According to a team of British scientists, the interaction between the region of the brain that processes sound – the auditory cortex – and the emotional part of the brain called the amygdala explains why sounds of chalk on a blackboard, a fork on a glass, and a knife on a bottle, are so unpleasant for us.

The team used functional magnetic resonance imaging to examine how the brains of 13 volunteers responded to a range of sounds. Listening to the noises inside the scanner the researchers rated them from the most unpleasant – the sound of knife on a bottle – to pleasing – babbling water. The team was then able to study the brain response to each type of sound.

The findings have been reported in the recent issue of the Journal of Neuroscience.

“It appears there is something very primitive kicking in,” said study lead author Dr Sukhbinder Kumar of University College London and Newcastle University. “It’s a possible distress signal from the amygdala to the auditory cortex.”

The study found that the activity of the amygdale and the auditory cortex varied in direct relation to the ratings of perceived unpleasantness given by the subjects. The emotional part of the brain, the amygdala, in effect takes charge and modulates the activity of the auditory part of the brain so that our perception of a highly unpleasant sound, such as a knife on a bottle, is heightened in comparison to a soothing sound, such as babbling water.

Analysis of the acoustic features of the sounds found that anything in the frequency range of around 2,000 to 5,000 Hz 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 said.

Scientifically, a better understanding of the brain’s reaction to noise could help our understanding of medical conditions where people have a decreased sound tolerance such as hyperacusis, misophonia and autism when there is sensitivity to noise.

“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,” concluded Prof Tim Griffiths from Newcastle University, senior author of the study.

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Bibliographic information: Sukhbinder Kumar et al. 2012. Features versus Feelings: Dissociable Representations of the Acoustic Features and Valence of Aversive Sounds. The Journal of Neuroscience, 32(41): 14184-14192; doi: 10.1523/JNEUROSCI.1759-12.2012