Why does our brain go on alert when it is disturbed by a nagging and loud noise like that of an alarm? Researchers from the University of Geneva have found the answer: according to them, these noises stimulate cortical areas linked to aversion and pain.
A fire alarm sounding, a jackhammer detonating, a child’s howl tearing the silence… These sounds have some points in common: they are loud and very unpleasant to the ear. Often, they also arouse in us a feeling of urgency and nervousness, even generate stress. But why do they have this effect on us?
Neuroscientists from the University of Geneva (UNIGE) and the University Hospitals of Geneva (HUG), Switzerland, have looked into the matter. In an article published in the journal NatureCommunicationsthey demonstrate that these are not only activating the conventional sound processing circuitry – like all other noises or voices that reach us – but that they also activate cortical and subcortical areas involved in the processing of aversion and pain.
Frequencies difficult to support
To reach this conclusion, the researchers recruited 16 volunteers and had them listen to repetitive sounds from 0 to 250 Hz, closer and closer to each other in order to define the frequencies that the brain finds unbearable. “We then asked the participants when they perceived the sounds as raw (distinct from each other) and when they perceived them as smooth (forming a continuous and unique sound)”, explains Luc Arnal, researcher in the Department of Fundamental Neurosciences of the UNIGE Faculty of Medicine.
Based on the responses of the participants, the scientists were able to establish that the upper limit of sound roughness is around 130 Hz. ‘one continuous sound.’
But why does the brain not support brutal sounds? To find out, the neuroscientists asked participants to listen to different frequencies which they had to rank on a scale of 1 to 5: 1 being bearable and 5 being unbearable. “The sounds considered intolerable were mainly between 40 and 80 Hz, that is to say in the range of frequencies used by alarms and human cries, including those of a baby”, explains Luc Arnal.
A way to better diagnose certain diseases
The researchers then tried to find out what is really going on in the brain: why are these harsh sounds so unbearable? “We used an intracranial EEG, which records cerebral activity inside the brain in response to sounds”, explains Pierre Mégevand, neurologist and researcher in the Department of Basic Neurosciences of the UNIGE Faculty of Medicine and at HUG .When the sound is perceived as continuous (above 130 Hz), the auditory cortex of the upper temporal lobe is activated. “It’s the conventional circuit of hearing”, underlines Professor Mégevand.
But when sounds are perceived as harsh (especially between 40 and 80 Hz), they also activate a large number of cortical and subcortical regions that are not part of the classical auditory system. “These sounds solicit the amygdala, the hippocampus and the insula in particular, all areas related to salience, aversion and pain. This explains why the participants experienced them as unbearable”, says Luc Arnal.
The latter admitted to being surprised that these regions are involved in sound processing. “We finally understand why the brain cannot ignore these sounds.” According to him, certain diseases such as Alzheimer’s, autism and schizophrenia produce these same atypical brain responses with sounds at 40 Hz. This discovery is therefore important to know if it is possible to detect these diseases earlier by soliciting the circuits activated by brutal sounds.
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