Brains Suppress Unwanted Memories: A Human Neuroimaging Study

A new study led by Maité Crespo García, PhD, a postdoctoral research associate at the MRC Cognition and Brain Sciences Unit, at the University of Cambridge, has demonstrated that the critical human adaptive ability to suppress unwanted memories depends on a region of the brain involved in cognitive control called the anterior cingulate cortex (ACC). The ACC detects the need to control memories and responds both proactively to early signals of unwelcome memories and reactively to intrusive thoughts that break through proactive mechanisms of suppression.

The study was published in the Journal of Neuroscience, in an article titled, “Anterior Cingulate Cortex Signals the Need to Control Intrusive Thoughts During Motivated Forgetting,” and used simultaneous EEG-fMRI recordings on humans of both sexes engaged in a task of active memory suppression to show that when unwanted memories or intrusive thoughts emerge in awareness, the ACC signals the prefrontal cortex (PFC) to trigger a top-down inhibitory control over the hippocampus—a C-shaped structure found in both hemispheres of the brain that helps consolidate memories—through specific neural networks.

Earlier studies have shown neural control mechanisms engage the right dorsolateral PFC (rDLPFC) to inhibit hippocampal activity when unwanted memories arise. But how the need for such neural control is detected and whether such control mechanisms act proactively to prevent unwelcome memories from being retrieved, or reactively to counteract intrusive thoughts, had been unclear.

In the current study, the researchers measured brain activity through both EEG (electroencephalography) and fMRI (functional magnetic resonance imaging) in 12 normal male and female participants with no history of psychiatric or neurological illness, while the subjects memorized sets of words (e.g., gate and train) and were asked to either recall a cue word’s pair (i.e., think about a train when shown the word “gate”) or only focus on the cue word (i.e., think about a gate when shown “gate”).

The proactive memory suppression task was based on the Think/No-Think (TNT) paradigm, where participants were presented with cues from studied pairs as reminders and directed to control the retrieval process, while the researchers acquired simultaneous EEG-fMRI recordings to track interactions between the dACC, rDLPFC, and the hippocampus during the task. The researchers analyzed the trial-by-trial variations in two EEG markers in the dorsal ACC (dACC)—theta power and N2 amplitude—that are believed to reflect the need for neural control.

“We found a clear role of dACC in detecting the need for memory control and upregulating prefrontal inhibition. Importantly, we identified distinct early (350–400 ms) and late (500–700 ms) dACC contributions, suggesting both proactive control prior to recollection, and reactive control in response to intrusions,” the authors noted.

Electrical activity and measures of blood oxygen dependent (BOLD) activity indicated that during strong early EEG activity, hippocampal activity and BOLD signals in the dACC and rDLPFC decreased. The authors claim that this suggests preempting retrieval reduces overall demands for neural control.

In the late window of activity, the activity in the dACC increased and effective connectivity analyses showed communication from dACC to rDLPFC and from rDLPFC to hippocampus. This the authors infer is tied to successful “motivated forgetting.”

Overall, the authors showed that during proactive memory suppression, activity in the ACC increases within 500 milliseconds of the task. The ACC then relays the “need for control” to the DLPFC, which in turn inhibits hippocampal activity and memory retrieval.

When the memory is stopped early, requiring no further suppression, the activity levels in the ACC and DLPFC decrease, whereas when the memory is not suppressed in time, the ACC generates a reactive alarm. This increases activity in the ACC, and subsequently in the DLPFC to stop intrusive thoughts.

“Our findings support a model in which dACC detects the emergence of unwanted content, triggering top-down inhibitory control, and in which rDLPFC countermands intruding thoughts that penetrate awareness,” the authors concluded.