By Emma Young
No one likes the sound of someone else chewing or drinking. But for some people, it’s enough to cause overwhelming feelings of anger or disgust — and in some cases, send them into a violent rage. People with “misophonia” (literally a hatred of sounds) over-react to some common everyday “trigger sounds” — typically, sounds made by another person. Though the phenomenon has been well documented, exactly what causes it hasn’t been clear. Now a new paper in the Journal of Neuroscience provides a compelling explanation: that misophonia isn’t related to hearing so much as to an “over-mirroring” of someone else’s physical actions. The team, led by Sukhbinder Kumar at Newcastle University, thinks that this excessive mirroring causes anger in some sufferers, and anxiety and distress in others.
So-called “mirror neurons” in the premotor cortex were first discovered in monkeys. Groups of these neurons fire when a monkey either performs a specific action, like opening a peanut shell, or watches another monkey doing the same action. This system is thought to aid learning from others, and also the coordination of actions, so that a troop can hunt together effectively, for example. In people, there is evidence for mirror systems for movement and also emotion. (I should note, though, that some researchers dispute the strength of this evidence.)
Misophonia has been thought of as a disorder of sound emotion processing — that sufferers over-react emotionally to trigger sounds. However, Kumar and his colleagues observed that most trigger sounds involve physical movements of the mouth and face. They wondered whether this might be important for understanding the disorder.
The team used fMRI to look at activity in the brains of 33 people with and without misophonia while they were in a “resting state” — when they were not actively engaged in doing anything in particular. They also analysed existing fMRI data on 42 people with and without misophonia while they listened to three categories of sounds: known trigger sounds (including gulping, slurping, eating food and sniffing); other generally unpleasant sounds (such as the sound of a baby crying or a dentist’s drill); and neutral sounds (like rain or a helicopter).
A number of intriguing differences emerged between sufferers vs non-sufferers. While in a resting state, people with misophonia showed stronger functional connectivity between the auditory and visual cortex and the ventral premotor cortex (functional connectivity is a measure of how closely correlated brain activity is in different regions). This latter region includes the orofacial motor area, which is responsible for movements of the face and mouth region. While listening to sounds of any kind, people with misophonia also showed stronger connectivity between the auditory cortex and the orofacial motor area. Finally, in response to known trigger sounds, they showed stronger activation of the orofacial motor area.
The fact that the misophonia group showed differences specifically involving facial motor regions points to misophonia being caused by aberrations in the motor mirror system — by excessive mirroring of someone else’s facial movements — rather than abnormal sound processing, per se. “Our results support a model of misophonia based on ‘hyper-mirroring’ of the orofacial actions of others with sounds being the ‘medium’ via which action of others is excessively mirrored,” the team explains.
It’s a fascinating idea. And it sits neatly with other work on different kinds of hyper-mirroring, such as “mirror pain” and “mirror touch“. However, people with these conditions feel at least some pain or touch when they see someone else being hurt or being touched, and these experiences don’t tend to make them angry. Why would hyper-mirroring of someone else’s mouth movements make misophonia sufferers so distressed, or induce rage?
The team suggests that if they’re not aware of what’s actually going on (and mirror pain would be a lot more noticeable than facial movement mirroring), someone with misophonia could experience a sense of a loss of control over their own actions and goals. “Since anger can be described as perceived goal interference, the drive to imitate others would make anger (or aggression) the dominating emotional response,” the team writes.
Clearly, there’s more to explore here. But this new conception of misophonia could have immediate practical impacts. For some sufferers and their families, misophonia is a very serious problem. It can lead to social isolation and even suicide. If sufferers could be trained to associate trigger sounds with the sound sources rather than orofacial movements specifically, perhaps this might work as an effective treatment.