Whether they take the form of worries, unpleasant memories or hallucinations, recurrent unwanted thoughts are a key feature of conditions such as anxiety, post-traumatic stress disorder (PTSD), depression and schizophrenia. Now, in a step towards better-treating these disorders, scientists from Britain's University of Cambridge have identified a chemical in the brain that lets most people suppress such thoughts.

Led by Prof. Michael Anderson and Dr. Taylor Schmitz, the researchers submitted a group of healthy test subjects to what's known as a Think/No-Think procedure.

This involved first teaching the subjects to associate a series of words with other unrelated words, such as associating the word "ordeal" with "roach." Next, they were asked to recall the associated word when shown the first word, but only if the letters of that first word were colored green. If the letters were red, they were instead supposed to suppress any thoughts of the associated word.

Utilizing both functional magnetic resonance imaging (fMRI) and magnetic resonance spectroscopy, the activity and chemistry of the subjects' brains were monitored as they performed this exercise. It was already known that the prefrontal cortex – the "command center" of the brain – played a role. What was found, however, was that a chemical known as GABA was more directly responsible for suppressing the unwanted thoughts.

GABA is the brain's main inhibitory neurotransmitter, and when released by one nerve cell can suppress activity in other connected cells. It was discovered that GABA concentrations within the brain's hippocampus, which is involved in the memory process, predicted how effectively the test subjects would be able to suppress the words.

"What's exciting about this is that now we're getting very specific," says Anderson. "Before, we could only say 'this part of the brain acts on that part,' but now we can say which neurotransmitters are likely important – and as a result, infer the role of inhibitory neurons – in enabling us to stop unwanted thoughts."

A paper on the research was recently published in the journal Nature Communications.