant, the participants found rough sounds with fluctuations in the range of 40-80 Hz particularly awful. This is “in the range of frequencies used by alarms and human screams, including those of a baby,” notes Arnal.
To explore why these sounds should trigger such a response, the team used a type of EEG in which electrodes are placed directly on the brain’s surface, usually to identify the source of seizures in patients with epilepsy. Eleven epilepsy patients had activity in their brains monitored while they listened to various sounds. The researchers found that the auditory cortex in the upper temporal lobe was activated regardless of whether the fluctuation in loudness was above 130 Hz, and so the sound was perceived as continuous, or in the 40-80 Hz range. This was completely expected.
However, when a “rough” sound was perceived as awful, and especially when it was in that 40-80 Hz range, the researchers were surprised to also see synchronised patterns of activity in a number of other brain areas, including the amygdala, hippocampus and insula. These areas are all related to salience, aversion and pain, comments Arnal. “This explains why participants perceived them as being unbearable.”
The results could also account for our aversion to other rough sounds like alarms or annoying vocal effects, the researcher add. “The negative percept induced by rough sounds … might result from their capacity to massively, exogenously hijack brain networks involved in aversion and pain processing,” they write.
All kinds of animals, from chimpanzees to birds, use rapid repetitive alarm calls to warn each other of danger. Our human scream is just one example. In exploiting the brain’s recognition of danger calls from other people, car alarms and sirens get their urgent messages across, too. “We now understand at last why the brain can’t ignore these sounds,” Arnal says.
Update 8th October 2019: This post was updated to clarify that the frequencies mentioned refer to the rate at which sounds change in loudness, rather than their pitch.