Harsh noises, such as car alarms, voices from construction sites and even human screams, are hard to ignore, mainly because they are unpleasant.
However, what happens to our brains when we hear such sounds? Why do we feel so unbearable?
A group of researchers from Geneva University and Geneva University Hospital in Switzerland tried to answer these questions in a recent study.
Why is this question so important? In their research paper published in the journal Nature Communication, the researchers explained that this is related to all aspects of communication.
The researchers wrote: "The primary purpose of communication is to attract the attention of the same kind, and this process can be optimized by adjusting the saliency of the signal, thus maximizing the sensory motor response of the receiver."
In neuroscience, salience is the quality that distinguishes things from similar things. The researchers pointed out in the paper: "In order to enhance the meaning of perception and ensure the effective response of the receiver, a common strategy is to increase the signal strength, such as screaming or shouting."
They added: "However, when we raise the sound level, the signal strength is not the only parameter that will change. Another important new feature is roughness, which is an acoustic texture caused by rapidly repeated acoustic transients. "
Therefore, in their research, scientists first determined the range of sounds, which are "rough" and unpleasant to the human brain. Then, they observed these brain regions activated by noise.
When did the noise become "unbearable"? The researchers recruited 20 healthy participants aged between 20 and 37, of whom 15 were women. The researchers conducted different experiments on different groups of participants.
In some of these experiments, researchers played repetitive sounds with frequencies between 0 and 250 Hz to participants. They also play these sounds at shorter and shorter intervals to determine when some of them become unpleasant.
Luc Arnal, one of the researchers, said: "We asked participants when they felt the sound was rough (different from each other) and smooth (forming a continuous single sound).
The research team found that when the stimulus reaches about 130 Hz, the upper limit of rough sound will appear. Anal explained: "Beyond this limit, the frequency people hear can only form a continuous sound."
In order to know exactly when rough sounds become unpleasant, the researchers also asked participants to rate these sounds from 1 to 5 when listening to different frequencies, where 5 means "unbearable".
Anal pointed out: "The sound that is considered unbearable is mainly between 40 and 80 Hz, that is, in the frequency range used by sirens and people's screams (including babies' screams). "
These unpleasant sounds can be perceived by human beings from a distance and really attract our attention. "That's why alarms use these rapid repetition frequencies to maximize the chances of detecting them and attracting our attention," Arnal added.
The researchers explained that when the repetition frequency of auditory stimuli exceeds every 25 milliseconds, the human brain cannot expect different stimuli and regard them as continuous noise that cannot be ignored.
Harsh sounds can trigger disgusting brain regions. When researchers monitored brain activity to find out why the brain found these rough noises so unbearable, they found something unexpected.
When participants heard sounds that exceeded the upper limit of roughness (above 130 Hz) and those that participants thought were particularly uncomfortable (40 to 80 Hz), the researchers monitored their brain activities.
In the former case, the researchers found that only the auditory cortex of the upper temporal lobe is active, which is the "traditional auditory circuit" observed by Megevand.
However, when participants heard sounds in the range of 40-80 Hz, other brain regions became active, which surprised the researchers.
Anal said: "These sounds will stimulate the amygdala, hippocampus and insula, which are all related to salience, disgust and pain. This explains why participants find it unbearable. "
"We finally understand why the brain can't ignore these sounds. Some things happen at these frequencies, and many diseases show atypical brain reactions to 40 Hz sound. These include Alzheimer's disease, autism and schizophrenia. "
In the future, researchers plan to conduct a more detailed study of the network in the brain that responds to harsh sounds. They hope to find out whether it is possible to detect specific nervous system conditions by monitoring the brain's response to specific sounds.