Researchers uncover a brain process that may help explain the curse of uncontrollable thoughts

Distressing conditions including PTSD, depression and anxiety have something in common: a difficulty in suppressing unwanted thoughts. Negative self-judgments and re-experienced traumas directly impact mental health and make recovery harder by intruding into the new experiences that should provide distance and a mental fresh start. Understanding what's involved in thought suppression may therefore be one key to helping people with these conditions. Now research in Nature Communications has uncovered an important new brain process that may help explain why some people struggle to control their thoughts.

The research, led by Taylor Schmitz, comes from the lab of Michael Anderson at Cambridge's MRC Cognition and Brain Sciences Unit. Anderson first began investigating thought suppression when the dominant position was that it's not possible to suppress thoughts voluntarily – "just try to not think of a white bear for five minutes and see what happens!" was the popular example of the time, derived from Daniel Wegner's ironic processing theory. However, Anderson used new methods, including the so-called "Think/No-Think paradigm" (more on this in a moment), to show that suppressed thoughts only rebound under certain conditions – in other words, it is sometimes possible to suppress thoughts successfully.

The new research used Anderson's Think/No-Think paradigm. Participants first studied pairs of words. Afterwards they were shown one word from each pair and – depending on the instruction for that pair – either had to recall the other word in the pair (a "Think" trial), or instead had to block the other word from coming to mind (a "No Think" trial) . Later, when participants tried to remember which words were partnered with which, they did worse for pairs from No-Think trials than Think trials, on average (in fact, they did even worse recalling the No-Think pairs than other pairs that hadn't featured at all since the initial study phase). This shows that No Think pairings had mostly been suppressed successfully.

These effects replicate established findings for the Think/No Think paradigm. The main purpose of the new study was to see what was happening in the brain during this process, and for this, Schmitz and his colleagues used two imaging techniques: functional magnetic resonance imaging (fMRI), to look at patterns of blood flow during the task, and 1H magnetic resonance spectroscopy (MRS), to look at the concentration of neurotransmitters in different parts of the brain. For comparison, the researchers also scanned the participants' brains during a task that involves suppressing movements, rather than thoughts (the "Stop Signal" task, in which a key must be pressed as fast as possible in response to certain colour cues, but the button press withheld for other colour cues).

During the suppression of thoughts (the No-Think trials), blood flow was reduced in the hippocampus, an area of the brain involved in memory. The extent of this change was related to the levels of the inhibitory neurotransmitter GABA. Participants with a higher concentration of GABA, specifically in the hippocampus, also showed a greater decrease in blood flow in that area. Moreover, the degree of thought suppression correlated with the amount of GABA in the hippocampus (not in the brain more widely). These same patterns were not observed in the Stop Signal task. These findings strongly suggests that hippocampal GABA is playing a specific role in the ability to suppress thoughts, by helping the area to dial-down its activity.

What triggers the involvement of GABA during No-Think trials? Previous work suggests that the thought suppression process involves a signal starting in the dorsolateral prefrontal cortex, and the more this frontal brain area is active, the more deactivated the hippocampus becomes – akin to the control area of the brain telling the hippocampus to cool its jets. The new findings suggest that this "negative coupling", as it's known, depends on GABA. For people low in hippocampal GABA, this process – which appears to be a key component of thought suppression – didn't happen.

These new findings have implications for our understanding of mental health conditions, suggesting that lower concentrations of GABA could be a risk factor for some. However, the research is correlational, so this conclusion has to be taken tentatively (causality could be the other way around, such that GABA is driven pathologically low by psychological issues). Bear in mind too that the sample were all mentally well participants. Future research could test the causal role of hippocampal GABA in uncontrolled thoughts (including in clinical samples), as well as refine our understanding of which psychological interventions will be of benefit – most likely those that don't just harness the involvement of frontal brain areas, but also produce downstream change in the soil health of the hippocampal range.

The study represents a breakthrough in bridging neurophysiology and psychology, and it looks likely that getting a grip on the role of molecular pathways in the hippocampus will be at least part of the solution to help those for whom issues from the past still plague their present.