When you hear the sound of a nail scratching a blackboard, the emotional and auditory part of your brain are interacting with one another, a new study reveals.

The heightened activity and interaction between the amygdala, which is active in processing negative emotions, and the auditory parts of the brain explain why some sounds are so unpleasant to hear, scientists at Newcastle University have found.

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

Researchers at the Wellcome Trust Centre for Neuroimaging at UCL and Newcastle University used functional magnetic resonance imaging (fMRI) to examine how the brains of 13 volunteers responded to a range of sounds.

Most unpleasant sounds Least unpleasant sounds Knife on bottle Applause Fork on glass Baby laughing Chalk on blackboard Thunder Ruler on bottle Water flowing Nails on chalkboard

Listening to the noises inside the scanner, the volunteers rated them from the most unpleasant, like the sound of knife on a bottle, to the most pleasing, like bubbling water. Researchers were then able to study the brain response to each type of sound.

"At the end of every sound, the volunteers told us by pressing a button how unpleasant they thought the sound was," Dr. Kumar said.

Researchers found that the activity of the amygdala and the auditory cortex were directly proportional to the ratings of perceived unpleasantness. They concluded that the emotional part of the brain, the amygdala, in effect takes charge and modulates the activity of the auditory part of the brain, provoking our negative reaction.

Analysis of the acoustic features of the sounds found that anything in the frequency range of around 2,000 to 5,000 Hz (cycles per second) was found to be 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 explained.

Professor Tim Griffiths from Newcastle University, who led the study, said, "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."

The study has been published in the Journal of Neuroscience.