RAS and What You See

I leave things out on purpose. If I need to take something upstairs later, I put it at the bottom of the stairs. I keep a handwritten list of the day’s tasks on my desk, not in an app, on paper, where I can see it and see the items on it. I do the dishes when I see them in the sink. Books I want to read stay on the counter, not the shelf. When I want to stay engaged with something, my first instinct is to make sure I’ll see it.

It’s a memory system that relies on the environment rather than on my own recall. The logic is simple: if it’s in my line of sight, I’ll think about it. If it’s put away, I probably won’t. What I’ve been curious about is what’s actually going on in the brain that makes that true.

The Reticular Activating System

The name comes from the Latin reticulum, little net. The reticular formation, the brainstem structure that houses the RAS, got that name because under a microscope it looks like a diffuse, woven mesh of neurons rather than anything with clean edges or obvious organization. Anatomists in the 19th century named it for what they could see, long before anyone understood what it did. The “activating system” part came much later, in the 1940s, when researchers figured out it was responsible for arousal and wakefulness. There’s something fitting about the name, a net is exactly what it is.

The word also has an unexpected connection. My first instinct was that reticular had something to do with the eye, which isn’t wrong, just one layer removed. The retina shares the same root: rete, Latin for net, named for its mesh of blood vessels and photoreceptors. The brainstem filter and the eye itself are both, etymologically, nets, one catching light from the world, the other deciding what in that world is worth registering.

Your senses are taking in somewhere around 11 million bits of information per second. Your conscious mind is processing about 50. Something has to decide what makes the cut.

That something is the reticular activating system, a cluster of neurons at the base of the brainstem that acts as a filter between the sensory world and conscious awareness. Everything you notice, every time your attention gets snagged by something in your environment, that’s the RAS deciding this one is worth flagging.

What it flags: anything novel, anything threatening, anything personally relevant. Your name in a crowded room. A familiar smell. An unresolved problem you’ve been turning over. The filter is dynamic, it’s calibrated around what your brain currently considers worth paying attention to, and it updates constantly.

The “new car” phenomenon is a clean illustration. After you buy a car, you start noticing that model everywhere. They were always there, the roads haven’t changed. What changed is that your brain updated the filter. The model is now personally relevant, so the RAS starts surfacing every instance of it that would previously have slid by unregistered.

It’s a genuinely strange thing to sit with: you are actively not seeing vast amounts of what’s in front of you, and you have almost no awareness that this is happening. The world you experience consciously is a heavily curated selection from a much larger feed.

How the Filter Gets Set

The RAS doesn’t decide relevance from scratch. It takes its cues from what the brain has been primed to care about, whatever’s currently active: goals, worries, things you’ve been thinking about repeatedly, things that are emotionally charged. These shape what gets flagged.

This is the actual mechanism behind a lot of things people talk about in vague terms. The idea that you start noticing opportunities when you’re actively pursuing something, or that anxious people seem to find confirmation of their fears everywhere, this isn’t just psychology in the loose sense. The filter is literally selecting for it. When something is on your mind, the RAS starts pulling related signals out of the background noise.

One direction this goes that I find interesting: vivid mental imagery seems to work as input to the filter in a similar way that actual visual experience does. Athletes have used visualization for decades, and part of what’s happening is that the brain doesn’t fully distinguish between imagining a performance and executing one, similar neural pathways, similar effect on the filter. Whether this extends into territory like vision boards is a question I hold more loosely, but the underlying mechanism isn’t mystical. The brain is doing pattern recognition, and what you’ve attended to vividly counts as signal.

What I’ve Noticed About Physical Objects

The dishes in the sink get done. The ones soaking in a basin under the counter, out of sight, less reliably.

This is the same dynamic as everything else I described at the start. If I need to remember to take something upstairs, putting it at the bottom of the stairs works. Things I want to remember to deal with get placed somewhere I’ll see them, not somewhere tidy. The handwritten list on the desk isn’t just a reminder system, it’s a way of externalizing thought into the visual field so the filter can keep working on it. Writing something down and leaving it visible is different from writing it into an app. One is in the environment. The other requires you to go looking.

There’s a concept in behavioral science called friction: objects that require retrieval steps have high friction, objects already in front of you have low friction. The RAS adds something to that explanation. It’s not just that an object is easier to grab when it’s visible. Visibility means the brain is actively registering it, noticing it again every time you pass, keeping it alive in awareness in a way that stored things aren’t. Putting something out of sight doesn’t just make it harder to access. It removes it from the filter entirely.

There’s an important caveat though, it doesn’t mean everything should just be out everywhere. A space full of unrelated stuff competing for attention is noise, not signal. The filter can’t do useful work when everything is equally visible and none of it is connected to what actually matters right now. The version of this that works is closer to a hierarchy: the things that deserve attention in the foreground, everything else stored. The goal isn’t maximum visibility. It’s selective visibility, curating what the filter gets to work with.

The Limits of Screens and Notifications

A notification is different from passive visibility, and the difference matters.

A notification interrupts, it pulls you out of whatever you were doing and creates a reactive moment. The thing on the counter just sits there, in your field of view, getting registered quietly every time you pass. No interruption, no context switch, just presence. The RAS works best with the second kind of signal, passive and continuous rather than momentary and alerting.

This is part of why phone reminders and digital lists don’t work as well for certain kinds of things. An app in the background isn’t visible. Your phone in your pocket isn’t cueing anything. The digital version of “I’ll remember this” requires you to remember to look at the right moment, which is exactly what you were hoping to offload.

But there’s a darker side to how digital systems interact with the filter. News, social media, and most algorithmically driven platforms are optimized for exactly what the RAS flags by default: novelty, threat, outrage, faces in distress, status signals. The filter was built to prioritize those things because they historically mattered for survival. The attention economy is essentially a business model built on exploiting that wiring, keeping the filter activated, keeping the signal stream full of things the brain is designed to find impossible to ignore.

Which leads somewhere uncomfortable: if the same system that makes the handwritten list on your desk work is the system being targeted by every app on your phone, then the question of what you put in your visual environment is also a question about what you’re choosing to prime the filter with. The physical environment you curate and the media environment you consume are both inputs into the same mechanism.

And the system itself isn’t neutral about visual information, it trusts it more than almost any other kind. We believe what we see in a way we don’t believe what we’re told. This makes vision our most useful sense and also our most exploitable one. Optical illusions, stage magic, deepfakes, political imagery, all of them work by feeding the visual system something it will process as real before the rest of the mind has time to evaluate it. The filter flags it as salient. The brain believes it. The correction, if it comes, comes later and with less force than the original impression.

How Visual We Actually Are

It’s worth pausing on how much of human cognition is built around visual processing, with the caveat that this plays out very differently from person to person.

More than half the cortex is involved in processing visual information, more than any other sense, by a significant margin. That’s the architecture. But the subjective experience of thinking varies enormously. Some people think primarily in images, running vivid internal scenes almost like a film. Others think almost entirely in words, or in abstract concepts that aren’t really either. The spectrum has names at its extremes: hyperphantasia, near-photographic mental imagery, and aphantasia, the complete absence of any mind’s eye. People with aphantasia cannot generate a mental image of an apple or replay a scene from memory visually. They tend to think in words, semantic descriptions, spatial relationships, or felt senses, knowing something is true without seeing it. Many only discover this is unusual as adults, because the assumption that everyone “sees” things internally is so embedded that it rarely comes up in conversation.

Francis Galton was studying this variation as early as the 1880s and was surprised to find that many scientists reported little to no mental imagery, while others described their inner experience as intensely visual. It’s one of those things people tend not to discover about each other because the default assumption is that inner experience is basically the same for everyone. It often isn’t.

The filter itself also varies. ADHD, for example, is partly a calibration issue, the gate is set differently, so more gets through, or the wrong things get flagged as relevant. People with ADHD often describe noticing everything, being pulled by every stimulus, unable to filter the environment down to what actually matters in the moment. It’s the RAS running with a different threshold, not a broken system, but it illustrates how much of what we think of as focus or attention is really the filter doing its job at the right sensitivity level.

Kids are a useful reference point here too. Children’s attention is much less filtered than adults’, they notice everything, which is why they learn so fast. The RAS becomes more selective over time, which is useful, but it also means we stop noticing things we once did. Part of what people call beginner’s mind, the quality of attention a novice brings to something, is probably the filter not yet having learned to screen that domain out.

The most vivid visual experience most people have happens with their eyes completely closed. Dreams are the visual system running full productions without any external input, the same machinery that processes the physical world generating scenes from scratch. What’s strange is that some people with aphantasia, who cannot generate mental images while awake, report that they do see images in dreams. Which raises genuinely interesting questions about where the gap is: whether waking visual imagination and dreaming use different pathways, or whether the gating mechanism that blocks imagery during waking hours relaxes during sleep.

The more defensible version of “we’re visual creatures” is that the visual processing infrastructure dominates the cortex regardless of how consciously visual someone’s thinking is. Even in people who think in words, visual cortex regions activate during reading and language processing in ways that don’t rise to experienced imagery. The machinery is visual even when the subjective experience isn’t.

Language reflects this in ways so embedded we don’t notice. We see a solution. We look at a problem from different angles. We have insight, foresight, vision. We illuminate an idea, shed light on confusion. The vocabulary of understanding is almost entirely visual, which holds even in people whose inner experience is predominantly verbal. Thinking and seeing seem to be, at some level, the same kind of thing.

The Filter and Other People

The same mechanism that decides whether you notice the thing on the counter is also shaping what you see in the people around you.

If the filter is calibrated around what’s personally relevant, other people are probably its oldest and most deeply embedded priority. For most of human history, reading the social environment correctly mattered more than almost anything else, who’s allied with whom, who has status, who’s watching you. Getting those reads wrong had real consequences. So the filter is exquisitely sensitive to social signals: faces, expressions, eye contact, shifts in group attention. Not as a deliberate process, but as a baseline condition of being in a room with other people.

Eye contact is one of the sharpest triggers. Direct gaze cuts through almost everything else in the environment, registered nearly instantly, before conscious processing has time to weigh in. It signals something: threat, interest, dominance, recognition. Across species, not just humans. The filter treats it as high-priority information essentially by default.

Status works similarly. High-status individuals in a room get tracked more, noticed more, remembered better, without any intention on the part of the observer. Part of what “commanding attention” actually means neurologically is reliably triggering RAS flags: through presence, unpredictability, the kind of stillness that reads as confidence. Charisma may be partly just a consistent ability to activate the filter in other people.

But the most interesting territory is how the filter shapes what you see once you’ve already looked at someone. Once you’ve formed a read on a person, the RAS starts selecting for confirming evidence. If you’ve decided someone is untrustworthy, neutral expressions start reading as evasive. If you’re primed to see hostility, ambiguous moments get classified as hostile. This isn’t a loose metaphor for confirmation bias, the filter is literally constructing the social reality you perceive. You don’t see people as they are. You see them through whatever the filter has already decided to surface.

The same priors that make you efficient in familiar situations make you slow to update in social ones. The filter that locked in a read about someone years ago is still running that pattern, still selecting evidence that confirms it, still screening out the evidence that doesn’t. The question of how those priors got set, and whether they’re still calibrated correctly, seems worth asking more often than most people do.

The Larger Thing About Vision

There’s a thread that goes further than the practical.

Vision is how we navigate space, but it’s also the medium through which we imagine what doesn’t exist yet. Every plan is a visual simulation. Every creative act starts with something someone saw in their mind before they made it real. Buildings, businesses, works of art, they existed as images before they existed as things. The ability to see what isn’t there yet, to hold a picture of something future or possible, is arguably the most distinctly human cognitive capacity we have.

And the language reflects this. We don’t say we heard the future or felt an insight. We say we envision, we see the path, we have vision for what we’re building. In almost every tradition across history, understanding is described as light, and confusion as darkness. Whether that’s poetic convention or points at something real about the structure of cognition, I’m not entirely sure. But it’s a striking convergence.

It makes the question of what we’re actually looking at every day feel like more than a productivity question. If visual processing is this central, not just to perception but to thought, memory, imagination, and understanding, then the visual environment is doing something more significant than background decoration. It’s a constant input into the most active cognitive system we have. What you put in front of yourself, what you keep out of sight, what you consume, who you look at and how, all of it is feeding a filter that is, in turn, constructing the world you experience.

I don’t have a tidy conclusion here. It’s more that I find the whole thread worth following, from how the RAS filters experience, to how thoroughly visual human thinking actually is, to what it implies about the spaces we inhabit, the objects we keep visible, and how we perceive the people around us. These feel like connected ideas, and I’m probably still working out where they lead.