The screech of chalk on a blackboard or running a knife on a bottle are some of the most unpleasant sounds in the world. An Indian-origin neuroscientist says this is because of heightened activity between the brain’s emotional and auditory regions. Sukhbinder Kumar, the co-author of a study from Newcastle University, reveals how unpleasant sounds heighten the interaction between two brain regions, the auditory cortex processing sound and the amygdala — set neurons in brains, which is active in the processing of negative emotions. “It’s a possible distress signal from the amygdala to the auditory cortex,” Sukhbinder says, the Journal of Neuroscience reports.
Researchers used functional magnetic resonance imaging (fMRI) to examine how the brains of volunteers responded to a range of sounds. 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. This process enabled researchers to study the brain response to each type of sound.
Dr Samir Dalwai, Developmental Pediatrician, explains, “The brain as a part of the sensory system, selectively shuts out some sounds and brings others to the forefront.” While the unhappy reaction to some sounds is a common trait in most adults, a more adverse reactions to sounds is often due to a sensory processing disorder, and particularly in autistic children, he says. They may even develop a repetitive and obsessive behaviour, as a result of this delicate sensory system.
The amygdala modulates the activity of the auditory part of the brain heightening our perception of a highly unpleasant sound, such as a knife on a bottle, as compared to a soothing sound of bubbling water. Analysis of the acoustic features found that sounds in the frequency range of 2,000 to 5,000 Hz were mostly voted as unpleasant by volunteers. Sukhbinder adds, “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.”