When an unwanted memory intrudes on the mind, it is a natural human reaction to want to block it out.
A hundred years ago, Freud suggested that humans have a mechanism that they can use to block unwanted memories out of consciousness.
More recently, scientists have started to understand how this works.
Neuroimaging studies have observed which brain systems play a part in deliberate forgetting, and studies have shown that it is possible for people deliberately to block memories from consciousness.
How do memories form?
Some memories can lead to fears and phobias.
For a person's mind to store a memory, proteins stimulate the brains cells to grow and form new connections.
The more we dwell on a memory or rehearse the specific events surrounding the memory, the stronger these neuronal connections become.
The memory remains there as long as we revisit it from time to time.
For a long time, people thought that the older the memory, the more fixed it is, but this is not necessarily true.
Each time we revisit a memory, it becomes flexible again. The connections appear to become malleable, and then they reset. The memory can change a little each time we recall it, and it resets stronger and more vividly with every recall.
Even long-term memories are not stable.
This process of strengthening is called reconsolidation. Reconsolidation can change our memories slightly for better or for worse. Manipulating this process can do the same.
If something frightens us when we are young, the memory of that event can become a little more frightening each time we recall it, leading to a fear that may be out of proportion with the real event.
A small spider that frightened us once may get bigger in our minds over time. A phobia can result.
In contrast, casting a humorous light on an embarrassing memory, for example, by weaving it into a funny story, can mean that in time, it loses its power to embarrass. A social gaff can become a party piece.
Why are bad memories so vivid?
Many people find that bad experiences stand out in the memory more than good ones. They intrude on our consciousness when we do not want them to.
Researchers have shown that bad memories really are more vivid than good ones, possibly due to the interaction between the emotions and the memories. This is particularly so when the emotions and memories are negative.
Neuroimaging has shown scientists that the process of encoding and retrieving bad memories involves the parts of the brain that process emotions, specifically the amygdala and the orbitofrontal cortex.
It seems the stronger the emotions associated with the memory, the more detail we will recall.
fMRI studies reveal greater cellular activity in these regions when someone is going through a bad experience.
Unwanted memories can lead to anxiety.
In 2012, scientists at the University of Cambridge showed for the first time which brain mechanisms are involved in substituting and suppressing memories.
They found that a person can suppress a memory, or force it out of awareness, by using a part of the brain, known as the dorsolateral prefrontal cortex, to inhibit activity in the hippocampus. The hippocampus plays a key role in remembering events.
To substitute a memory, people can redirect their consciousness towards an alternative memory.
They can do this by using two regions called the caudal prefrontal cortex and the mid-ventrolateral prefrontal cortex. These areas are important for bringing specific memories into the conscious mind, in the presence of distracting memories.
Suppressing a memory involves shutting down parts of the brain that are involved in recall. To substitute a memory, those same regions must be actively engaged in redirecting the memory way towards a more attractive target.
One of the report's authors, Dr. Michael Anderson, likens this to either slamming on the brakes in a car or steering to avoid a hazard.
The researchers used functional magnetic resonance imaging (fMRI) to observe the brain activity of participants during an activity.
This activity involved learning associations between pairs of words, and then trying to forget the memories by either recalling alternative ones to substitute them or blocking them out.
Results showed that both strategies are equally effective, but that different neural circuits are activated.
In post-traumatic stress disorder (PTSD), people who have experienced a traumatic life event are troubled by unwanted memories that insist on intruding into the consciousness.
Knowing more about how a memory can be substituted or suppressed might help people with this debilitating condition.
The mental context in which a person perceives an event affects how the mind organizes the memories of that event.
We remember events in relation to other events, where it occurred, and so on. This, in turn, affects what triggers those later memories, or how we can choose to recall them.
Context can be anything that is associated with a memory. It could include sense-related cues, such as smell or taste, the external environment, events, thoughts or feelings around the time of the event, incidental features of the item, for example, where it appears on a page, and so on.
As we use contextual clues to recall information about past events, scientists have suggested that any process that changes our perception of that context can increase or reduce our ability to retrieve specific memories.
To test this, a team of researchers set participants a task of memorizing sets of words, while viewing images of nature, such as beaches or forests. The aim of the images was to create the contextual memories.
Some participants were then told to forget the words on the first list before studying the second.
When the time came to recall the words, the group that had been asked to forget were able to recall fewer words.
More interestingly, fMRI tracking showed that they also had fewer thoughts of the images.
In deliberately trying to forget the words, they had discarded the context in which they had memorized them. In addition, the greater the detachment from the context, the fewer words they remembered. This suggests that we can intentionally forget.
The researchers then instructed the group to remember the words did not "flush out" the scenes from their minds, and continued to remember the words and think of the images.
The findings could be useful for helping people either to remember things, for example, when studying, or to reduce unwanted memories, for example, in treating PTSD.
Weakening memories that cause phobias
Treatment for people with phobias includes exposure to the item that causes fear. Exposure therapy aims to create a "safe" memory of the feared item, which overshadows the old memory. While this works temporarily, the fear often returns in time.
In August 2016, researchers from Uppsala University and Karolinska Institutet in Sweden showed that disrupting a memory can reduce its strength.
In their experiment, people who were afraid of spiders were exposed to pictures of their eight-legged friends in three sessions. The aim was to disrupt the memory by disturbing it and then resetting it.
First, the research team activated the participants' fear by presenting a mini-exposure to spider images.
Then, 10 minutes later, the participants viewed the images for longer. The next day, they saw the pictures again.
By the third viewing, the researchers noticed that there was less activity in the part of the brain known as the amygdala.
This reflected a lower level of emotional interference and a lesser tendency in the participants to avoid spiders.
The scientists concluded that the first exposure made the memory unstable. When the longer exposure occurred, the memory was re-saved in a weaker form. This, they say, stops the fear from returning so easily.
A drug for forgetting?
Some drugs show promise for treating or preventing PTSD by removing bad memories.
To complement cognitive approaches, some scientists have suggested using drugs to remove bad memories or the fear-inducing aspect that is associated with them.
D-cycloserine is an antibiotic, and it also boosts the activity of glutamate, an "excitatory" neurotransmitter that activates brain cells.
In other research, when a group of people with PTSD took propranolol at the time of consolidating a memory, for example, just after recounting a bad experience, they had fewer stress symptoms the next time the memory was activated.
Propanolol blocks norepinephrine, a chemical that plays a role in the "fight or flight" mechanism and gives rise to stress symptoms.
Researchers in New York carried out tests on rats that showed it is possible to erase single memories from the brain, by delivering a drug known as U0126, while leaving the rest of the brain intact.
In a mouse study published in Nature in 2014, scientists used a drug known as an HDACi to erase epigenetic markers in the DNA that enable bad memories to live on. This could help people, for example, with PTSD.
However, more research is needed into how to use these drugs safely and effectively.
Implanting false memories
Taking memory manipulation one step further, memory experts like Julia Shaw, author of "The Memory Illusion," have worked out how to implant false memories.
She starts, she says, by telling someone that when they were young, they committed a crime, then adding layers of information until the person can no longer decipher reality from imagination.
Shaw says she does this to highlight how some interrogation methods can be abused.
Such techniques are not without ethical concerns.
Healthy people could use them to erase an inconvenient event from the mind. Perpetrators of crimes could give memory-erasing drugs to people to make them forget events.
After all, some bad memories serve a purpose. They can prevent people from making the same mistakes again, or guide their actions on similar occasions in the future. How much do we want to forget?