A knife scraping against a glass bottle is the most unpleasant sound for most human beings, researchers from the Newcastle University and Wellcome Trust Centre for Neuroimaging at UCL, both in England, reported in the Journal of Neuroscience.
When activity between the auditory and emotional parts of the brain increases, it may cause a sensation of revulsion or disgust at a sound, as typically occurs to most of us when we hear somebody scrape chalk or their nails on a blackboard.
When we hear unpleasant sounds, the auditory cortex and the amygdala interact more intensely and process the negative emotions. The amygdala is a small almond shaped part of the brain that processes our emotions and aggression. It also controls fear responses and forms emotional memories.
The scientists used brain imaging to see what goes on in the brain when we are exposed to unpleasant sounds. The imaging showed that when we hear something we don’t like, the amygdala becomes much more active. They believe it processes the data from the auditory nerve in such a way as to provoke a negative reaction.
Study author, Dr Sukhbinder Kumar, said, “it appears there is something very primitive kicking in. It’s a possible distress signal from the amygdala to the auditory cortex.”
Study leader, Professor Tim Griffiths, of Newcastle University, and colleagues used fMRI (functional magnetic resonance imaging) to find out how the brains of 13 participants responded to a variety of different sounds.
A knife scraping against a bottle versus the sound of bubbling water
The volunteers listened to the sounds while inside the scanner and rated them from most unpleasant to the most pleasing (least unpleasant). The sound of a knife scraping against a bottle was the most hated sound, while bubbling water was rated the most pleasing. The scientists studied the brain responses to each type of noise.
There appeared to be a correlation between the type of sound the participants heard and the levels of activity in the amygdala and the auditory cortex – the activity varied according to the ratings of the sounds. The higher the activity, the greater the revulsion.
The amygdala, which is the emotional part of the brain, appears to take charge and modulate the activity of the auditory part of the brain, making our perception of a very disagreeable noise feel even worse, compared to nice sounds, such as bubbling water or applause.
Unpleasant sounds are between 2,000 to 5,000 Hz frequency range
After analyzing all the sounds the participants liked and hated, and placing them in order of unpleasantness, the researchers found that disagreeable sounds tend to be between the frequency range of about 2,000 to 5,000 Hz.
Dr. 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.”
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