"Our ability to control our thoughts is fundamental to our wellbeing," said Michael Anderson from the University of Cambridge in the UK.
"When this capacity breaks down, it causes some of the most debilitating symptoms of psychiatric diseases: intrusive memories, images, hallucinations, ruminations, and pathological and persistent worries," said Anderson.
"These are all key symptoms of mental illnesses such as PTSD, schizophrenia, depression, and anxiety," he said.
Anderson likens our ability to intervene and stop ourselves retrieving particular memories and thoughts to stopping a physical action.
A region at the front of the brain known as the prefrontal cortex is known to play a key role in controlling our actions and has more recently been shown to play a similarly important role in stopping our thoughts.
The prefrontal cortex acts as a master regulator, controlling other brain regions - the motor cortex for actions and the hippocampus for memories.
In the study published in the journal Nature Communications, scientists used a task known as the 'Think/No-Think' procedure to identify a significant new brain process that enables the prefrontal cortex to successfully inhibit our thoughts.
In the task, participants learn to associate a series of words with a paired, but otherwise unconnected, word, for example ordeal/roach and moss/north.
In the next stage, they were asked to recall the associated word if the cue is green or to suppress it if the cue is red; in other words, when shown 'ordeal' in red, they were asked to stare at the word but to stop themselves thinking about the associated thought 'roach'.
Using a combination of functional magnetic resonance imaging (fMRI) and magnetic resonance spectroscopy, the researchers were able to observe what was happening within key regions of the brain as the participants tried to inhibit their thoughts.
Researchers showed that the ability to inhibit unwanted thoughts relies on a neurotransmitter - a chemical within the brain that allows messages to pass between nerve cells - known as GABA.
GABA is the main 'inhibitory' neurotransmitter in the brain, and its release by one nerve cell can suppress activity in other cells to which it is connected.