Understanding forgetting

Students are returning to school after a long summer break. What will they remember from last year? What happens if they’ve forgotten a lot of it? In this blog, let’s focus on forgetting: what it is and what to do about it.*

But first, we need to reset our understanding of forgetting.

Forgetting is a loaded concept (Ryan & Frankland, 2022, p.183) usually characterised as a bad thing.

What if we viewed forgetting differently?

Forgetting as memory management

Our brain is bombarded with stimuli. It has to decide what to store, what to make accessible and inaccessible. If it didn’t, one cue from the environment could overwhelm us with every related memory we’ve ever made. Our brains need some kind of memory management system (Davis & Zhong, 2017, p.1) – enter forgetting.

Forgetting as adaptive

It’s likely that the brain’s memory management system works in response to the environment. It’s adaptive, serving an important goal: maintaining memories that produce accurate predictions of the world (Kroes & Fernández, 2012). Accurate predictions must be current. To achieve this, memory needs to be able to update.

Learning updates our memories by adding to what we know.

Forgetting can update our memories by making less helpful information less accessible.

Imagine you use your memory to make a prediction about the environment and that prediction turns out to be wrong. The connections in the memory trace may weaken. This is helpful: it means you’re less likely to access this poor prediction in future.

This means learning and forgetting may be two sides of the memory updating coin (Ryan & Frankland, 2022).

But what exactly do we mean by forgetting?

What is forgetting?

Memories are formed when information is encoded as a memory trace. Forgetting is thought to be the loss of access to the memory trace. In other words, it’s when you can’t retrieve the memory with the cues you have.

There are multiple theories and many mechanisms to explain forgetting (Ryan & Frankland, 2022; Wixted, 2004). Let’s look at some of these mechanisms and what they may indicate about how we should teach.

Use it or lose it

Memories rarely used become prime candidates for forgetting. Weaker synaptic connections are thought to get pruned in the brain and this weakening could occur when memories aren’t being accessed (Ryan & Frankland, 2022). This means students who haven’t studied a topic over the summer may appear to have forgotten it. But ‘lose it’ doesn’t mean the memory is erased. The trace is still there; it’s just inaccessible. Which leads us to our next point…

Reactivate not reteach

We can despair when students have forgotten knowledge we thought was secure. It’s tempting to reteach the topic from scratch.

But forgetting is actually a lack of access to the memory.

We’ve all experienced the spontaneous recovery of a memory we thought we’d forgotten when we have the right cues, e.g. we hear a song that cues a memory of a party we thought we’d long forgotten.

This tells us that with the right cues, memories can be recovered (Davis & Zhong, 2017).

An article by Bjork, Jones & Wiliam explains brilliantly why, instead of testing (or reteaching from scratch) we should begin with a “refresher review” of previous material students haven’t accessed for a while. This provides students with the cues to see if they can retrieve the knowledge.

Connect it

Why are some memories stored for the long term yet others seem to disappear immediately?

When a memory is encoded, two competing processes may start: consolidation (long-term storage) and forgetting (Davis & Zhong, 2017; Ryan & Frankland, 2022). Which process prevails partly depend on how the memory was first formed.

Not all memories are created equal. This means they aren’t forgotten at the same rate or in the same way.

To understand how memory formation affects forgetting, we delve into memory consolidation…

Memories are generally encoded in the hippocampus. The memory trace is usually fragile at this point. Memories can then be stabilised through a process called consolidation: the hippocampus replays some memories to the neocortex (outer layer of the brain) where they are stabilised and stored in networks.

The problem is, consolidation is a process that usually requires rest/sleep. This means memories are fragile for a while. Fragile new memories processed in the hippocampus are prone to a type of forgetting called interference (Davis & Zhong). Interference is where access to the memory is disrupted, altering it or preventing it from consolidating properly (Wixted, 2004).

Imagine you teach students the meaning of ‘osmosis’ and they form a fragile memory in their hippocampi. The memories they have formed before and after the osmosis memory have the potential to interfere with this new memory because it’s fragile. These other memories can therefore disrupt the osmosis memory trace and, by taking up capacity in the hippocampus, can interfere with its consolidation (Wixted, 2004; Alonso et al., 2020).**

How do we help students avoid interference and put memories on the path to consolidation?

By ensuring they connect new information to what they already know.

If the brain perceives new information as relevant to knowledge it already has stored in its networks, memory consolidation is improved (Tse et al., 2007; van Kesteren et al., 2020). This may happen because the new information bypasses the fragile hippocampal storage (to an extent) and is processed more directly in neocortical networks (van Kesteren et al., 2012).

This means we must help students perceive and form links between new material and what they already know. This puts memories on the path to consolidation.

Harness forgetting: create the right gists

We just looked at how memory consolidation stabilises memory. Consolidation also changes the quality of the memory trace. When memories consolidate, our brains extract common aspects of our experiences, storing them as generalisations or gists. Gists are useful as they apply to multiple situations allowing us to make overall better predictions and think flexibly. To create these gists, our brains must make idiosyncratic details of our experiences harder to access i.e. forgotten.*** This is usually useful: individual details don’t help us make better predictions.

We can harness the nature of forgetting by building curricula and instruction around the important ideas/gists students need to retain. We must make sure students think hard about them and return to them.

However, the detail that surrounds these gists is still important for making this information interesting and meaningful.**** We need to be aware that detail is more easily forgotten (van Kesteren & Meeter, 2020). Checking for loss of important details in students’ memories and revisiting important details are therefore also important.

Harness forgetting: spaced practice

Spacing study episodes over time benefits long-term memory whereas massing episodes leads to faster forgetting (e.g. Sobel et al., 2011).***** This may link to our brains wanting to make the best predictions. Spacing gives the brain the sense that this information is remaining stable over time and is ripe for storage since it will continue to help us make the best predictions. With massed study, the brain can’t get this sense of stability over time (Ryan & Frankland, 2022).

Waiting longer for forgetting to set in before revisiting material may be most beneficial for memory and have lower costs than revisiting material too soon (Cepeda et al., 2008). Clearly this affects our curricula sequencing. To make learning efficient, important ideas should be spaced over time. Forgetting is harnessed by leaving enough of an interval for it to properly set in before students revisit the material.

So we see that forgetting is a necessary part of the brain’s memory management. It is powerful, inevitable and adaptive. The best thing we can do is understand it and work with it.

*This blog focusses on neurotypical brain development rather than those with less typical cognitive variation or, as we are talking about forgetting, pathological memory-related conditions such as dementia and Alzheimer’s disease.

**It’s a lot more complex than this in reality as we are using what we already know to make sense of new ideas. However, if we are failing to make links to relevant prior knowledge, memory traces may remain fragile for longer.

***This is a different type of forgetting. There seems to be many mechanisms the brain can use to weaken access to memories.

****I think this relates to Counsell’s (2018) concepts of core and hinterland.

*****There are different theories as to why including a consolidation account (Smith & Scarf, 2017).


Alonso, A., van der Meij, J., Tse, D., & Genzel, L. (2020). Naïve to expert: Considering the role of previous knowledge in memory. Brain and neuroscience advances, 4, 2398212820948686.

Bjork, R. A., Jones, K., Wiliam, D. (NY). Why Testing Shouldn’t Be the First Response to Last Year’s Learning Gaps. ascd. https://www.ascd.org/el/articles/why-testing-shouldnt-be-the-first-response-to-last-years-learning-gaps

Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological science, 19(11), 1095-1102.

Counsell, C. (2018, April 12). Senior Curriculum Leadership 1: The indirect manifestation of knowledge: (B) final performance as deceiver and guide. The Dignity of the Thing. https://thedignityofthethingblog.wordpress.com/

Davis, R. L., & Zhong, Y. (2017). The biology of forgetting—a perspective. Neuron, 95(3), 490-503.

Kroes, M. C., & Fernández, G. (2012). Dynamic neural systems enable adaptive, flexible memories. Neuroscience & Biobehavioral Reviews, 36(7), 1646-1666.

Ryan, T. J., & Frankland, P. W. (2022). Forgetting as a form of adaptive engram cell plasticity. Nature Reviews Neuroscience, 23(3), 173-186.

Smith, C. D., & Scarf, D. (2017). Spacing repetitions over long timescales: a review and a reconsolidation explanation. Frontiers in Psychology, 8, 962.

Sobel, H. S., Cepeda, N. J., & Kapler, I. V. (2011). Spacing effects in real‐world classroom vocabulary learning. Applied Cognitive Psychology, 25(5), 763-767.

Tse, D., Langston, R. F., Kakeyama, M., Bethus, I., Spooner, P. A., Wood, E. R., … & Morris, R. G. (2007). Schemas and memory consolidation. Science, 316(5821), 76-82.

van Kesteren, M. T. R., & Meeter, M. (2020). How to optimize knowledge construction in the brain. npj Science of Learning, 5(1), 1-7.

Van Kesteren, M. T., Ruiter, D. J., Fernández, G., & Henson, R. N. (2012). How schema and novelty augment memory formation. Trends in neurosciences, 35(4), 211-219.

Wixted, J. T. (2004). The psychology and neuroscience of forgetting. Annual review of psychology, 55(1), 235-269.

3 responses to “Understanding forgetting”

  1. Thank you, very helpful.

    Liked by 1 person

  2. […] Understanding forgetting Forget everything you think you know about forgetting. […]


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