Why we cringe at nails on the chalkboard

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The reason we cringe at the sound of scratching on a chalkboard is because the sound triggers an uptick in communication between a part of the brain involved in hearing and a part involved in emotions, a new study from England says.

In the study, 13 participants listened to 74 sounds and rated them according to their pleasantness. Researchers used functional magnetic resonance imaging (fMRI) to examine how the participants' brains responded to the sounds.

When the participants heard an unpleasant sound, there was an interaction between the auditory cortex, which processes sound, and the amygdala, which processes negative emotions.

"It appears there is something very primitive kicking in," said study researcher Sukhbinder Kumar, a research fellow at Newcastle University. "It's a possible distress signal from the amygdala to the auditory cortex."

Moreover, the more unpleasant the sound,, the greater the activity between these two brain regions, the researchers said.

The 10 most unpleasant sounds, according to the participants' ratings, were:

Knife on a bottle
Fork on a glass
Chalk on a blackboard
Ruler on a bottle
Nails on a blackboard
Female scream
Angle grinder (a power tool)
Brakes on a cycle squealing
Baby crying
Electric drill

The least unpleasant sounds were:

Applause
Baby laughing
Thunder
Water flowing

An analysis of the sounds showed that those with frequencies between 2,000 and 5,000 Hertz were found to be unpleasant. "This is the frequency range where our ears are most sensitive," Kumar said. The reason for such sensitivity is not exactly understood, but this range includes the sounds of screams, which people find intrinsically unpleasant, he said.

A better understanding of the brain's reaction to noise could help researchers to treat medical conditions in which people have a decreased tolerance to sound, people with autism who are sensitive to noise, or disorders such as tinnitus and migraine, , the researchers said.

The study was published Oct. 10 in the Journal of Neuroscience, and was funded by the Wellcome Trust.