Two ways to forget at will

Although neuroimaging studies have shown in the past that we can choose to block unwanted memories from our minds, these investigations have not been able to expose the cognitive strategies that we employ to do so or the neural processes that underlie these strategies.

A study which appeared in the Cell Press journal Neuron earlier this week offers evidence that there are in fact two, apparently opposite, tactics for forgetting which nonetheless have a similar effect on our subsequent ability to recall the memory.

Dr Roland Benoit, a researcher in the MRC Cognition and Brain Sciences Unit at the University of Cambridge, used functional magnetic resonance imaging (functional MRI) to investigate the precise mechanisms involved in voluntary memory suppression along with his colleague Dr Michael Anderson. Prior work on which Dr Anderson collaborated (reported in Nature in 2001) has demonstrated that the effort to exclude a memory of a past experience can harm recollection of that memory later.

Dr Benoit took the time to answer some questions from ScienceOmega.com, beginning with the motivations behind the research. As he pointed out, we have all had the feeling of trying to put an unpleasant memory from our mind.

"Intuitively, it feels as though we solve this problem both by voluntarily pushing the intruding memory out of mind, but also by strategically ‘changing the subject’; that is, by training ourselves to think of something else in response to the reminder," Dr Benoit went on.

"After that fight with your spouse, the next time you see them, you may attempt to push the memory out of mind to get over things, and you might also try to remind yourselves of all the lovely things they have done for you. If we compare the brain to a car, it’s the difference between slamming on the mental brakes to stop remembering, versus steering towards another memory to avoid the unwanted memory of the fight."

The researchers hypothesised that memory exclusion was possible via either of these two mechanisms. Actively pushing a memory out of awareness can be referred to as ‘direct suppression’. On the other hand, when someone attempts to focus on a distracting thought or alternative memory, a type of mental redirection can occur which is referred to as ‘thought substitution’. Interestingly, these two mechanisms would work in opposite ways.

"Direct suppression works by shutting down the ‘remembering’ process of the unwanted memory – in effect, turning off the unwanted memory," explained Dr Benoit. "In contrast, thought substitution acts by ‘turning on’ the remembering process selectively for a substitute memory. At first blush, these might seem similar; after all, when one ‘turns on’ a substitute memory, it might seem to ‘turn off’ the unwanted memory. For these reasons, it was unclear whether these processes can be isolated, and which, if either, would actually cause forgetting."

The experiment itself built on the think/no-think procedure developed by Dr Anderson and Dr Collin Green for the 2001 paper. I asked Dr Benoit if it was it difficult to adapt the methodology in order to test their hypothesis.

"The critical part was to develop a technique to encourage people to use one or the other of the two hypothesized processes, but not both," he replied. "Therefore, we tested two groups of people who were treated in exactly the same way with the only difference being that one group eventually followed instructions designed to induce direct suppression, whereas the other group performed a task that induced thought substitution."

The participants were trained to associate particular reminders with particular memories, for example BEACH – AFRICA. In some cases, a substitute was provided in a pairing, eg, BEACH – SNORKEL. Later, in the suppression phase, the volunteers were scanned with functional MRI.

In this phase, the participants were presented with the reminders. In some cases they were asked to recall the associated memory and in others they were asked to try suppressing the memory. For instance, when BEACH appeared on the screen, they were asked to not think of the other word that went with it.

The critical part of the experiment was in asking the thought substitution group to avoid thinking of the associated item by recalling and concentrating on the substitute memory instead – SNORKEL, in this case. Thus, they tried to concentrate on the substitute to help them not think about the unwanted associate.

"We carefully explained to them that we did not want them to try to distract themselves with alternative thoughts, but simply to stare at the reminder and prevent the memory from coming into mind," Dr Benoit reiterated. "We told them that if the memory happened to come to mind anyway, that they should push it out of their mind."

Rather than trying to think of a distracting memory, members of this group simply tried to halt the remembering process altogether. After engaging in this think/no-think task as best as they could, the volunteers were asked to remember all of the memories, irrespective of whether they had previously tried to suppress or recall the respective words. This allowed the team to assess whether attempting not to think about certain of the memories had caused forgetting.

"Critically, we also tested reminder-memory pairs that people had studied initially, but were set aside during the think/no-think phase, and thus had neither been retrieved nor suppressed," said Dr Benoit. "People’s ability to recall those memories lets us know how well they would be able to recall the memories after doing nothing with them in the interim. Importantly, previously suppressed memories were recalled worse than those baseline memories, demonstrating that thought substitution and direct suppression attempts both induced forgetting."

Dr Benoit describes this as the first remarkable finding of the study; that both suppression and substitution lead to a similar degree of forgetting.

"Both are good solutions to avoiding and forgetting an unwanted memory. In fact, if you were to only look at how well people forget the unwanted memories, you couldn’t tell whether participants engaged in suppression or substitution. You could only tell by looking at their brain data.

"When we measured neural activity during direct suppression, we found that the right prefrontal cortex suppresses activation in a memory structure critical for conscious memory of past experience (ie, the hippocampus). Thus it effectively breaks the remembering process. This, in turn, disrupts the memory representations that would be necessary for recalling the unwanted memory later on."

By contrast, when neural activity was monitored during thought substitution, it was found that two left prefrontal regions form a neural network which works with this same memory structure to selectively steer the remembering process towards the distracting thought.

"Thus, whereas direct suppression reduces memory activity in the hippocampus, thought substitution increases memory related processing. Basically, you can forget unwanted memories by engaging either of two opposite mechanisms that are based on different neural networks."

In this particular study, one group of participants engaged the direct suppression mechanism and a different group engaged the thought substitution mechanism. Hence there is no evidence that an individual might find one strategy more effective than the other.

"Overall, both mechanisms led to the same degree of forgetting," Dr Benoit remarked. "However, we did observe that participants varied in how good they were at suppressing memories. It is possible that those who struggled to use direct suppression might have found it easier to use thought substitution instead, or vice versa."

According to the researchers, this study greatly increases our understanding of how voluntary forgetting is achieved, revealing two distinct and opposing neural processes that produce it. A better understanding of how these mechanisms function and ways in which they could break down may ultimately help in understanding disorders that are characterised by deficient regulation of memories, such as post-traumatic stress disorder (PTSD).

"We assume that these mechanisms are involved in forgetting unwanted memories in healthy people, and a number of recent studies have shown that the types of effects and processes we are studying generalise to emotionally negative experiences and personal autobiographical memories.¹

"Knowing that distinct processes contribute to forgetting may be helpful, for instance, as people may naturally be better at one approach or the other. In fact, we have just started a research project to examine the direct suppression mechanism in people suffering from PTSD."


¹ For example, ‘It's All in the Detail: Intentional Forgetting of Autobiographical Memories Using the Autobiographical Think/No-Think Task’, Noreen and MacLeod, Journal of Experimental Psychology: Learning, Memory and Cognition, 2012