Why are nails on a blackboard unbearable?

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If the mere thought of fingernails scraping along a blackboard makes you cringe, blame your amygdala.

Scientists have discovered that this primitive almond-shaped brain region is behind our aversion to high-pitched sounds.

They say our reaction to the sound of scraping nails - which is in the same frequency range as screams and babies cries - could be an ancient survival instinct.

The study scanned the brains of 13 volunteers while they listened to a range of sounds before rating how much they liked them.

The more unpleasant a sound, the greater the amygdala - one of the first brain structures to evolve - lit up. It then activated the auditory cortex, which processes sound, leading the volunteers to perceive the noise more keenly.

The sound of a knife on a glass bottle was rated the most unpleasant, followed by a fork on a glass. Chalk on blackboard came third, a ruler on a bottle fourth and nails on a blackboard fifth, the Journal of Neuroscience reports.

Further analysis found that these high-pitched sounds were more easily picked up by the ear.

Researcher Dr Sukhbinder Kumar, of Newcastle University, said: "It appears there is something very primitive kicking in.

"It's a possible distress signal from the amygdala to the auditory cortex. There is a frequency range where our ears are the 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.

- DAILY MAIL