The sound of people chewing, slurping, tapping, or humming can drive some people into a rage, and now scientists have discovered the neurological wiring responsible for this strange condition.
Called misophonia, it describes the unreasonable emotions that well up inside some of us when we hear certain repetitive noises being produced by those around us. People with this condition experience annoyance or even anger at the clacking of a keyboard, the rustling of a chip packet, or the smacking of lips.
While it's been recognised as a condition since 2000, research into its cause and its prevalence has been limited. There are no official criteria in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), and those who experience it often find it difficult to be taken seriously.
But a study published in the Journal of Clinical Psychology in 2014 suggested that it could affect as much as 20 percent of the population, while a 2015 study in Australasian Psychiatry argued that it was associated with obsessive compulsive disorder and anxiety, and could potentially be considered a disorder in its own right.
Now, a team led by researchers from Newcastle University in the UK has found evidence of changes to the brain's frontal lobe that could account for the emotional response triggered by sounds in those with misophonia.
A test group of 20 volunteers who said they experienced the condition listened to neutral, repetitive sounds, such as a boiling kettle; annoying sounds such as a baby's cry; and 'triggering' sounds, including breathing noises or loud chewing.
Their neurological and physiological responses were compared with those from a control group of 22 volunteers who felt they didn't have misophonia.
Neither group reacted much to the neutral or annoying sounds. When it came to the 'triggering' sounds, however, those in the test group experienced significantly increased heart rates and skin conductivity.
Brain scans also revealed a marked difference in the subjects' neurology. In those with misophonia, the triggering noises correlated with increased activity in various regions of the brain, including the frontal lobe and the anterior insular cortex (AIC).
The AIC is buried deep in the fold separating the frontal lobe and parietal lobe from the temporal lobe of the brain. It's responsible for a bunch of mediation tasks, including managing emotional experience. It also plays a role in integrating signals from the outside world with information inside the body.
While the triggering sounds also sparked a reaction in the AIC of those without misophonia, the fact there was no marked increase in the activity of areas such as the frontal lobe indicates a higher level of control between the two parts of the brain.
Those with misophonia not only had increased AIC and frontal lobe activity, but also in the ventromedial prefrontal cortex (vmPFC), hippocampus, and amygdala. Measurements taken of the structure of the vmPFC indicated they had thicker insulating myelin sheaths, which helps nerves carry messages.
Taken together, the evidence suggests that those with misophonia have brains that struggle to control the spread of messages associated with certain sounds.
While we all might feel a twinge of bother, having misophonia turns an annoying sound into an enraging experience, as it spreads through different parts of the brain associated with 'fight or flight' responses.
Team leader Sukhbinder Kumar described the impact of their discovery: "For many people with misophonia, this will come as welcome news, as for the first time, we have demonstrated a difference in brain structure and function in sufferers."
Sadly for those with misophonia, the discovery doesn't come with an easy fix. It might help the rest of us sympathise, however, and consider chewing with our mouths closed.
This research was published in Current Biology.