Imagine chalk scraping against a blackboard, or teeth against a fork — why do we cringe at these sounds?

In a new study, scientists reveal what is actually going on in the brain to make us recoil at certain noises.

Brain imaging shows that when we hear an unpleasant noise, the amygdala (active in processing emotions) adjusts the response of the auditory cortex (part of the brain that processes sound) which heightens activity and triggers a negative emotional reaction.

“It appears there is something very primitive kicking in,” said author Dr. Sukhbinder Kumar, who has a joint appointment at the Wellcome Trust Centre for Neuroimaging at UCL and Newcastle University. “It’s a possible distress signal from the amygdala to the auditory cortex.”

The researchers used functional magnetic resonance imaging (fMRI) to watch how the brains of 13 participants responded to a variety of sounds. Volunteers listened to noises inside the scanner and then rated them from the most unpleasant—the sound of a knife on a bottle — to the most pleasing — babbling water.

Researchers were then able to study the brain response to each type of sound.

The results showed that the activity of the amygdale and the auditory cortex varied in direct relation to the negative ratings given by the participants.

It appears that the emotional part of the brain, the amygdala, takes charge and modulates the activity of the auditory part of the brain so that our perception of a negative noise is heightened in comparison to a soothing sound, such as a babbling brook.

Acoustic analysis found that anything in the frequency range of around 2,000 to 5,000 Hz was perceived as 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,” said Kumar.

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 (literally a “hatred of sound”) 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,” said study leader Tim Griffiths, Ph.D., from Newcastle University.

The study is published in the Journal of Neuroscience.

Source: University College London