How the Brain Purges Intrusive Thoughts
A neuroimaging study reveals the role of hippocampal GABA in inhibitory control.
We have all experienced events better left forgotten, whether it was an embarrassing performance onstage or an argument over a holiday dinner. But some people simply cannot suppress intrusive memories, hallucinations, ruminations, or persistent worries, no matter how hard they try. This cognitive impairment is a hallmark of a range of psychiatric disorders, including post-traumatic stress disorder, schizophrenia, depression and anxiety.
Clues to the neural networks involved in this process, known as inhibitory control, have come from past human imaging studies. When people are instructed to try to suppress unwanted thoughts, a brain region called the right dorsolateral prefrontal cortex reduces activity in another brain region called the hippocampus, inducing forgetting of the suppressed content. Studies in patients and animals have suggested that hippocampal levels of a chemical messenger called GABA could play an important role in memory and psychiatric symptoms.
A study published Nov. 3 in Nature Communications provided novel evidence regarding the role of hippocampal GABA in stopping unwanted thoughts. The researchers, from England, Spain and the U.S., combined functional magnetic resonance imaging with a noninvasive imaging technique known as 1H magnetic resonance spectroscopy that provides sensitive measures of brain metabolites such as GABA. Prior to scanning, 24 healthy young adults were drilled on 60 cue-memory word pairs (e.g., BEACH-AFRICA). During the practice phase, they were instructed to recall memories for cues presented in green font, but to avoid thinking of cues presented in red.
The participants performed the Think/No-Think task during the fMRI session, while their resting GABA concentrations in the hippocampus were measured in a separate magnetic resonance spectroscopy session. Their memory performance was also assessed in a final surprise recall test after the scan sessions.
Participants with higher hippocampal GABA concentrations exhibited better suppression of unwanted content, as reflected by superior suppression-induced forgetting on the surprise memory test. Moreover, higher hippocampal GABA predicted stronger coupling between the right dorsolateral prefrontal cortex and the hippocampus during thought suppression. Taken together, the findings suggest that hippocampal GABA may play an important role in enabling the prefrontal cortex to suppress unwanted thoughts.
According to the authors, the study sheds light on how the content of awareness is voluntarily controlled and provides a framework for studying psychiatric disorders that share persistent intrusive thoughts as a common symptom.