Why is the brain disturbed by hard sounds?

Why are the harsh sounds of alarms or human cries attracting our attention? What happens in the brain when it detects these frequencies? Neuroscientists from the University of Geneva (UNIGE) and University Hospitals of Geneva (HUG), Switzerland, analyze the reaction of people when they listen to a range of different sounds, the aim being to determine how repetitive sound frequencies are considered unpleasant. . Scientists also studied stimulated brain areas when listening to these frequencies. Surprisingly, their results, published in Nature Communications– showed not only that the conventional sound processing circuit is activated, but also that the cortical and subcortical areas involved in the treatment of salience and aversion are also solicited. This is a first and explains why the brain goes on alert when hearing this type of sound.

Alarm sounds, whether artificial (horn of a car) or natural (human cries), are characterized by repetitive sound fluctuations, usually located at frequencies between 40 and 80 Hz. But why these have frequencies been selected to indicate a hazard? And what is happening in the brain to hold our attention at this point? Researchers from UNIGE and HUG played repetitive sounds from 0 to 250 Hz to 16 more and more participants 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 homogeneous (forming a continuous and unique sound)", explains Luc Arnal, researcher in the Department of Fundamental Neuroscience # 39; University of Geneva. Medical School.

On the basis of the participants' responses, the scientists were able to establish that the upper limit of the roughness of the sound was around 130 Hz. "Above this limit", continues Arnal, "the frequencies are heard only as a only its continuous ". But why does the brain judge unpleasant raw sounds? In an attempt to answer this question, the neuroscientists asked the participants to listen to different frequencies that they had to rank on a scale of 1 to 5, 1 being bearable and 5 unbearable. "The sounds considered intolerable were mainly between 40 and 80 Hz, that is to say in the frequency range used by alarms and human cries, including those of a baby," explains Arnal. Since these sounds are noticeable at a distance, unlike a visual stimulus, it is essential to be able to capture attention from the point of view of survival. "That's why alarms use these fast repetitive frequencies to maximize the chances that they will be detected and get our attention," says the researcher. In fact, when repetitions are spaced less than 25 milliseconds apart, the brain can not anticipate them and therefore suppress them. He is constantly alert and attentive to the stimulus.

Hard sounds are not part of the conventional auditory system

The researchers then tried to find out what was really happening in the brain: why are these hard sounds so unbearable? "We used an intracranial EEG, which records the activity of the brain inside the brain in response to sounds," says Pierre Mégevand, neurologist and researcher at the Department of Fundamental Neuroscience of the Faculty of Medicine at the University of Montreal. UNIGE and HUG.

When the sound is perceived as continuous (greater than 130 Hz), the auditory cortex in the upper temporal lobe is activated. "This is the conventional circuit of hearing," says Mégevand. But when sounds are perceived to be hard (especially between 40 and 80 Hz), they induce a persistent response that also recruits a large number of cortical and subcortical regions that are not part of the conventional auditory system. "These sounds appeal to the amygdala, the hippocampus and the insula in particular, all areas related to salience, aversion and pain." That's why the Participants found them unbearable, "said Arnal, surprised to learn that these areas were involved in sound processing. .

This is the first time that sounds between 40 and 80 Hz mobilize these networks of neurons, although the frequencies have been used for a long time in the alarm systems. "We finally understand why the brain can not ignore these sounds," says Arnal. "Something special happens at these frequencies, and there are also many diseases that present atypical brain responses to sounds at 40 Hz. These include Alzheimer's disease, autism, and schizophrenia. . " Neuroscientists will now study the networks stimulated by these frequencies to see if it would be possible to detect these diseases sooner by soliciting the circuit activated by the sounds.