Have you ever considered why loud sounds such as alarms or screams quickly make you take notice? Neuroscientists are curious, and they are analyzing how we listen to a range of different sounds. The central objective of the research is to examine the extent to which repetitive sound frequencies are considered to be unpleasant. The team is also exploring the areas inside the brain that receive stimulation when one is listening to these sounds. The results are explaining why the brain goes into a state of alert when hearing harsh sounds.
An artificial alarm such as a car horn, and a natural alarm such as a scream, consist of repetitive sound fluctuations which range between 40 and 80 Hz. Why do these frequencies signal danger? Furthermore, what is going on inside our brains to hold our attention to such an extent? The research team played repetitive sounds between 0 and 250 Hz to 16 study participants. Playing the sounds closer and closer together to identify frequencies that the brain finds unbearable. The study participants identified when they perceived the sounds as rough and separate from each other and when they were smooth and continuous.
The research team established the upper limit of sound roughness based on the responses of the participants. Above 130 Hz, the sound is a single continuous one. Now the question is, why does the brain recognize rough sounds to be unpleasant? The team urged participants to listen to different frequencies and classify them on a scale of 1 to 5, with one being bearable and five being unbearable. The unbearable sounds reside between 40 and 80 Hz, which are the frequencies that alarms, and human screams produce. Because these frequencies are perceptible from a distance, it makes sense that alerts use these tones to gain our attention.
The next task for the research team is finding the answer to why these harsh sounds are so unbearable. To find the answer, intracranial EEG is utilized to record activity within the brain. When sounds are continuous, above the 130 Hz level, the auditory cortex within the upper temporal lobe activates, which is the circuit for hearing. When the sounds are harsh, falling in the range of 40 to 80 Hz, they induce a response that brings in a high number of cortical and subcortical regions that are not part of the conventional auditory system. Areas within these regions, such as the amygdala, hippocampus, and insula, relate to aversion and pain. This recruitment response is the reason the participants experience the sounds as unbearable.
This research demonstrates that sounds between 40 and 80 Hz recruit these new neural networks as a response to harsh sounds like alarms. The research team plans to investigate the networks to determine if they can detect certain illnesses early by canvassing the circuit activated by the sounds. The research team is confident that their research may detect diseases in their early stages.