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Once in this state, how does it sort through external stimuli, particularly sounds? Scientists from the University of Geneva (UNIGE) and the Institut Pasteur analyzed the brain's response to so-called "rough" sounds such as screams or alarms.
The team discovered that these sounds are systematically processed unlike other sounds, and they generate a specific brainwave. These findings, published in the journal Scientific Reports, help better understand certain perception disorders, such as hyperacusis—hypersensitivity and/or intolerance to certain sounds—as well as the impact of nighttime auditory stimulation on brain function.
Roughness is an acoustic characteristic defined by rapid modulations in sound intensity, between 40 and 100 times per second. "Unlike speech, whose syllabic rhythm varies between 4 and 8 Hz, rough sounds bombard the auditory system at high frequency, creating a sensation of harshness that is often unpleasant," explains Luc Arnal, a researcher at the Institut Pasteur who co-led this work.
"This property, typical of alarm sounds but also of human screams and infant cries, is precisely what makes them effective: they automatically capture our attention to signal imminent danger." These sounds directly activate the amygdala, a brain region involved in emotional reactions and attention.
It is essential to decipher how sounds affect our brain activity during sleep and, consequently, the physical and mental health of each of us.
Different sounds tested during sleep
While the effect of sound roughness on the awake brain is now well explored, what happens during sleep was still largely unknown. "Our investigations are not only central to understanding certain pathologies like hyperacusis but also to describing and explaining the very serious impact of the sound environment on health, particularly at night," emphasizes Sophie Schwartz, full professor in the Department of Fundamental Neurosciences at the UNIGE Faculty of Medicine and co-director of the study.
Neuroscientists recruited 17 volunteers who agreed to sleep in specially equipped rooms to measure their brain activity during sleep via electroencephalography (EEG). "We then played various types of human screams and 'fake screams' at low volume, varying pitch and roughness, to provoke a brain reaction without fully waking them," details Guillaume Legendre, a researcher in Sophie Schwartz's team and the study's first author. "And it is indeed the roughness of the sounds that acts on the brain's alert systems, regardless of whether they are high- or low-pitched."
The research team also observed two other phenomena. First, rough sounds systematically trigger a response in the brain, unlike other sounds. Second, sound roughness correlates with an increase in the power of sleep spindles (or spindles). "These are brief brainwaves that the brain generates in response to a disturbance, in this case an auditory stimulus, during sleep," explains Guillaume Legendre.
Triggers of intense emotions
"Sound roughness is a property that is usually rare in the environment, reserved for humans and animals to send an emergency message," says Luc Arnal. "However, overexposure to these sound frequencies triggers very different emotional reactions depending on the individual, ranging up to completely irrational and even potentially violent reactions."
More broadly, this work helps better characterize the emotional circuits linked to sounds involved in psychiatric or neurological pathologies: hyperacusis, tinnitus, but also epilepsy or Alzheimer's disease, where patients sometimes exhibit abnormalities in auditory processing.
"Moreover, we live in increasingly noisy environments, especially in urban areas. In this context, it is essential to decipher how sounds affect our brain activity during sleep and, consequently, the physical and mental health of each of us," concludes Sophie Schwartz.