Why we forget: the science behind our lost memories

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Forgetting has more advantages than we could remember

Writer: Karolay Lorenty
Editor: Denis Duagi

We often see forgetting as a failure, the inability to retain fragments of our lives. Experiences, images and words sneakily slip out of our minds to either disappear or to return in the form of a nebulous replica. If we were given the opportunity, many of us would choose to have a flawless memory. But before making this decision, we should first contemplate the real consequences of a brain that cannot forget.

People with highly superior autobiographical memory (HSAM) have the unique ability to remember every day of their lives with precision. But this extraordinary talent has a downside. “Whenever I see a date flash on the television I automatically go back to that day and remember where I was, what I was doing, what day it fell on and on and on and on and on”. This uncontrollable surge of memories results in rumination of past experiences. Can you imagine being able to recall every mistake, every moment of shame, sadness and loneliness? Trapped in a mind that cannot let go, these individuals usually suffer from anxiety and depression. Is it possible that forgetting is not a failure, but a protection? 

In the last decade, researchers have found evidence that forgetting is not just a slow passive fading of memories, but an active process. For memories to be formed, synapses must undergo structural remodelling. One of the pathways involved in this synaptic plasticity is that of the small GTPase Rac1, which is highly expressed in the hippocampus (brain region involved in memory). Researchers investigating contextual fear, where a series of electrical footshocks condition rodents to freeze, have found that inhibiting Rac1 activity after conditioning results in memory formation, as indicated by heightened responses. By contrast, injecting a Rac1 activator before conditioning results in forgetting of the contextual fear. This can be explained by the role of Rac1 in synaptic remodeling. Fear conditioning activates Rac1 to encode the memory. But for consolidation, Rac1 must be inhibited. If synaptic remodeling is continued by persistent Rac1 activation, the memory is forgotten. 

This evidence reveals an active intrinsic mechanism for forgetting. But what is its function? Contextual fear conditioning is used to model post-traumatic syndrome disorder (PTSD). It is possible that while most people experiencing traumatic events can successfully suppress the unpleasant memories, the forgetting mechanism of PTSD patients is impaired. Thus, forgetting is an important protective mechanism. But so far, we have focused on forgetting unpleasant events. Does forgetting ordinary memories present an advantage? Although we may think that an impeccable memory comes in handy for an exam, forgetting seems to play an important role in learning. In fact, it seems to provide an evolutionary advantage. We need to take into account that the goal of learning is not to merely preserve information, but make the right decisions. 

In a dynamic environment, it is important to adapt to rapidly-changing circumstances. Obsolete information needs to be forgotten as new experiences are acquired. Interestingly, increasing neurogenesis in adult mice causes forgetting. When a new neuron is integrated in an established network, it disrupts the circuit, and therefore, the information that was stored. However, it also gives room for encoding new memories, thus increasing cognitive flexibility. Ablation of adult neurogenesis in mice results in decreased performance in a shock zone task, where mice were trained to avoid a region of a circular platform by receiving foot-shocks. When the shock zone was moved 180 degrees, mice with ablated neurogenesis and reduced forgetting were not able to adapt to the new environment.

But what about those details that are not disrupted by new information, those that are simply washed out from our memories? Like a great movie or book that we can clearly remember for a week, and that eventually  becomes, at best, a few images and broken sentences in our minds? It has been suggested that memories are broken down into simpler forms that contain the gist of our experiences. Our brains seem to get rid of unnecessary details and make a summary. But why? Wouldn’t it be beneficial to preserve as much information as possible? Unfortunately, remembering details would also entail difficulties in generalising across experiences. And often, it is the application of what we have learned to other contexts that is valuable. In machine learning, this phenomenon is called regularisation, a process whereby information is simplified to promote generalisation, as an overly precise model leads to inaccurate predictions. So maybe, even if we can’t remember the details, the main messages stay with us, ready to be used in our lives.

Every species with a memory forgets, and so it seems that forgetting is not a failure, but essential for cognition. And as other important functions, it must be tightly regulated. While we’ve seen that an inability to forget can result in less adaptive responses and mood disorders like PTSD and depression, what happens if the opposite is true? Alzheimer’s disease (AD) is characterised by memory loss, and patients present aberrantly increased levels of Rac1 activity. Is it possible that the memory loss seen in AD is triggered by uncontrolled forgetting? A recent study has provided evidence that Rac1 inhibitors could potentially rescue memory decay. Although this is a recent field and its potential is yet to be fully uncovered, this shift in our understanding of memory will not be forgotten. 

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