Unravelling the mysteries of unpleasant scrapping sounds

Unravelling the mysteries of unpleasant scrapping sounds.

A team of researchers led by Professor Tim Griffiths from Newcastle University have found out that higher activation in the brain regions which processed emotion and sound, and the correlation between these two are the main reason why the sound of knife on the bottle or nails on the blackboard are so unpleasant.

In their recent study published in the Journal of Neuroscience October 2012, the scientists at Institute of Neuroscience, Newcastle and Wellcome Trust Centre, London explained the model of interaction between the auditory cortex, the part of the brain that processes sound, and the amygdala which processes negative emotions. 

When we hear a nasty noise, the sound is first processed to a high level in the auditory cortex. The information is then relayed to the amygdala where not only it determines the acoustic features of the stimulus, it will also decide how unpleasant the noise is. If the unpleasantness of the sound is perceived as higher, the amygdala will then return the information to the auditory cortex causing a heightened activity in the auditory cortex in response to this sound compared to a less unpleasant sound.

A total of sixteen volunteers participated in the study where functional magnetic resonance imaging (fMRI) was used to examine their brain activity in response to unpleasant sounds played in the MRI scanner. The choices of sounds were based on the scientists’ previous work in 2008 in which 50 volunteers were recruited to rank a set of 74 sounds according to their unpleasantness. These included the most unpleasant knife on a bottle, chalk scratched on blackboard, and bubbling water which was deemed least unpleasant.

The MRI data collected was then analysed and a connectivity analysis was conducted to determine the interactions between different brain regions, how a brain area may respond to a change in activity another area. For the purpose of this research, four brain regions were specifically studied which were the right and left auditory cortex and right and left amygdala.

Image 1: MRI image showing the response in amygdala which correlates with acoustic features and rating of unpleasantness. The red areas indicate where the acoustic features are processed while blue areas indicate the valence (or degree of unpleasantness). 

 The study found that sounds in the frequency range of 2000 to 5000 Hz were found to be unpleasant. Based on the relationship models tested in this research, it was also found that the brain parts did not work independently from one another, but in a very integrated manner. Stimulus received by the auditory cortex was processed and forwarded to the amygdala. A backward connection from amygdala to the auditory cortex subsequently takes place in accordance to the interpretation of the amygdala of the characteristic of the stimulus.

 Although the study is rather limited in the sense that there is very small sample size, such findings is significant to pave our ways to a better understanding of certain medical conditions in which there is lack of tolerance to sound. This includes hyperacusis (decreased sound tolerance), misophonia (“hatred” of sound) and phonophobia (fear of sound).

"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," says Professor Tom Griffiths in a press release in last October.

Future studies are necessary to gain more understanding regarding the subject and to investigate further on the effect of different sounds, both pleasant and unpleasant and to answer whether similar pathway plays the role in processing nice sounds compared to these nasty scraping noises that makes us grind our teeth.

(This article is adapted from The Institute of Neuroscience press release, Nasty noises: Why do we recoil at unpleasant sounds?, published on 10^th October 2012, accessible at http://www.ncl.ac.uk/ion/news/news/item/nasty-noises-why-do-we-recoil-at-unpleasant-sounds-copy)

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