11 Active Forgetting
Grace Rheault
Learning Objectives
- Understand the neural mechanisms of forgetting.
- Describe of the importance of forgetting as an essential process of a healthy brain, as well as the potential implications of forgetting for treating maladaptive behaviours, like substance use disorder.
What if we never forgot?
Imagine you could remember everything you’ve ever learned and every memory you’ve ever had. You could read over your notes just once to study for a test, and you could perfectly recount stories from your past. Wouldn’t it be great to never forget anything? There are some people who have been observed to have an exceptional ability to remember things. One famous example is that of Solomon Shereshevsky, a Russian newspaper reporter born in 1896 (Benjamin et al., 2018). At work one day, the editor of the newspaper was angry that Solomon was not taking notes in their morning meetings, which involved extensive details about the reporters’ assignments for the day. The editor later realized that Solomon could recount the assignments perfectly, word-for-word. Solomon underwent psychological testing where it was found that he could memorize whole pages from books, series of numbers and digits, and people’s faces. Time did not seem to erode his memories. Sounds great, doesn’t it? Unfortunately, Solomon’s apparent gift of never forgetting ended up causing him significant challenges in his day-to-day life. Because his memory did not differentiate between mundane and essential information, he was constantly overwhelmed with all kinds of information when he didn’t need it. Additionally, he could remember in detail every person he had ever met, making him distracted in social settings. Solomon’s exact condition remains unclear; however, it is agreed that he had a kind of hyper memory.
A phenomenon labelled highly superior autobiographical memory (HSAM) is a similar condition in which people have difficulty forgetting. People with this condition are able to recount events in their lives with extreme accuracy (LePort, Dickinson-Anson, et al., 2012). Some studies have found that, people with this condition often suffer from obsessive tendencies significantly more than people without this condition (LePort, Dickinson-Anson, et al., 2012). However, this is still under speculation, and neuroscientific explanations of personality and mental health differences in people with enhanced memory are scarce. Despite this, there are many anecdotal reports from people with enhanced memories that their condition can be a burden due to the overwhelming amount of information that they are dealing with, particularly if some of the information remembered is traumatic.
The importance of forgetting
Although forgetting is often thought of as negative, or as a failure of memory, it is actually an essential function of memory. The lived experiences of people with exceptional memory demonstrate that not being able to forget may pose a burden on an individual. There are pieces of information in our brains that we don’t want, nor do we need. Therefore, our brains must work to forget some information to “clean things up” and to make our brains more efficient. If we cannot forget, we would have to sort through all the information one has ever come across each time something must be recalled (Castillo at al., 2021). Forgetting was previously thought of as a passive process of decay that occurs over time. However, our brains are now thought to undergo active processes of forgetting to be more efficient.
Forgetting improves our neural processing efficiency. Khan, Dudukovic, et al. (2007) discuss how forgetting can contribute to a healthy memory by reducing demands on the cognitive mechanisms required for remembering. Remembering only what information is pertinent facilitates the retrieval of important memories- by cutting down on what memories are competing for attention. To test to what extent forgetting may decrease demands on neural processing related to selective retrieval, fMRI was conducted during selective retrieval tasks. The findings indicated that greater forgetting of memories that were not relevant reduced demands on both neural mechanisms that detect conflict, and neural mechanisms that overcome memory competition through selection and inhibition. Additionally, forgetting can contribute to emotional regulation by helping discard traumatic information (Nørby, 2017).
How does forgetting occur?
Active Forgetting
Wang et al. (2019) investigated whether people could intentionally forget by way of a mechanism in the sensory cortex. They tested this by showing participants a series of images, followed by instructions to either remember or forget the image, and measured brain activity with fMRI. They found that there was a U-shaped relationship between neural activation and memory. Therefore, this suggests that items more likely to be forgotten elicited moderate activation in the brain, specifically in the motor cortex. This was the case for both intentionally and incidentally forgotten items. This research suggests that forgetting can be an activation-driven process, rather than a passive one.
Inhibitory processes
When retrieving specific memories, the brain must sort through information. fMRI has shown that there is activation in the lateral prefrontal cortex (PFC) during selective memory retrieval. With increasing retrieval practice, activation in the PFC decreases (Kuhl, et al., 2007). The PFC seems to reduce access of competing memories through inhibition during selective memory retrieval, i.e., contributes to forgetting. This suggests that with repeated ‘blocking’ of irrelevant memories during retrieval, retrieval of important memories becomes more efficient.
Dopamine
Dopamine is known to play a role in memory processes. However, more recently it has been found to contribute to forgetting. There is thought to be dopamine signaling in the PFC that contributes to forgetting (Castillo et al., 2021). A study published in August 2022 found that active forgetting in rats was improved by increased dopamine activity in the prefrontal cortex (Gallo et al., 2022). Rats injected with drugs that either blocked dopamine receptors or suppressed release of dopamine into the prefrontal cortex demonstrated a lack of retrieval-induced forgetting on a memory task as compared to rats that received a dopamine-enhancing drug (Gallo et al., 2022).
Sleep
While a person is asleep, the brain is thought to go a sort of “rewiring” that helps to organize thoughts and information from the day (Poe, 2017). Some of this brain organization that occurs while asleep likely involves processes of forgetting to promote optimal adaptability to new situations, and to help prioritize important information (Hoedlmoser et al., 2015). A study conducted in 2015 looked at the potential for sleep mechanisms to participate in the forgetting of irrelevant information. Experienced bike riders were trained to ride a bike with reversed handlebars in three 15-minute sessions. Riders that were given a two-hour sleep (i.e., enough time to have a full sleep cycle) following the training sessions were more likely to forget the irrelevant skill of biking with reversed handlebars. Participants who had more rapid eye movement (REM) sleep were even more likely to forget how to ride improperly (Hoedlmoser et al., 2015). The researchers conducting this study interpreted that REM sleep could be important in forgetting some information to preserve important information important for everyday life (i.e., riding a bike properly) (Hoedlmoser et al., 2015).
Implications of forgetting
Memory disorders
A better understanding of how forgetting occurs has implications in many areas, including memory disorders. If diseases like Alzheimer’s are thought of as a disorder of memory, we must also think of them as a disorder of forgetting (i.e., in which too much forgetting occurs). If we can understand how forgetting occurs, we may be able to advance our knowledge of memory disorders and how to treat them (Castillo et al., 2021).
Substance use disorder
Knowing the mechanisms underlying forgetting could have potential implications in treating maladaptive memory behaviours, such as substance use disorder. Experiments done with mice have shown that dopaminergic activation in specific pathways were able to erase cocaine-related memories, therefore demonstrating possible implications in treating substance use disorder (Castillo et al., 2021).
Post-traumatic stress disorder
Learning how to forget traumatic memories could help in treating people with disorders like post-traumatic stress disorder (PTSD). PTSD occurs when people are exposed to traumatic events, and later suffer from “intense or prolonged psychological distress at exposure to internal or external cues that symbolize or resemble an aspect of the traumatic event(s)” (American Psychiatric Association, 2013). PTSD is associated with the hyperactivity of certain brain systems during sleep that are not conducive to forgetting processes (van Liempt, Arends, et al., 2013; Poe, 2017). This suggests that an inability to forget could be contributing to symptoms of PTSD. If the mechanisms of forgetting through these systems are better understood, it could allow for research into effective treatment for PTSD.
References
American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (5th ed.). https://doi.org/10.1176/appi.books.9780890425596
Benjamin, S., MacGillivray, L., Schildkrout, B., Cohen-Oram, A., Lauterbach, M. D., & Levin, L. L. (2018). Six Landmark Case Reports Essential for Neuropsychiatric Literacy. The Journal of Neuropsychiatry and Clinical Neurosciences, 30(4), 279–290. https://doi.org/10.1176/appi.neuropsych.18020027
Castillo Díaz, Caffino, L., & Fumagalli, F. (2021). Bidirectional role of dopamine in learning and memory-active forgetting. Neuroscience and Biobehavioral Reviews, 131, 953–963. https://doi.org/10.1016/j.neubiorev.2021.10.011
Hoedlmoser, K., Birklbauer, J., Schabus, M., Eibenberger, P., Rigler, S., & Mueller, E. (2015). The impact of diurnal sleep on the consolidation of a complex gross motor adaptation task. Journal of Sleep Research, 24(1), 100–109. https://doi.org/10.1111/jsr.12207
Kahn, I., Dudukovic, N. M., Kuhl, B. A., & Wagner, A. D. (2007). Decreased demands on cognitive control reveal the neural processing benefits of forgetting. Nature Neuroscience, 10(7), 908–914. https://doi.org/10.1038/nn1918
LePort, A., Mattfeld, A. T., Dickinson-Anson, H., Fallon, J. H., Stark, C. E. L., Kruggel, F., Cahill, L., & McGaugh, J. L. (2012). Behavioral and neuroanatomical investigation of Highly Superior Autobiographical Memory (HSAM). Neurobiology of Learning and Memory, 98(1), 78–92. https://doi.org/10.1016/j.nlm.2012.05.002
Nørby S. (2018). Forgetting and emotion regulation in mental health, anxiety and depression. Memory (Hove, England), 26(3), 342–363. https://doi.org/10.1080/09658211.2017.1346130
Poe, G. (2017). Sleep Is for Forgetting. The Journal of Neuroscience, 37(3), 464–473. https://doi.org/10.1523/JNEUROSCI.0820-16.2017
van Liempt, S., Arends, J., Cluitmans, P. J. ., Westenberg, H. G. ., Kahn, R. S., & Vermetten, E. (2013). Sympathetic activity and hypothalamo-pituitary–adrenal axis activity during sleep in post-traumatic stress disorder: A study assessing polysomnography with simultaneous blood sampling. Psychoneuroendocrinology, 38(1), 155–165. https://doi.org/10.1016/j.psyneuen.2012.05.015
Wang,T., Placek, K., & Lewis-Peacock, J. A. (2019). More Is Less: Increased Processing of Unwanted Memories Facilitates Forgetting. The Journal of Neuroscience, 39(18), 3551–3560. https://doi.org/10.1523/JNEUROSCI.2033-18.2019
