The brain, with its 100 billion neurons, allows us to do amazing things like learn multiple languages, or build things that send people into outer space. Yet despite this astonishing capacity, we routinely can’t remember where we put our keys, we forget why we went to the grocery store, and we fail when trying to recall personal life events.

This apparent contradiction in functionality opens up the question of why we forget some things but remember others. Or, more fundamentally, what causes forgetting?

This week my book ‘The Memory Illusion’ drops in Canada, and as a Canadian girl I want to celebrate this by showcasing some Canadian researchers who have given us insight into precisely this question.

An article published recently in Psychological Science by Talya Sadeh and colleagues at the Rotman Research institute in Toronto addresses a long-running debate in the world of memory science; do we forget things because of decay or interference?

A message in the sand

Decay. Advocates of the decay account posit that our memories slowly disappear, fading because of a passage of time during which they have not been accessed.

You can picture this much like a message written in sand, with every ocean wave that flows over the shore making the writing less legible until it eventually disappears entirely. The sand represents the web of brain cells that form a memory in the brain, and the ocean waves represent time passing.

Interference. Often contrasted with this is the interference account, where it is thought that “memories are made less accessible because of interference from similar information acquired before or after their formation”, according to Sadeh and colleagues.

In our beach example, this means that instead of waves slowly corroding the message, a child comes along and writes over it. This makes the message making harder, or even impossible, to read. The child in this example represents a new experience, and the message it writes is the information left behind in the brain by that experience. This leads to forgetting because it essentially overwrites the original memory. This is a process that can also lead to false memories, my favorite topic.

Representation theory

What Sadeh and her Canadian colleagues help to illustrate is that these theories need not stand in competition with one another. Both decay and interference are important for understanding forgetting.

According to their paper, which makes a case for the ‘representation theory of forgetting’, “the primary cause of forgetting… depends on the nature of the initial memory.” The researchers found support for their theory of forgetting by conducting a word memory experiment with 272 students from the University of Toronto. Here, participants were randomly assigned to experimental conditions that varied in terms of how long they waited between learning words and having to remember them, and the extent to which their memory of the words was interfered with by the things they had to do between learning and remembering.

According to the authors, they found support for the idea that a memory can take the form of two different representations in the brain; familiarity or recollection.

‘Familiarity’ is a memory process that allows us to remember something, but without specific details. It is the idea that we ‘know’ something happened although we cannot remember the original context. This is like when you feel you recognize a face, that guy looks so familiar, but you cannot put your finger on where you know the person from.

In contrast to this, if you have a ‘recollection’ of something, you also remember the context of the memory. In this process you recognize that guy, and you remember his name or other defining details. That’s Ed.

Blame the hippocampus

Our Canadian research team suggests that these two types of memory representations act differently, and look different in the brain. Each of them is thought to rely differently on a key part of the brain, called the hippocampus, which is important for making memories; “Recollection-based memories, supported by the hippocampus, are… relatively resistant to interference from one another. Decay should be a major source of their forgetting. By contrast, familiarity-based memories, supported by extrahippocampal structures… [are] sensitive to interference."

Combining our metaphors, that guy who looks so familiar is information that is likely to be forgotten because the child writes over it in the sand, while recalling that’s Ed is more likely to disappear due to waves washing the memory away over time.

So, what have we learned? Why we forget seems to depend on how a memory is stored in the brain. Things we recollect are prone to interference. Things that feel familiar decay over time. The combination of both forgetting processes means that any message is unlikely to ever remain exactly the way you wrote it.