Certain sounds, such as a car horn or a human shriek, are jarring to the listener. Alarm sounds, too, are characterised by repetitive sound fluctuations, which are usually situated in frequencies within a certain range. But when a sound is heard within these frequencies, what happens in the brain to hold the listener’s attention to such an extent?
Neuroscientists from the University of Geneva (UNIGE) and Geneva University Hospitals (HUG) have now sought to analyse what goes on in the brain when people hear these frequencies. Their results, published in Nature Communications, have identified which frequencies are perceived as rough.
The researchers conducted experiments with participants who were made to listen to various sounds with frequencies between 0 and 250 Hz. “We then asked participants when they perceived the sounds as being rough (distinct from each other) and when they perceived them as smooth (forming one continuous and single sound),” researcher Luc Arnal said in a statement released by UNIGE.
The researchers were able to establish that the upper limit of sound roughness is around 130 Hz, while the sounds considered intolerable were mainly between 40 and 80 Hz. When sounds are in the smooth range, the conventional auditory system is activated. But when sounds are perceived as harsh (especially in the 40-80 Hz range), they induce a persistent response that additionally recruits a large number of cortical and sub-cortical regions that are not part of the conventional auditory system. These regions are related to aversion and pain, Arnal said in the university statement.
This is the first time that sounds between 40 and 80 Hz have been shown to mobilise these neural networks, although these frequencies have been used for a long time in alarm systems.