NEWCASTLE, England, Oct. 12 (UPI) -- Increased activity between two separate parts of the brain explains why the sound of chalk on a blackboard is so unpleasant, British researchers say.
Newcastle University scientists said the interaction between the region of the brain that processes sound, the auditory cortex, and the amygdala, which is active in the processing of negative emotions, is heightened when people hear unpleasant sounds.
The amygdala modulates the response of the auditory cortex and the amplified activity provokes our negative reaction, they said.
"It appears there is something very primitive kicking in," lead Newcastle researcher Sukhbinder Kumar said. "It's a possible distress signal from the amygdala to the auditory cortex."
Researchers used magnetic resonance imaging to examine how the brains of 13 volunteers responded to a range of sounds they rated from the most unpleasant -- the sound of a knife on a bottle -- to the most pleasing, the sound of bubbling water.
The researchers were 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 the perception of a highly unpleasant sound is heightened as compared to a soothing sound, they said.
Anything in the frequency range of around 2,000-5,000 Hertz was found to be unpleasant, Kumar said.
"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," he said.