Why are harsh sounds so unbearable for the human brain?


Most, if not all, of us find harsh noises, such as those that car alarms make, difficult to bear. New research looks at what happens in the brain during exposure to these auditory stimuli.
Harsh noises activate brain regions that play a role in pain and aversion, a new study has found.
Grating noises, such as car alarms, the sounds of a construction site, or even human screams, are very difficult if not impossible to ignore, chiefly because they are unpleasant.
What happens in our brains when we hear such noises, though, and why do we find them so unbearable?
These are the questions that a team of researchers from the University of Geneva and Geneva University Hospitals in Switzerland has aimed to answer in a recent study.
Why is this issue important in the first place? In their study paper — which appears in the journal Nature Communications — the researchers explain that it ties into aspects of communication.
“A first and foremost purpose of communication is to catch the attention of conspecifics [individuals of the same species],” the researchers write, “a process that can be optimized by adapting signal salience to maximize the receiver’s sensory-motor responses.”
In neuroscience, salience is the quality that sets something apart from items of the same kind. “To amplify sensory salience and ensure efficient reactions on the receiver’s end, a generic strategy is to increase signal intensity, e.g., by screaming or shouting,” the authors note in their paper.
“However, signal magnitude is not the only parameter that changes when we increase vocal sound levels. Another important emerging feature is roughness, an acoustic texture that arises from fast repetitive acoustic transients,” they add.
So, in their study, the scientists first established the range of sounds that are “rough” and unpleasant for the human brain. They then looked at the brain areas that these noises activate.
When does noise become ‘intolerable?’
The researchers recruited 27 healthy participants between the ages of 20 and 37 years, 15 of whom were female. The researchers worked with various groups of these participants for different experiments.
For some of these experiments, the researchers played the participants repetitive sounds with frequencies between 0 and 250 hertz (Hz). They also played these sounds at progressively shorter intervals to determine the point at which some of these sounds became unpleasant.The team found that the upper limit of sound roughness occurs when the stimulus reaches about 130 Hz.

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