The sound of someone munching on an apple or slurping soda through a straw is enough to make certain people see red.
Known medically as misophonia – literally "hatred of sounds" – the "common disorder" can trigger anger, anxiety or disgust in response to everyday sounds such as people eating, drinking or even breathing.
With the disorder "devastating" for some, scientists from Newcastle University set out to uncover how these intense reactions come about.
The team analysed the MRI brain scans of people with and without misophonia, during both a "resting state" and after being "sound evoked".
Results – published in The Journal of Neruoscience – suggest misophonia is brought about by "supersensitised" connections between different regions of the brain.
With misophonia relatively misunderstood, a fuller grasp of the disorder could have "important consequences for devising effective therapies".
"Our findings indicate that for people with misophonia there is abnormal communication between the auditory and motor brain regions – you could describe it as a 'supersensitised connection'," said study author Dr Sukhbinder Kumar.
"This is the first time such a connection in the brain has been identified for the condition."
More than one in 10 (15%) adults are thought to endure misophonia, with many patients "written off as being grouchy, cranky or irritable".
Prior to the Newcastle study, misophonia was thought to be "a disorder of sound-emotion processing".
The scientists noticed, however, most of the "trigger sounds" relate to the face and mouth, like chewing. They therefore wondered whether the nerve cells that underly these movements may be to blame.
To learn more, the team analysed the brain scans of 33 people – made up of both misophonia patients and "controls" – while they were in a "resting state". These were compared against the "sound evoked" MRI responses of 42 patients and controls.
Watch: 'Sound hatred' a genuine problem, says UK scientists
The results reveal the misophonia patients and control volunteers had "no difference in auditory cortex responses to trigger sounds".
Misophonia was linked to "stronger connectivity" between the brain regions involved in hearing and seeing relative to that which controls facial movements, however.
The connectivity between the so-called auditory cortex and "facial motor area" was also stronger "during sound perception".
In addition, the patients' facial motor area was more strongly activated "in response to trigger sounds", the results show.
"Misophonia is therefore not an abreaction to sounds...but a manifestation of activity in parts of the motor system involved in producing those sounds," wrote the scientists.
The team believes the "action" of the "trigger-person" is "mirrored" by the misophonia patient, causing their discomfort, with the sound being just a "byproduct of that action".
The so-called mirror system "helps us process movements made by other individuals by activating our own brain in a similar way – as if we were making that movement ourselves," said Dr Kumar.
"We think in people with misophonia, involuntary over-activation of the mirror system leads to some kind of sense that sounds made by other people are intruding into their bodies, outside of their control," he said.
"Interestingly, some people with misophonia can lessen their symptoms by mimicking the action generating the trigger sound, which might indicate restoring a sense of control.
"Using this knowledge may help us develop new therapies for people with the condition."
Misophonia patients are generally advised to keep a diary of their triggers, to help them identify and avoid noises they find uncomfortable. These sounds could also be masked via background music. Some may also benefit from therapy.
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