An old photograph, the smell of warm apple pie, or even a long-forgotten name overheard at a party all have the potential to unleash a flood of related memories. In many ways, our conscious experience is dictated by the reminders we encounter in daily life. Sometimes, however, we are motivated to exert conscious control over which thoughts are permitted to rise to awareness, either by actively seeking to remember particular instances or by suppressing inappropriate, unpleasant, or otherwise distracting thoughts. While everyday life presents numerous opportunities to engage in these control behaviors, individuals coping in the aftermath of a major life trauma may be especially likely to turn to memory suppression to curb flashbacks. Just how effective are these suppression attempts? Do they reduce the likelihood that the unwanted memories will threaten to rise to the surface in the future? And, importantly, are there unintended consequences of memory suppression?

Our recent research has revealed that trying to suppress the past has both direct effects on the unwanted memories themselves and profound side effects for unrelated memories. When confronted by strong and specific reminders of unwanted memories, individuals tend to engage portions of the brain's frontal lobe known to help with stopping inappropriate behaviors (like remembering something that we don't want to remember)¹. The prefrontal cortex does so by sending inhibitory ("stop") signals to the hippocampus - a region of the brain associated with retrieving old memories, as well as forming new ones^2,3. We can directly measure the associated decreases in hippocampal activity with functional magnetic resonance imaging (fMRI) and establish the lingering difficulty in retrieving memories that had been suppressed previously using surprise recall tests at later points in time.

Such targeted memory suppression effects may be all well and good if one is trying to prevent a particular unwanted memory from coming back to mind. The issue is that the brain's response can be rather sluggish, meaning that other memories - innocent bystanders that later we might like to remember - can get lost in the process of ramping up or down the hippocampal memory control system. In other words, suppressing unwanted memories not only makes it harder to remember those particular memories later, it also has the power to induce amnesia for other experiences that happen just before or after one tries to suppress unrelated memories.

To test for this side effect, we first asked participants to study a number of arbitrary word pairs (e.g., LEAP-BALLET). Participants were then required to either retrieve or suppress the second word from these pairs, given the first. In between these tasks, we presented participants with novel experiences, like a picture of a peacock standing in a parking lot. While participants weren't aware of this at the time, our primary research question was whether suppressing words (like BALLET) around the introduction of the bystander pictures would impair subsequent memory for details about those unrelated experiences. This was assessed in a surprise final memory test, in which participants were asked to recall the associated bystander objects (e.g., the peacock) for each presented background scene (e.g., the parking lot).

We were amazed to find that participants' memory for these bystander pictures was reduced by nearly half, simply because they had engaged in an unrelated word-suppression task seconds before or after an image of the peacock in the parking lot appeared. Interestingly, they had no problem remembering that they had seen a peacock - they just couldn't remember where they had seen it: Was it in a parking lot or a zoo, for instance? The observed amnesia effect proved robust in our series of seven experiments with nearly 400 participants, surviving long delays and growing with suppression practice. Moreover, it was possible to predict the magnitude of the amnesia effect by observing hippocampal activity in participants' brains.

Together with previously reported forms of forgetting (e.g. inattention, decay over time, interference from related memories, and brain damage), this new type of suppression-induced amnesia may add to explanations as to why individuals recovering from major traumas often have generalized memory impairments for experiences unrelated to the traumatic events themselves^4-6. To the extent that suppressing memories of the past creates a temporary learning impairment for the present, individuals regularly employing such a suppression strategy to cope with chronic memory intrusions may be particularly susceptible to the amnesic side effects.

By learning more about the neural and cognitive mechanisms that allow us to remember when we want to remember and forget when we want to forget, we can identify memory control strategies that could help us to let go of traumatic experiences of the past without losing memories of the present.

Next read: How Does Memory Guide Learning? Birds Can Answer by Wenchan Zhao , Todd F. Roberts

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