Studies on Human Responses to Sound
Have you ever wondered why harsh noises from an alarm or a loud screeching sound will immediately get your attention?
Neuroscientists from the University of Geneva (UNIGE) and Geneva University Hospitals (HUG) in Switzerland have been studying how people respond when they hear a scope of different sounds. The main objective of the study was to determine which repetitive noise frequencies were deemed unpleasant.
The neuroscientists also analyzed regions in the brain that were activated when hearing these frequencies. Results revealed that the traditional sound-processing area became activated. It also showed that the cortical (part of the cerebral cortex), the subcortical areas that are associated with processing salience, and aversion are sought out. This is why the brain becomes alert when hearing this type of sound.
Alarms have repetitive sound variations that range between 40 to 80 Hz. To determine why these frequencies are perceived to be dangerous and intolerable, repetitive sounds between 0 and 250 Hz were played for 16 test subjects. The sounds were played closer and closer together to figure out which frequencies the brain finds unbearable. Test subjects were asked which sounds they recognize as sounding rough (distinct from each other) and when they are recognized as sounding smooth (producing one continuous single sound).
The scientists confirmed that the higher limit of sound roughness is approximately 130 Hz. Any sounds above these frequencies are heard as one continuous sound.
Neuroscientists asked test subjects to listen to a variety of frequencies. They were to classify the sounds on a scope of 1 to 5, where 1 was considered bearable and 5 was considered unbearable. Sounds that were regarded as intolerable ranged between 40 to 80 Hz. This is the range that alarms and human screams fall under. These frequencies can be heard from a long distance. From a survival point of view, it's important that these sounds can be heard in order to gain the attention of others. Noises that occur in rapid succession cannot be anticipated or suppressed by the brain. As long as the noises continue, those who hear them are steadily alert and aware of the stimulation.
The Science of Harsh Sounds
Analysts discovered what happens to the brain when it comes in contact with harsh sounds. They recorded brain activity and how it reacted to the sound.
When continuous sound is recognized, the auditory cortex is triggered. This is the typical route that is used in order to hear. When sounds are recognized as harsh (between 40 to 80 Hz), they bring about an unrelenting response that uses up areas of the brain that are not part of the normal auditory system. These noises seek out the amygdala, hippocampus, and insula, which are all areas connected to salience, aversion, and pain. This explains why test subjects perceived the sounds as being unbearable.
This is the first case study to demonstrate that sounds between 40 and 80 Hz have been shown to organize these neural structures, but the frequencies have been used for years in alarm systems.
There have also been a number of illnesses that show abnormal responses from the brain to sounds that reach 40 Hz. These diseases include Alzheimer's, autism and schizophrenia. Neuroscientists plan to study the neural structures that are triggered by these frequencies in order to find out if it's possible to identify these illnesses early by seeking the circuit that is stimulated by the sounds.
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