Aiming to forget – human frailty or strength?

What does it really mean to forget? Michael Anderson (University of Cambridge) has spent his career challenging the belief that forgetting is a human frailty and exploring the neurological mechanisms behind so-called motivated forgetting.

He began his talk, at the British Academy in a joint event with the British Psychological Society, speaking of Eternal Sunshine of the Spotless Mind, a film that follows a man's mission to have memories of his ex-partner erased. He said that although people think forgetting is a negative thing, there are many situations where people may be motivated to actively forget. For example, one may wish to alleviate negative feelings, to protect one's own self-image. He added: 'More often than we realise, forgetting is what we want and need to do. Forgetting is the goal while remembering is the human frailty.'

Dr Anderson told a story of knocking a cactus from a windowsill and inhibiting himself from catching it. He said it occurred to him that if nature saw fit to give us mechanisms to prevent actions, some of those mechanisms might be able to be turned inward – for example in the case of memory retrieval.

He went on to investigate this idea in the lab. Participants were trained to memorise word pairs. When faced with one of the cue words, could people actively prevent themselves retrieving the associated word? Anderson looked further into the moment where a person stops themselves from remembering. A 'think – don't think' paradigm presented subjects with words either in green or red: when shown green they retrieved the associated word, when shown red they had to refrain from remembering the associated word, not allowing it to enter their consciousness. In the final phase of the test participants had to try to remember all word pairs. The words presented on 'think' trials were remembered well, while those on 'don't think' trials were remembered more poorly. Anderson used the term 'suppression induced forgetting' to describe this phenomenon – constantly trying to keep something from one's mind results in worse memory for that stimuli.

But what are the neurological mechanisms behind this process? Anderson found activity in the dorsolateral prefrontal cortex during motivated forgetting, an area also heavily involved in inhibiting action. Interestingly, Anderson also saw suppressed activity in the hippocampus – an area with a well-established role in memory.

Anderson wanted to find out in these 'don't think' trials whether people were simply waiting and not thinking of the associated word, or 'slamming on the mental brakes', as it were, to stop themselves remembering. So he asked subjects in the scanner to report on the 'don't think' trials whether the associated word had crept into consciousness or hadn't. He said when a participant experienced the word coming to mind and had to push it out he saw huge down-regulation of hippocampus activity.

Potentially, Anderson said, down-regulating hippocampus activity does not target specific memories but has a more global effect on memory – causing people to briefly become amnesic. He described the case of one woman who suggested to him that she had experienced organic amnesia. It later emerged that she had witnessed a mass school shooting and later had to return to the same school. She found that her memory for her school years was badly affected. Anderson suggested that while suppressing memories triggered by stimuli related to the incident, this down-regulation of the hippocampus would have affected other memories that were unrelated to the trauma but that happened to the woman just before or after the incident that triggered the suppression.

In his future work, Anderson and his team are hoping to assess whether people can be trained to suppress certain memories and whether people can be trained to down-regulate the hippocampus using neurofeedback. He concluded: 'Forgetting is much more active than we realise. We do it on purpose more often than we realise.'