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Humans may have a warning system in the brain that helps suppress unwanted thoughts. Having a better understanding of how this system works could one day lead to treatments for obsessive compulsive disorder (OCD) or PTSD.

at the University of Cambridge and his colleagues analysed the brain scans of 24 Chinese-speaking people as they completed a memory suppression task.

The researchers asked them to memorise 48 pairs of Chinese words. Each person then had electroencephalography (EEG) electrodes attached to their head and was put into a functional magnetic resonance imaging (fMRI) machine.

鈥淲e wanted to analyse the participants鈥� brains in two ways simultaneously because fMRI is really good at telling you where things are happening in the brain, but it鈥檚 no good at telling you about the timing of brain activity, whereas EEG is really good at that,鈥� says Anderson.

A screen displayed the words the participants had memorised, one at a time, alongside either a green, red or no light. When the green light was shown, they were instructed to think about the word it had initially been paired with. A red light indicated they should try not to recall the paired word. No action was required for words that appeared with no light.

Each word pair was repeated 12 times in a random order and was always shown with the same 鈥� or no 鈥� coloured light.

About 350 milliseconds after participants saw a red light, a brain region known for managing attention, called the dorsal anterior cingulate cortex, sent a signal to an area of the brain involved in working memory, called the dorsolateral prefrontal cortex.

Previous studies have suggested these regions are involved in memory control, but their specific roles have been unclear. The signal the team observed might be an alert for an unwanted memory, Anderson says, which the dorsolateral prefrontal cortex then works to suppress.

鈥淚t takes about 500 milliseconds to consciously remember something that鈥檚 associated with something else 鈥� so this signal is happening in your subconscious,鈥� says Anderson.

The researchers also tested how well the participants could recall the paired words and found that people with stronger warning signal were more likely to struggle with this task.

Anderson compares the way the suppression signal works to seeing an ex-partner鈥檚 car parked outside a nearby shop. The first time you see the car you may be flooded with unwanted memories, but if you see it in the same spot over and over, the warning signal gets stronger and you are better able to suppress the memories.

The bigger the warning signal fired, the less activity the researchers saw in the dorsolateral prefrontal cortex. This suggests that the stronger the warning signal, the less effort required by the brain to suppress the memory, says Anderson.

鈥淭he hope is that by tracing out how these mechanisms work in detail, we鈥檒l have the model system that we could apply to disorders like OCD, anxiety and depression, which all feature intrusive thoughts,鈥� says Anderson.

at the University of Leicester in the UK says a limitation of this study is that the word pairs used might not be relevant to mental health conditions. 鈥淲ith post-traumatic stress disorder (PTSD) and OCD, unwanted thoughts are very distressing,鈥� she says.

Anderson says that other similar studies conducted by his team using distressing stimuli have elicited activity in the same regions of the brain.

鈥淭he neural circuit identified in this intriguing study can provide a potential target for the treatment of intrusive thoughts in PTSD and OCD,鈥� says at University College London. Using a neurofeedback procedure, which gives real-time feedback from brain activity, 鈥減atients might be able, in the near future, to train parts of their prefrontal cortex to exert a more stringent top-down control on unwanted memories鈥�, he says.