Squeaking chalk triggers alarm in the brain

Emotionszentrum reacts particularly strong at high and little fluctuating T nen

Chalk (stick-slip effect) © Jean-Jacques MILAN / CC BY-SA 3.0
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The squeaking of chalk on a blackboard or the scratching of a knife on glass make most people extremely uncomfortable. Why this is so and what is happening in the brain, now British and German researchers have enlightened. With the help of brain scans, the scientists found that especially tones between 2, 000 and 5, 000 hertz - a high beeping sound - trigger an alarm signal in the brain. In this pitch are also high screeching and screaming, which often indicate a danger in nature, the researchers report in the journal "Journal of Neuroscience". If we perceive such sounds, this triggers an alarm signal in the brain. This makes our hearing center even more sensitive to the potentially dangerous sound, but at the same time instinctively causes negative feelings: we jerk back, perhaps even get goose bumps and would rather cover our ears.

"There is something very primitive about this reaction, " says lead author Sukhbinder Kumar of Newcastle University, who collaborated with colleagues from the Wellcome Trust Center for Neuroimaging in London and the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, In the case of the sounds we perceive as unpleasant, the brain center responsible for emotions, the amygdala, engages directly. It then takes over the control of the auditory cortex and directly affect our sense of hearing such high frequency squeaks.

The new findings could also help clarify why, for example, people with migraine or autism are often particularly sensitive to noise. It is possible that the amygdala reacts very strongly to them, prematurely triggering the immune response and hypersensitivity.

From screaming to water splashing

For their study, the researchers had auditioned thirteen subjects for 74 different, short sounds - from scratching a knife or fork to glass to the scream of a woman, to rather pleasant noises such as the lapping of water. The participants were meanwhile in a magnetic resonance tomograph, this brain scanner recorded the activity of different brain areas. After each tone, participants should indicate on a scale of 1 to 5 how pleasant or unpleasant they felt the sound.

Of the 74 noises, the participants rated the noise of a knife or a fork on glass the most negative. Close behind in third place lay the notorious squeak of chalk on a blackboard. In sixth place lay the screeching of a woman, at the eighth the squeaking of bicycle brakes and at least the ninth cry of a baby. On the other hand, the subjects rated baby laughter, applause and, strangely, thunder as well. The analysis of the acoustic properties of these sounds revealed a clear pattern: "Tons that were perceived as very negative had high frequencies and only slight fluctuations in the course of sound, " the researchers write. The most pronounced negative reactions were at high temperatures between 2, 000 and 5, 000 hertz. display

The analysis of the brain scans have also shown a clear connection, the scientists report: The higher the tone and the lower the modulation, the stronger the reaction of the amygdala in the right brain half had failed. This emotional center processes the acoustic properties of the sound as well as its possibly threatening meaning. It ultimately triggers the instinctive defense reaction. This explains why, in the case of sounds that subjectively rated subjects as unpleasant, their amygdala was also particularly active. (Doi: 10.1523 / JNEUROSCI.1759-12.2012)

(Journal of Neuroscience, October 16, 2012 - NPO)