My husband was recently describing something that happened on a past holiday. It wasn’t a significant event, but it sounded pleasant. I, however, had no recollection of what he was telling me. He couldn’t quite believe it.
We know that “recollections may differ”, but how can it be so different? And why do I not have this memory? I’m busy at work – have I simply run out of space?
It’s a tempting explanation. We talk about “full heads”, “information overload”, and “too much to take in” as though the brain were a container that eventually reaches capacity. But the brain does not fill up. Instead, it filters.
At any given moment, far more information is available to us than we could ever realistically store. The sights, sounds and conversations of even a single day would overwhelm any system that attempted to record them in full. Instead, the brain relies on selection. Attention determines what is noticed. Emotion helps determine what matters. Then, structures such as the hippocampus decide what is worth committing to longer-term memory.
If your attention is elsewhere, the process falters at the first step.
On that holiday, my husband may have paused long enough to register the moment. I may have been thinking about where we were going next, checking timings, or simply moving through the day without stopping to take it in. The difference is subtle, but it matters. Without focused attention, experiences are only weakly encoded, if at all. In that sense, the memory was not lost. It was never fully formed.
Even when memories are successfully encoded, they are not stored as fixed records. Each time we recall an event, we reconstruct it, drawing on fragments of sensory detail, prior knowledge and expectation. With repetition – through conversation, reflection or retelling – those reconstructions become stronger and more coherent. Over time, they can feel increasingly vivid and certain.
This helps explain why shared experiences can diverge so dramatically. We assume that living through the same moment should produce the same memory, but the brain does not work that way. It does not passively record experience. It actively selects, prioritises and, just as importantly, discards.
The feeling that our brains are “full” arises not because we have run out of storage, but because we have reached the limits of what we can process at once. Attention is finite. Working memory – the small amount of information we can actively hold in mind – is even more limited. When these systems are saturated, new information struggles to gain a foothold. This is the mental equivalent of too many tabs open: nothing has been permanently lost, but everything becomes harder to manage.
Where the computer analogy breaks down
Computing analogies are useful up to a point. If working memory resembles RAM – fast, temporary, limited – then long-term memory is often compared to a hard drive. But this is where the parallel breaks down. A hard drive stores files in fixed locations, retrievable in exactly the same form in which they were saved. The brain does not work this way.
Memories are not stored as discrete files. They are distributed across networks of neurons, overlapping, reshaped, and reassembled each time they are recalled. New experiences do not simply add to what is already there – they interact with it, altering both the new and the old.
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Attempts have been made to estimate how much the brain could theoretically hold. One widely cited figure from the Salk Institute puts it at around a petabyte – roughly equivalent to hundreds of years of continuous video. It is an impressive number, but also a somewhat misleading one. It implies a storage system that fills up over time, when in reality the brain is constantly reorganising itself. Capacity is not fixed, and information is not stored in isolation. It is integrated, modified, and, when no longer useful, allowed to fade.
Which raises a slightly uncomfortable question: what happens to the memories we would like to keep?
Some of them will fade – not because the brain has run out of space, but because they are not continually reinforced. Memory is not preserved simply because it matters to us. It is preserved when it is revisited, retold, or reconnected to other experiences. Without that reinforcement, even meaningful moments can become harder to access over time.
What is lost, in most cases, is not the memory itself but our ability to retrieve it. A familiar smell, a piece of music, or an unexpected detail can bring something back that seemed entirely gone. The trace remains, but it has slipped out of reach. And the absence of a memory is rarely evidence of a system at capacity – more often, it is the trace of a moment that was never fully stored, or one that has simply not been called upon.
Michelle Spear does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.