Have you ever experienced the shrieking sound of an unexpected car alarm? How about the ear splintering beep of a smoke detector after leaving dinner in the oven for too long? We understand these alerts to be necessary, but why are these loud noises chosen to warn us of danger or grab our attention? These sounds do more than just jar us out of a deep slumber according to research conducted by neuroscientists from the University of Geneva (UNIGUE) and Geneva University Hospitals (HUG) in Switzerland. After analyzing the brain’s reaction to certain frequencies, scientists have discovered why these particular frequencies are used to alert the brain to danger, and exactly how we are conditioned to find these noises painful or unpleasant.
Alarm sounds, such as car horns or human screams, generally fall within frequencies between 40hz and 80hz and are characterized by “repetitive sound fluctuations”, but why are these frequencies chosen to alert us of danger? Researchers at UNIGE and HUG sought to find the answer by playing repetitive sounds between 0 and 250hz to 16 participants, moving closer and closer together in frequencies to identify the sounds that your brain will find unbearable. Luc Arnal, a researcher in the Department of Basic Neurosciences in UNIGE’s Faculty of Medicine, explained the process, “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).” Based on their responses, researchers were able to pinpoint the upper limit of sound roughness around 130hz. “Above this limit,” notes Arnal, “the frequencies are heard as forming only one continuous sound.” Sounds that fell under 130hz, specifically between 40-80hz (the range in which car alarms and human screams fall), were perceived as intolerable.
Researchers had found which frequencies were most distinct from one another, but why are these “rough” frequencies perceived as unpleasant to our brains? Using an intracranial EEG, a device that records brain activity inside the brain itself in response to sounds, neurologists discovered that these harsh sounds do not fall under our conventional auditory system. “When frequencies are above 130hz, they are perceived as continuous and activate your conventional auditory system: the auditory cortex in the upper temporal lobe,” explains Pierre Mégevand, a neurologist and researcher in the Department of Basic Neurosciences at the UNIGE Faculty of Medicine and at HUG. When sounds that are perceived as harsh (falling between 40-80hz) are observed, researchers had found that they provoke responses from sections of the brain that are not part of the conventional auditory system. “These sounds solicit the amygdala, hippocampus and insula in particular, all areas related to salience, aversion, and pain. This explains why participants experienced them as being unbearable,” Arnal explains, the first time that sounds between 40 and 80hz have been found to invoke such a response in these neural networks. “We now understand at last why the brain can’t ignore these sounds.”