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People who still believe the outdated notion that mental health conditions are "all in a person's head" have yet another reason to stop believing the myth: According to a new study in the journal Current Biology, those with anxiety perceive the world differently -- and it stems from a variance in their brains.

It all comes down to the brain's plasticity, or its ability to change and reorganize itself by forming new connections. These inherent changes in the brain dictate how a person responds to stimuli, and researchers from the Weizmann Institute of Science in Israel found that people diagnosed with anxiety are less likely to be able to differentiate neutral or "safe" stimuli from threatening ones.

The scientists found that those with anxiety experienced lasting plasticity long after an emotional experience (aka a "stimulus") ended. This means the brain was unable to distinguish new, irrelevant situations from something that's familiar or non-threatening, resulting in anxiety. In other words, anxious individuals tend to overgeneralize emotional experiences, whether they are threatening or not.

Most importantly, researchers noted, this reaction is not something that an anxious individual can control, because it's a fundamental brain difference.

For the study, researchers trained individuals to associate three specific sounds with one of three outcomes: money loss, money gain or no consequence. In the next phase of the study, participants listened to approximately 15 tones and were asked to identify whether or not they'd heard them before.

The best way to "win" the tone-identifying game was for participants to not confuse or overgeneralize the new sounds with the ones they heard in the first phase of the study. The study authors found that subjects with anxiety were more likely than non-anxious subjects to think a new sound was one that they heard earlier.

This occurrence wasn't due to an impairment in learning or hearing. It happened because they perceived the earlier tones, which were linked to an emotional experience of money loss or gain, differently than the other participants.

Researchers also discovered that during the exercise, people with anxiety displayed differences in the amygdala, the region of the brain that's associated with fear. The findings may explain why the disorder develops for some people and not others, according to the authors.

"Anxiety traits can be completely normal, and even beneficial evolutionarily. Yet an emotional event, even minor sometimes, can induce brain changes that might lead to full-blown anxiety," lead researcher Rony Paz said in a statement.

The new research is a sound reminder that a person is hardly responsible for having a mental illness; surmounting evidence shows mental health conditions have genetic and physiological underpinnings. A 2015 study found that anxiety may be hereditary, while other research suggests depression may be an inflammatory disease.