(CNN) Though the sound of chewing may be unpleasant, most of us do not respond with a fight-or-flight reaction, a condition known as misophonia. A new study explored this phenomenon and discovered that, compared with most people, sufferers of misophonia have structural differences in the areas of their brains tasked with regulating emotions.
The research, published Friday in the journal Current Biology, also showed how trigger sounds can provoke a physiological response such as increased heart rate and sweating in people with misophonia -- a term coined only in 2001.
"Misophonia, as a term, means 'hatred of sound,' " said Sukhbinder Kumar, lead researcher and a research fellow in the Institute of Neuroscience at Newcastle University, England. "It shouldn't be taken to mean that people with misophonia hate all sounds. No, only certain specific sounds evokes a strong negative emotional response."
Common trigger sounds for people with misophonia include the small noises that occur when eating or drinking as well as repeated pen clicking, explained Kumar, who is part of a research team focused on how the brain processes emotion, particularly from sound.
"Misophonia was quite intriguing for us, because we know that some of the sounds, for example the sounds of nails on the blackboard, can easily evoke negative reactions, and most of us would find those sounds very annoying," he said.
However, since people with misophonia respond with extreme negativity when hearing "very normal" sounds, said Kumar, many essentially cannot lead a normal family life or interact normally in social situations. Maddened by everyday sounds, sufferers can be debilitated.
Due to difficulties reporting and diagnosing the condition, researchers don't know how many people have it.
Inspiration for the new study came when, after publishing a very popular study about how people in an MRI react to sounds, Kumar began to get emails from people describing their intense response to certain noises. Kumar did not immediately take these to heart, but the emails didn't stop and so could not be ignored.
For preliminary research, Kumar's lab invited four people to the clinic.
"We were really surprised how similar the symptoms were across people and how trigger signs were very similar," he said.
So for the next step, they developed a questionnaire to uncover more details. Ultimately, that led to the current study, a head-to-head comparison, so to speak, of people with the condition and those without.
The study examined three categories of sound, explained Kumar. The first included trigger sounds for people with misophonia: eating, breathing, drinking water. The second category included "normally annoying sounds, such as somebody screaming or sounds of a baby crying," he said. "And then we had a third category of sounds, a neutral sound such as the sound of the rain."
The researchers played all three categories of sounds for two separate groups of participants in an MRI brain scanner: 20 people who have misophonia and 22 people who do not. The researchers compared the brain activity between these two groups and measured heart rates and skin responses, said Kumar.
Hearing the usual annoying sounds, people with misophonia showed no differences in reaction from the group without this disorder.
However, listening to the trigger sounds, people with misophonia began to sweat, and their heart rates went up.
"When they hear these sounds, they have a very severe anxiety, and their usual tendency is to just leave the situation, basically," Kumar said.
Brain activity also divided one group from the other. For people with misophonia, "the trigger sounds evoked much larger activity in a part of the brain called anterior insula cortex," he said, adding that this region is mostly involved in the processing of emotions and how you feel.
Next, the researchers tried to find out how this area was connected to other parts of the brain. According to Kumar, "this region was more strongly connected to parts of the brain which are involved in recalling your past experiences": the ventromedial prefrontal cortex, which plays a role in decision-making; the posteromedial cortex, which helps regulate emotions; the hippocampus, which is involved in processing emotions; and the amygdala, which is involved in experiencing emotion.
"Kumar's research clarifies the neurological mechanisms involved in misophonia," said Miren Edelstein, a doctoral candidate at the University of California, San Diego who has researched the condition, though she did not participate in this study. "The work is methodically conducted and as a result has greatly advanced our understanding of an under-researched and debilitating condition."
According to Ewan McNay, a neuroscientist and associate professor in the department of psychology at the University at Albany, "It's quite possible that all individuals have a certain set of sounds they would respond strongly to. It's just that these individuals have a set of sounds that happen to be common everyday sounds."
The study is well-designed, said McNay, who was not involved in the research. "These folks were careful with the data." Still, he noted that the conclusion about functional connectivity in the brain may be "overdrawn."
Though the researchers definitely see a "heightened response" to a set of trigger sounds, what they don't show is how someone who does not have misophonia responds to what is for them an "equally aversive sound," said McNay. In other words: Could the same brain connectivity be seen when someone who doesn't have misophonia overreacts to, say, the sound of car tires in the rain?
McNay is not saying that each of us definitely has a trigger sound; however, each of us might, and that was not explored by the study.
David Baguley, president of the British Tinnitus Association and a professor of hearing sciences at the University of Nottingham and Anglia Ruskin University, said "the next stage in research has to be the design and validation of truly effective treatments. This study is an important step on that journey." He was not involved in the research.
Although Kumar and his colleagues plan to continue studying misophonia along with other disorders involving sound and emotion, an obvious question remains: What are the origins of misophonia?
It may be that the unique brain structure, resulting from biology or genetics, comes first, or this abnormal brain architecture follows early experiences, Kumar said. "It's a mystery. How it is acquired and how it develops, we don't understand."