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Why we all hate the sound of nails on a blackboard

EVEN the thought of hearing nails scraping down a blackboard makes most of us wince.

Now scientists have found out just why the sound is so unpleasant.

Heightened activity between the emotional and auditory parts of the brain explains why people grimace when a knife is scratched against a bottle.

Scientists at Newcastle University revealed the interaction between the region of the brain that processes sound, the auditory cortex, and the amygdala, which is active in the processing of negative emotions when we hear unpleasant sounds.

Brain imaging has shown that when we hear an unpleasant noise the amygdala modulates the response of the auditory cortex, heightening activity and provoking our negative reaction.

"It appears there is something very primitive kicking in. It's a possible distress signal from the amygdala to the auditory cortex," said study author Dr Sukhbinder Kumar.


This might be linked to humans' dislike of screaming.

Analysis of the sounds found that anything in the frequency range of around 2,000-5,000Hz is unpleasant.

"This is the frequency range where our ears are most sensitive. Although there's still much debate as to why our ears are most sensitive in this range, it does include sounds of screams which we find intrinsically unpleasant," Dr Kumar said.

MRI scans were carried out on 13 volunteers' brains as they listened to a range of sounds. While in the scanner they rated them from the most unpleasant, the sound of knife on a bottle, to the nicest, the sound of babbling water.

Researchers then studied the brain response to each type of sound.

They found that the activity of the amygdala and the auditory cortex varied in direct relation to the ratings of perceived unpleasantness given by the volunteers.

The emotional part of the brain, the amygdala, in effect takes charge and modulates the activity of the auditory part of the brain so that our perception of a highly unpleasant sound is heightened as compared with a soothing sound.

A better understanding of the brain's reaction to noise could help our understanding of medical conditions with which people have a decreased sound tolerance, such as autism.

Professor Tim Griffiths from Newcastle University, who led the study, said: "This might be a new inroad into emotional disorders and disorders like tinnitus and migraine in which there seems to be heightened perception of the unpleasant aspects of sounds."

The study is published in the Journal of Neuroscience.

Out of 74 sounds, volunteers rated the five most unpleasant as a knife on a bottle, a fork on a glass, chalk on a blackboard, a ruler on a bottle and nails on a blackboard.