Having unwanted thoughts? Scientists find out why some people can not suppress it

Most of us have probably spent many a night up late, re-playing an awkward moment that happened ten years ago over and over again in our heads. This does not often happen when the thoughts are positive. That's true for returning service members and victims of abuse who suffer from post-traumatic stress, as well as for patients with debilitating anxiety, depression or even schizophrenia.

Everyone has memories or thoughts that they would rather suppress than think about. "There must be a similar mechanism for helping us stop unwanted thoughts from occurring".

Previous studies looked primarily to the prefrontal cortex instead of the hippocampus.

A team of researchers led by Dr Taylor Schmitz and Professor Anderson set volunteers tasks involving memory recall and inhibition.

To understand this process of suppressing unwanted thoughts more clearly, participants of the study engaged in a task called "Think/No-Think" where each participant first learned to associate a series of unconnected word pairings. In some, participants were shown colored circles associated with two separate buttons to push, and after identifying the correct buttons were given two new colors and continued the process while the researchers recorded success rates.

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 at the University of Cambridge in the United Kingdom wanted to study how intrusive thoughts get caught in a loop in the brain. It's a neurotransmitter called GABA - a chemical in the brain that sends messages between nerve cells, according the study's author, Michael Anderson, a professor at the University of Cambridge. When there was lots of GABA in the subjects' hippocampi - the area of the brain that controls memory - it predicted whether or not the subject was able to suppress the word association, implying that they would, too, be better at suppressing unwanted thoughts.

"What's exciting about this is that now we're getting very specific", said Prof 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".

"Most of the focus has been on improving functioning of the prefrontal cortex", Anderson said. Among the participants, those who had less hippocampal GABA were not quite strong in suppressing the thought as per the prefrontal cortex's commands. "If an army's foot-soldiers are poorly equipped, then its commanders' orders can not be implemented well".

The discovery may answer one of the long-standing questions about schizophrenia. Post-mortem studies have revealed that the inhibitory neurons (which use GABA) in the hippocampi of these individuals are compromised, possibly making it harder for the prefrontal cortex to regulate activity in this structure. Perhaps in the near future there will be medication that redeems people from the obsessive thoughts. Their mission? To block your intrusive thoughts.

A striking hallmark of mental health problems such as PTSD, anxiety, and depression is the inability to control negative or unpleasant thoughts from surfacing.