Just why is the sound of nails on a chalkboard so annoying? Researchers from Newcastle University say that being annoyed by certain sounds comes from high levels of activity between certain brain regions that process emotion (the amygdala) and the auditory cortex, a region that processes sound.

Researchers found a link between the amygdala and the auditory cortex by using fMRI scans to see what happens when people are exposed to annoying sounds. Their study included 13 participants who were asked to listen to 74 different sounds and rate them based on the degree of unpleasantness. Out of the 74 sounds, "knife on a bottle" was rated the worst sound.

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

Researchers studied the effects of the sounds on the brain and found that activity in the amygdala and the auditory cortex had a direct relation to the unpleasantness of a sound, meaning nasty sounds increased activity in these regions.

They found that when we hear a fork scraping a bottle, the emotional part of the brain takes over and enhances the sound making it really unpleasant. On the other hand, the sound of bubbling water does not increase the activity in the amygdala, making it more soothing. The study also found that sounds that have frequency range of around 2,000 to 5,000 Hz were considered unpleasant.

"This is the frequency range (2,000 to 5,000 Hz) 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. 

The research may help find treatments for conditions that increase people's reaction to sounds like autism, hyperacusis (where a person feels certain sounds to be extremely loud) and misophonia (where a person is disturbed by specific sounds that may be soft or loud).

"This work sheds new light on the interaction of the amygdala and the auditory cortex. 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," lead author Professor Tim Griffiths from Newcastle University, said in a press release.

The study was published in the Journal of Neuroscience.