That Ear-splitting Screech
Why do we recoil at the sound of chalk or fingernails on a blackboard, or the sound of a knife on a bottle?
A team of scientists from Newcastle University found that this is the result of heightened activity between the emotional and auditory parts of our brains. Their findings were published recently in Journal of Neuroscience. They found that humans find such sounds unpleasant because of 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 when we hear unpleasant sounds.
A total of 13 volunteers participated in the study. The researchers, using functional magnetic resonance imaging (fMRI), examined how the brains of volunteers responded to a range of sounds. The participants were asked to rate these sounds, from the most unpleasant to the most pleasing. Researchers then studied the brain response to each type of sound. The sound of a knife on a bottle was found to be the most unpleasant, and the sound of bubbling water, the most pleasing.
The researchers found that the activity of the amygdala and the auditory cortex varied in relation to the ratings of perceived unpleasantness given by the subjects. The amygdala was found to take charge and modulate the activity of the auditory cortex so that our perception of an unpleasant sound was heightened, in comparison to a pleasant one. When the acoustic features of the sounds were analysed, it was found that those in the frequency range of around 2,000 to 5,000 Hz were found to be unpleasant. Our ears are known to be most sensitive at this range.
According to the researchers, the reason that the amygdala modulates the activity of the auditory cortex, heightening our perception of an unpleasant sound, could be an evolutionary tool kicking in—in all likelihood the amygdala is sending a distress signal to the auditory cortex.
It is believed that the findings will enable a better understanding of the brain’s response to noise. Scientists involved in the study also claim that this will also help us understand medical conditions where people have decreased sound tolerance, and even autism, which entails high sensitivity to noise.