The brain does what it can to suppress unwanted thoughts, but we often end up wrestling with our demons in the middle of the night anyway. Still, the brain offers us at least some control, which is thought to be an attempt to stabilize us and maintain our well being. Without that control, unwanted memories can become debilitating, which is why intrusive memories are classic symptoms of mental illnesses like schizophrenia, PTSD, and depression.
Scientists aren’t exactly sure how the brain subdues these thoughts. They know that the prefrontal cortex plays a key role in controlling our actions and stopping our thoughts, but the exact mechanism that triggers this safety system is a mystery. In a study released Friday in Nature Communications, scientists from the University of Cambridge reveal that we can largely blame sleepless nights spent contemplating unwelcome thoughts on a neurotransmitter called GABA.
GABA, short for gamma-aminobutyric acid, is a chemical in the brain that allows messages to pass between nerve cells and acts as the brain’s main ‘inhibitory’ neurotransmitter. When one nerve cell releases GABA, it suppresses the activity of other connected cells. In the new study, the Cambridge researchers showed that the amount of GABA in the brain’s hippocampus can predict how well people’s brains can block the retrieval process of memory, which in turn blocks bad memories from resurfacing.
To determine this, study participants learned to do a word exercise in which they had to associate paired but unrelated words with each other, such as “moss” and “north.” These word pairs made up the “memories” the participants had to either retrieve or suppress in the next step of the experiment, in which they had to respond to red and green prompts on a screen. If they saw a green circle with a word, they had to recall the word’s pair, but if they saw a red circle with that word, they had to try to avoid thinking of its paired word. So, if they saw the “moss” with a red circle, they would have to do their best to not think of “north.”
Meanwhile, the researchers scanned the brains of the participants with functional magnetic resonance imaging, which measured brain activity, and magnetic resonance spectroscopy, which analyzed brain chemistry.
People with less hippocampal GABA turned out to be less likely to be able to suppress memories, which the researchers linked to lower hippocampal activity in the prefrontal cortex. These observations are consistent with previous research showing that people with schizophrenia have compromised inhibitory neurons — the cells that rely on GABA for signaling. The prefrontal cortex in the brains of these people has a harder time regulating activity, making it more difficult to suppress errant thoughts and unwanted memories. Sometimes, this loss of control leads to hallucinations.
“The environmental and genetic influences that give rise to hyperactivity in the hippocampus might underlie a range of disorders with intrusive thoughts as a common symptom,” explains study co-author Taylor Schmitz, Ph.D. in a statement. If a treatment can be developed to improve GABA activity within the hippocampus, then the severity of this condition may be able to be lessened in the future.