Why sounds can be unpleasant?
The screech of chalk on a blackboard or running a knife on a bottle are some of the most unpleasant sounds in the world, all because of the heightened activity between the brain's emotional and auditory regions, says an Indian-origin neuroscientist.
Sukhbinder Kumar, study co-author from Newcastle University, reveals how unpleasant sounds heighten the interaction between two brain regions, the auditory cortex processing sound and the amygdala - set neorons in brains, which is active in the processing of negative emotions.
"It appears there is something very primitive kicking in. It's a possible distress signal from the amygdala to the auditory cortex," Kumar says, the Journal of Neuroscience reports.
Researchers at the Wellcome Trust Centre for Neuroimaging at the University College of London and Newcastle used functional magnetic resonance imaging (fMRI) to examine how the brains of volunteers responded to a range of sounds, according to a Newcastle statement.
Listening to the noises inside the scanner they rated them from the most unpleasant - the sound of knife on a bottle - to pleasing - bubbling water. Researchers were then able to study the brain response to each type of sound.
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 as compared to a soothing sound, such as bubbling 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.
Kumar explains: "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."