People with anxiety may not be able to distinguish between neutral and threatening stimuli as well as individuals without this mental health condition, a small study suggests.

Researchers designed a sound experiment to explore the brain's "plasticity," or its ability to change and respond to new situations. These changes influence how the brain reacts to stimuli and whether the person is able to distinguish between safe or dangerous circumstances, said lead author Rony Paz of the Weizmann Institute of Science in Rehovot, Israel.

"Our study suggests that people with anxiety cannot discriminate, at the most basic level, between stimuli that have an emotional content and similar mundane or daily stimuli," Paz said by email.

"This in turn might explain the anxious response that they exhibit to scenarios that seem regular, normal or non-emotional to anyone else - their brain cannot discriminate and responds as if it is the anxious stimulus," Paz added.

In the first part of the experiment, Paz and colleagues trained 28 volunteers with anxiety to associate three distinct tones with three outcomes - money loss, money gain, or no consequences.

Then, in the second phase, researchers played 15 different tones and asked participants to identify whether they had heard the sound during the first part of the experiment.

The goal was to see if people could avoid over-generalizing a new tone and mistaking it for a different sound they heard before.

But the people with anxiety were much more likely to make this mistake than 16 healthy controls who previously participated in a similar experiment, researchers report in the journal Current Biology.

Researchers also compared magnetic resonance imaging (MRI) scans of the anxious people to those of the healthy participants in the previous experiment.

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With anxiety, people had more activity visible in the amygdala, a brain region tied to fear and anxiety, and also in primary sensory regions of the brain, the study found.

The study is small, and more research involving more people is still needed to understand how or whether anxiety may directly cause shifts in how people perceive the world around them, Paz said.

It's possible, however, that the inability to distinguish sounds may extend to other senses, Paz added.

"Our world is comprised of simple features that build the complex picture," Paz said. "If we cannot discriminate these building blocks or simple features, we would not be able to discriminate the big picture."

The results also underscore that anxiety is a disease, not a voluntary way of perceiving the world, said Dr. Damiaan Denys, a psychiatry researcher at the University of Amsterdam who wasn't involved in the study.

"If you're anxious, you're hardly to blame," Denys said by email. "It is not a choice or a lack of free willpower."

In an ideal world, people would want to have less generalization and more ability to detect nuances differences in distinct situations, said Dr. Amit Etkin, a psychiatry researcher at Stanford University who wasn't involved in the study.

The findings add to a growing body of evidence suggesting that people with anxiety respond with fear to situations that only partially resemble previous frightening encounters, Etkin said by email.

"This paper goes a long way in providing a biological explanation and shows how the bias towards over-generalization that is seen in anxiety is evident even in how their brain encodes the perceptual information itself," Etkin added.

"More and more, I see over-generalization as a new and important domain for understanding anxiety, which may furthermore offer us new and different insights into how we might better treat these patients," Etkin said.