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Scientists studying the brain's reaction to sound have determined the piercing screech of a knife against a glass bottle to be the human ear's worst nightmare.

A research team at the University of Newcastle examined brain activity in a group of volunteers, who listened to 74 recorded sounds under an MRI machine and ranked each experience.

The 13 subjects found the sound of a fork against glass to be the second most repulsive, while sounds like flowing water were much easier for the brain to bear.

In order to establish why, the researchers focused on the ear's acoustic rather than associative reactions. Sukhbinder Kumar, the study's author, said he and co-author Tim Griffiths chose sounds that do not call to mind any negative emotional experiences, unlike a separate 2007 study that tested more evocative sounds and found that of vomiting to be the most universally appalling.

The scans revealed that the unpleasant noises not only increase blood flow in the brain's auditory cortex, where sound is processed, but also trigger the amygdala, a primitive region of the brain that processes emotions.

"The new finding in this study is to precisely define the loop between the auditory cortex and the amygdala," Griffiths said. "We were able to define a network within the brain that constitutes sound unpleasantness."

The most disagreeable sounds had frequencies of 2000-5000 Hz, the part of the spectrum at which our ears are most sensitive, Kumar said.

While it is unknown why this range is most difficult for the human ear to tolerate, specific sensitivities like this one could have evolutionary advantages, Griffiths pointed out.

"Because this is such a fundamental level of unpleasantness, it could be a sort of alerting and arousing system in the brain," he said.

The study, which was published in the Journal of Neuroscience , will help scientists understand what makes certain people more sensitive to disagreeable sounds than others, Kumar said, potentially leading to new insight into disorders like misophonia, a severe sound intolerance, and the more common tinnitus, which Kumar is currently studying.