To live is to forget – account numbers, names, the precise locations of keys and wallets, friends from childhood, peripheral characters from prestige TV shows, inside jokes, past ambitions, U.S. history, much else. Goldfish with guns: that’s the human race. But every frailty, we know, serves some larger adaptive purpose. So it is worth asking, as we wrack our brains for whatever it was we know we were supposed to do today: why must things be like this? Why do we forget? For this week’s Giz Asks, we reached out to a number of neuroscientists and psychologists to find out.
Assistant Professor of Brain and Cognitive Sciences and Head of the Memory and Forgetting Lab at Ben Gurion University of the Negev
Why do we forget things we once remembered? For years, researchers have attempted to adjudicate between two possibilities. The first is that forgetting occurs because, like colours that fade, memories decay over time. The second possibility is that similar memories interfere with one another. For instance, when meeting many new people on the same day, we might forget some of the faces because we have many similar memories of faces interfering with one another.
Recent scientific evidence has shown that forgetting can occur either due to decay or due to interference, depending on the brain structure supporting memory. The hippocampus, a structure important for memory, has unique properties which enable it to distinguish between similar memories. Therefore, memories relying on the hippocampus are not likely to interfere with one another. However, these memories fade and decay quickly. Another structure which supports memory, the Perirhinal cortex, does not have a good ability to distinguish between similar memories. Therefore, memories relying on this structure are more likely to interfere with each other.
“Like colours that fade, memories decay over time.”
Assistant Professor of Psychology and Head of the Long Term Memory Lab at the University of Virginia
Exactly why some experiences stand the test of time and others fade from memory is an on-going topic of research across a number of psychology, cognitive, and neuroscience labs as experiences can be forgotten for a number of reasons.
Why do we forget people’s names right after being introduced? The cause of that forgetting is likely due to inattention. We’re so familiar with the “procedure” of an introduction that we start to “tune out” or stop paying attention, even before the other person has said their name. This is why strategies such as repeating the person’s name back to them are recommended during interviews – repeating a name will increase the likelihood that you attend to it, helping you remember it later.
Why do you forget where you’ve parked your car? The cause of that forgetting is likely interference. Every day, you park your car in a particular location, maybe having to change it depending on available spaces. Your brain forms a link or an association between the car and the location. When you next think of your car, your brain retrieves, or brings to mind, many of those past associations. You then have to sift through all of those associations to find the right one, making it more difficult to remember. This process can be made more challenging – if not impossible – if you didn’t pay attention to the location when you first parked the car, analogous to the introduction example. If you didn’t form that association in the first place, it won’t even be there to retrieve.
Finally, why do you forget things that you used to know? We know we went to grade school, and surely at some point had to remember a few things in order to pass tests and graduate, but why can’t we remember those things now? It’s a great question, but a really tricky one to answer. Is the original memory gone, or is that you just can’t access it? You might have forgotten your 4th grade teacher, but if you went back to your elementary school and re-experienced being there, you might be able to remember. Many instances of forgetting are like this, in that the memory might be stored in your brain, but you have insufficient cues or lack the information needed to access it.
Despite the benefits that might come with being able to remember every single experience in our lives, research suggests that some forgetting is actually beneficial. For instance, if your usual parking lot is demolished and replaced with a high rise, retaining those old car-location associations simply isn’t useful anymore and will actually get in the way (in the form of interference) of forming new car-location associations. Thus, although we may lament forgetting experiences, ultimately such a process enables us to adapt flexibly to the world in which we live.
“Emotional events tend to be better remembered than neutral, every day ones – we remember them simply because they may guide future behaviour back to positive rewards, or away from threatening places.”
Associate Professor, Psychology, NYU and Head of the Davachi Memory Lab
I think you are asking the wrong question. The only way to answer the question ‘Why do we forget?’ is to know the answer to ‘Why do we remember?’. Once we have a full understanding of why we remember, only then can we begin to understand why we forget.
But we can start with the fact that forgetting is one of the most reliable effects in memory. We forget most of what we encounter every day, and forgetting is not picky: we forget places, things, colours, sounds, names. Whole days and sometimes years of our lives become inaccessible. Why? Well, my first assumption is that this must be a clue to how the memory system is supposed to work. Forgetting must be adaptive. There are many theories about why forgetting is a good thing – perhaps we don’t have enough room to store all of our experiences, so we save room for only the most important, the ones that we will likely need in the future. That could explain why emotional events tend to be better remembered than neutral, every day ones – we remember them simply because they may guide future behaviour back to positive rewards, or away from threatening places.
However, even if one assumes an endless amount of storage space for memories, does it even make sense to store everything? Will I need to remember my specific grocery list from yesterday? What about from 30 days ago? We experience a lot in the world that simply will not be relevant or important very soon, so the brain dumps it. Over time, what the brain tends to be good at storing for the very long term are the elements of our experiences that are repeated over and over again, or even that are approximated. We can recall our walking path to our first job, or to our college dormitories. We may be able to recall the layout of our childhood homes. These are the elements that were a part of several dozen or hundreds of experiences – and the brain finds a way to extract and represent these regularities pretty faithfully and for a very long time. So, why do we have this very detailed episodic memory that seems to decay almost immediately? As I mentioned at the outset, that is the burning question…
“When the brain experiences new information, it tries to overlay that new information on top of previous experiences that it’s had.”
Assistant Professor, Psychology, SUNY Geneseo, whose work examines the cognitive and neuropsychological properties of human memory
The main reason we forget is due to a phenomenon called interference, which is when new information interferes with previous things you’ve learned.
Let’s say you watch a new episode of your favourite TV show. Chances are, you’ll remember a lot of what happened in the episode before it’s over. But if, immediately after you finished watching it, you then watched the second episode, your memory for the first one would be worse. That’s because, as you’re encoding the second episode in your brain, it’s overlapping and interfering with what you’d learned before, in the first one.
When the brain experiences new information, it tries to overlay that new information on top of previous experiences that it’s had. This is efficient for the brain, because when you encode new experiences in the context of older things, you can sort of piggyback your new experiences on top of your older memories. But you’re kind of changing things as you go, and that can lead you to forget elements of what you’ve experienced before.
Rehearsal is another important element in all of this. One of the reasons why we forget a given memory – what makes a memory vulnerable to forgetting – is how well we’ve rehearsed it. If you watch that first episode five times in a row before watching the second, you’re going to retain a lot more than you would have otherwise.
Forgetting isn’t always a bad thing. Sometimes we actually want to forget. Studies have shown that people can intentionally forget things that are no longer useful to them, seemingly by just not rehearsing those things. If you do experience something that you don’t want to remember later on, for whatever reason, you can just try not to think about it, and that memory – because it’s not being rehearsed – will end up interfered with and forgotten.
“Forgetting in such instances happens when memories interfere with each other and memories that are unique are less subject to forgetting due to interference.”
Postdoctoral Fellow at the Dynamic Memory Lab at UC Davis
One way we forgot is when we try to access information from our memory, but fail at it. This is, essentially, a retrieval failure, and in these cases we know what we want to access (an email password, for instance). The goal of accessing the email requires the password, and this starts a search in our memory, using a cue: “password.” This search activates not only the target information – i.e., the correct password – but also non-target information, like old passwords for the same email address. Now we have password candidates from our search process, and one of these need to be selected. During this selection process, these password candidates compete with each other. In the end, we choose one of these candidates. If it is not the correct candidate, it means we failed at retrieving the target information which we describe as “forgetting our password”. This type of retrieval failure happens because memory representations that are activated by a certain retrieval cue interfere with each other. This type of forgetting especially happens when we reset our password. The connection between the cue and the old password is very strong compared to the newly formed connection between the cue and the new password which makes the old password a very strong candidate.
In my research, I have investigated how forgetting due to interference happens for emotional memories compared to neutral ones. I found that although the mechanisms behind forgetting work similarly for both types of memories, there is consistently less interference, less forgetting, for emotional memories. So you might not forget emotional memories such as the 2016 U.S. presidential election at all because there are not many memories that can compete with that. It is also impossible to create memories that can compete with that even if we wanted to. On the other hand, if you try to remember where you parked your car yesterday, good luck with that! There will be hundreds of memories up for competition.
In summary, forgetting in such instances happens when memories interfere with each other and memories that are unique are less subject to forgetting due to interference.
“Neurons that fire together wire together.”
Graduate Student, Cognition, Brain & Behaviour, University of Notre Dame, whose research focuses on memory and event cognition
From my perspective, this question can be answered from either a cognitive or a physiological standpoint. Of course, both standpoints should be mutually supportive of one another. First, however, the reader should keep in mind that forgetting can be adaptive. It is important to forget irrelevant information that would otherwise clog up the memory system.
Cognitively, there has been a longstanding debate as to whether forgetting is due to (a) the decay of information in the memory stores or (b) interference from competing information at retrieval. In terms of the interference hypothesis, memory researchers make a distinction between availability (i.e., is the information still in the memory system?) and accessibility (i.e., can the existing information be presently reached at this moment of retrieval?). To illustrate, consider the tip-of-the tongue phenomenon where you know the word is in your mind somewhere, but you just can’t get in the right frame of mind to grab that word. Or consider the case where you visit your old high school, see your old locker, and suddenly a series of old memories spring back into mind. From the interference perspective, these old memories were still preserved in some manner and form, and they can now be utilised for retrieval processes.
Physiologically, the dominant theory is the synaptic plasticity hypothesis that can be summed up with the common phrase, “neurons that fire together wire together”. That is, learning (or encoding new information) is facilitated by strengthened synapses in our neural circuitry. For the events of our lives, this process is thought to largely be dependent on the hippocampus at first. However, over time, this information is thought to be translated into cortical areas. Forgetting can occur when the specific synaptic circuitry that holds a memory is weakened.
“Over very short intervals memories require changes in the strength of connections at synapses, the meeting points between neurons; over longer periods information is moved around the brain.”
Professor, Cognitive and Behavioural Neurology, University of Exeter
There are many reasons why we forget. One way to think about these is in terms of the stages involved in ‘memory processing,’ which include perception, memory formation, storage and retrieval. Failure to perceive is a common reason for (apparent) forgetting – the last time you forgot something your partner told you, it may well have been because you weren’t listening in the first place! This also helps to explain why we forget many mundane events, like brushing our teeth – we pay them little attention.
But assuming we are paying attention, memory formation is a complex process, extending over seconds to hours at least and, probably, much longer. Over very short intervals memories require changes in the strength of connections at synapses, the meeting points between neurons; over longer periods information is moved around the brain. This process of building a memory can be disrupted dramatically by such things as a significant head injury: it’s common to lose the ‘fragile’ memories for events occurring just before them. There is also evidence for a natural ‘decay’ of memories for events of little importance: these ‘consolidate’ less well than emotionally important ones – and are forgotten.
But even memories which have been well consolidated can be forgotten. I study people with epilepsy who ‘lose’ memories for salient personal events: in some cases, these seem to be ‘deleted’ by the electrical storm that occurs in an epileptic seizure. More commonly, we lose such memories transiently, because we are unable to ‘locate’ them – this is a failure of retrieval rather than storage, which we can overcome by finding the right cue to fish the memory to the surface. And finally, we can forget because we decide to: the surprising phenomenon of directed forgetting. I’m sure there are other reasons too – but I forget.