tl;dr:
- The bizarre otherworldliness of dreams makes them seem foreign, almost as though they came from some weird place "out there", not of our own making.
- This in turn makes recurring dreams puzzling, especially when they are separated by long intervals (sometimes years); if dreams come from "out there", where the heck is that and why can it store these recurring patterns and places stably over time?
- These false intuitions are dispelled once we realize that our brains are all about storing patterns, and the same mechanisms that allow us to form waking memories and mental models are the ones that provide us with a stable pool of patterns from which we build recurring dreams.
- This simple fact is occluded and obscured by the apparent forgetting that happens on waking.
- This all seems bleedingly obvious in retrospect, and it makes one wonder why I even needed to write it down.
Over a period of many years now I've had a series of recurring dreams, or at least, recurring themes within dreams. When you're in the dream world, it seems detailed and real, yet at the same time unreal or surreal because of the way in which improbable or impossible things occur. You find places and people morphing from one into another in a way that seems to simultaneously escape your notice while also registering in way that causes you to remark on it later on. The laws of physics are defied. Rules of causality are suspended. Events are reordered and incompatible facts are juxtaposed. These bizarre ensembles of characters, interactions, and locations are so unexpected, so novel — even if they are stitched out of a patchwork of people and things you know or can imagine — that it is almost like they're being delivered to you from the outside, by a Christopher Nolan-esque cinematic auteur of unbounded and inimitable creativity.
The thing that has struck me about these recurrences is just how complicated the dream world seems to be. Your brain appears to synthesize these fantastical locations, not just randomly — like a procedural generator would create terrain in an open-world video game — but in a way that has permanence, because you can find yourself back in those same places months or years later.
At first this ability to recall these impossible, intricate places puzzled me, but I have a theory now about what makes them come back. I'm not going to get into how dreams get put together, because I honestly have no idea, but I want to explain how it is that these recurring patterns are more or less durably stored and made available for reuse over long periods.
So, let's start from the basics. How do brains work? Let me give you my horribly imprecise understanding of this, hopefully one that is just vague enough to be compatible with how things actually work in there. Brains are massive networks of interconnected neurons. Synapses fire. Signals are transmitted between neurons. There are activation thresholds that dictate whether signals get through. Importantly for the purposes of this discussion, brains are not merely deciding machines that control the systems and actions in the organism that they inhabit; they are dynamic, evolving, self-modifying receptors and recorders of information and patterns. That's how memories are made: the act of "recording" a memory is in some sense a rewiring of the brain in such a way as to capture a pattern and make it available for latter retrieval.
There is a curious difference between memory and memorization. We form memories effortlessly all the time as experiences wash over us. In contrast, memorization is an intentional, effortful act aimed at creating a memory for later recall. Perhaps frustratingly, this latter activity can be quite hard. But the automatic formation of memories is as easy as breathing; in fact, it's something we can't help but do. This is not to say that our memory is infalible. Details may escape us, memories may fade (become harder or imposible to access), facts may be switched. But in the absence of dementia or other pathologies, the brain is a marvelously flexible, capacious, and impressively reliable self-updating reservoir of information.
A particular kind of memory is the "mental model". We typically use that word to describe things that are a little more abstract that a vanilla memory. For example, we might refer to a mental model of "how the economy works", or "what Alice thinks about Bob". But we also have spatial models of the environment around us — the neighborhood we live in, for example — and these are much more akin to memories, in the sense that we acquire them and build them up automatically, without even intending to, by the mere fact of experiencing the environment around us.
And this memory formation, this rewiring of the brain, it becomes more and more accurate, detailed and durable as we repeatedly move through that environment, in a sense "wearing in" the connections in a way that makes them stronger and more complete. (As an aside, this is the power of positive thinking too — and the harm of negative thought patterns — because when we repeatedly activate the same neural pathways we "burn in" in a way that makes it all the more likely that we'll fall into following the same pathways again in the future.) After a while, your mental model of the area you live in becomes so detailed and comprehensive that you can name and visualize countless details about it, large and small, involving distances, textures, smells, relationships, and all manner of patterns and symbols.
I think that it's this that's happening when you visit places in dreams. Your brain is conjuring up these fantastic places, making a pastiche of experiences, patterns, places, and symbols — both lived or perhaps experienced in fictional worlds of movies, TV shows, video games, comics, magazines — by some mechanism that I haven't even begun to understand. But regardless of the source for this material and its combinations, it winds up leaving an impression on your brain. It leaves a mark, not quite the same as a waking memory, but a nearby cousin to it. Something is recorded in your brain somewhere. Neurons are rewired. Patterns are inscribed. Symbols are persisted.
The fact that we usually have trouble remembering our dreams when we wake up obscures all the pattern-recording that's actually going on behind the scenes. I don't know the mechanism for this amnesia-on-waking either, but I feel sure that it is only a surface-level illusion. The reason why we revisit places and relive patterns in recurring dreams is because we accumulate a corpus of stored material, a set of "mental models", in our brains that's very close to the kinds of structures we use to store other memories. The dreamscape only seems otherworldly; actually, it's made of the same stuff from which we build our internal representations of the real world. And now that I've written all that out, I'm rather embarrassed that it took me this long to figure out.