Neuroscientists have analyzed how people react when they listen to a range of different sounds, the aim being to establish the extent to which repetitive sound frequencies are considered unpleasant. Their results showed that the conventional sound-processing circuit is activated but that the cortical and sub-cortical areas involved in the processing of salience and aversion are also solicited. This explains why the brain goes into a state of alert on hearing this type of sound.
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Source: ScienceDaily