Our Brains Are Not Multi-Threaded

Photo by Anders Sandberg.

In computer programming, it’s common to split your program into multiple different threads that run simultaneously, as this often simplifies application design.

Imagine, for example, you’re creating a basic game. You might have one thread dedicated to updating the graphics on the screen, another thread dedicated to calculating the next move, and another monitoring the mouse to see if the user is trying to click.

You could, of course, write a single-threaded program that explicitly switches back and forth between working on these different tasks, but it’s often much easier for the programmer to write independent threads, each dedicated to its own part of the larger system.

In a world before multi-core processors, these threads weren’t actually running simultaneously, as the underlying processor could only execute one instruction at a time. What it did instead was switch rapidly between the threads, executing a few instructions from one, before moving on to the next, then the next, and then back to the first, and so on — providing a staccato-paced pseudo-simultaneity that was close enough to true parallel processing to serve the desired purpose.

Something I’ve noticed is that many modern knowledge workers approach their work like a multi-threaded computer program. They’ve agreed to many, many different projects, investigations, queries and small tasks, and attempt, each day, to keep advancing them all in parallel by turning their attention rapidly from one to another — replying to an email here, dashing off a quick message there, and so on — like a CPU dividing its cycles between different pieces of code.

The problem with this analogy is that the human brain is not a computer processor. A silicon chip etched with microscopic circuits switches cleanly from instruction to instruction, agnostic to the greater context from which the current instruction arrived: op codes are executed; electrons flow; the circuit clears; the next op code is loaded.

The human brain is messier.

When you switch your brain to a new “thread,” a whole complicated mess of neural activity begins to activate the proper sub-networks and suppress others. This takes time. When you then rapidly switch to another “thread,” that work doesn’t clear instantaneously like electrons emptying from a circuit, but instead lingers, causing conflict with the new task.

To make matters worse, the idle “threads” don’t sit passively in memory, waiting quietly to be summoned by your neural processor, they’re instead an active presence, generating middle-of-the-night anxiety, and pulling at your attention. To paraphrase David Allen, the more commitments lurking in your mind, the more psychic toll they exert.

This is all to say that the closer I look at the evidence regarding how our brains function, the more I’m convinced that we’re designed to be single-threaded, working on things one at a time, waiting to reach a natural stopping point before moving on to what’s next.

So why do we tolerate all the negative side-effects generated from trying to force our neurons to poorly simulate parallel programs? Because it’s easier, in the moment, than trying to develop professional workflows that better respect our brains’ fundamental nature.

This explanation is understandable. But to a computer scientist trained in optimality, it also seems far from acceptable.

42 thoughts on “Our Brains Are Not Multi-Threaded”

  1. Thank you for articulating what I’ve come to experience first hand here at grad school. All my deliverables are essay-style take home exams or research/seminar papers and I also have a thesis to knock out. Even on days on which I have multiple blocks of time to devote to working on deliverables, I’ve found that it’s easier to focus on just one per day instead of trying to chip away at a few. That, or I need to allow myself a long enough break between the academic tasks to let the first one empty out. Only wish I’d figured this out sooner!

    • Yes! I realized at some point that certain tasks (writing, primarily) require a large amount of what I call buffer time. I can’t just start and stop them, but have to really book spaces of time to get to these projects.

    • That’s exactly how I’m wired. And (to me) similarly I could take one hard class and 1 or 2 easy classes while working full time, but taking 2 or 3 hard classes was too much for me, limited study time made it impossible to change gears smoothly enough to devote the attention to each class that it deserved.

  2. I have few projects that I work at dame time, but what I split the day into blocks or sprints, where I work only at one project during this block. And I must thank Cal for his advice to use expensive notebook for scheduling.

  3. I think this is related, too, to why adults with ADD struggle in the workplace if the managers embrace open offices, encourage constant communication, and so on. ADD adults need the respect of uninterrupted time blocks to allow them to focus as much as possible given that their minds are already prone to multi-threading whether the ADD individual wants their brain to or not.

      • Someone did a study on the human brain and multi-tasking and they found out that people who multi-task are physically unable to concentrate on one thing at a time. The point being multi-tasking is not good for our brains. leave multi-tasking to the computers.

  4. Cal’s work has ‘saved’ me in a way over the past couple of years. Just finishing up a double masters program while working full time. I can’t say it was easy to juggle, just that the combined wisdom of Deep Work and Digital Minimalism gave me the confidence to tackle work projects more ‘one at a time’. My productivity increased in spite of a heavy school load, and the interesting thing was that absolutely no one cared that I protected (and continue to protect) my time fairly carefully. My boss only cares that my work gets done, my program supervisors only care that my courses are complete and my thesis got written. In some environments, we have more control over our work habits than we think.

    • For the book I’m currently writing, I ended up diving deeply into the literature on network switching and its cost on performance, and interviewed three of the top scholars in the general field. The reality of what we’re doing to our brains in our frenetic office environments is pretty grim once you look closer…

    • Neuroscientist here: I am a PhD and know tons of PhD’s who themselves still seem too have personal difficulty with the concepts that Cal espouses here and within his books. They live in the ‘do more things now!’ lifestyle, deadlines constantly in motion. You don’t have to be a biologist to study how things affect our attention, there’s plenty of resources at our disposal these days.

        • Just realize that specializing in one area doesn’t disqualify from working in another. My area is educational technology, but I have to at least be conversant in some areas of educational psychology, pedagogy, web design, and neuroscience. I’m co-authoring a paper in the Communication field that was just submitted last week. The idea of a narrow focus/knowledge base isn’t the reality of most academics.

    • “Many who are self-taught far excel the doctors, masters, and bachelors of the most renowned universities.”
      ? Ludwig von Mises (1881-1973)

  5. I think it would be more accurate to say that consciousness is not multi-threaded, although the brain most certainly is. The multi-threading metaphor is useful when considering the work that the unconscious can do when we “seed” it through our conscious thoughts and actions.

  6. I’ll say I agree with this when it comes to focused attention, but I do think our brains have subprocesses going on—they just don’t rise to the level of conscious awareness until we have a “flash of inspiration”.

    I’ll intentionally use that to my advantage. If I feel stuck on a problem, I’ll “file that away” in my brain and determine that I’ll give it more attention later, then I’ll go do something else (often a physical activity like walking or cleaning up or whatever). It’s amazing how many times a solution will pop up suddenly after a short while, and I didn’t have to waste expensive “conscious” energy working on it!

    All that to say: yes, it’s better to focus on one thing at a time, but I believe the brain is able to pull at multiple threads at once if we empower it to go off and do its thing without undue pressure.

  7. Well …. it seems that you are mostly familiar with software for which the cost of a context switch (that is, changing from one thread/task to another) is of essentially zero cost.

    However, if you work with realtime software, and in particular audio processing software, the cost of context switches ends up being deeply significant in many situations. It can just steal so much time from the “real task” (processing audio data) that it becomes a design problem.

    In this sense, brains and computers are not actually that different. You’re just aware of the cost of mental context switches, and not aware of the cost of the computational ones.

  8. The prefrontal cortex is single threaded, but the brain is not. The brain process the visual info come from the eyes and interpret the signal form you hears at the same time while monitoring you hearts and lungs. That is to say, the part of the brain which we have control of is certainly single threaded. In another word, the body just gave one thread to us.

  9. Thanks Cal. I just finished reading GTD again and tried to apply projects and next actions approach. When I was focussing on one project and next action my output was not only great but also I found some depth. This week I tried 2 projects and I already feel the difference. I may have made some ground on next actions on both but no depth or fulfillment and somehow I feel cognitive dissonance at the end of it all. Your article somehow verifies my subjective experience. One thing I am not sure with the GTD methodology is as per GTD you can do many projects next actions as long as you have externalized the next actions but in my experience it creates more loops (unless the action is less than 2 minutes or simple chores). My question is how to work on multiple projects with deadlines looming near. Do you suggest sprinting and finishing one “thread” at a time ? any insights ?

  10. Maybe it’s an opinion about the “conscious” part of brainwork. Hopefully you can walk and chew, walk and phone etc…. The brain has many parallel networks and vital functions to handle and all these networks provides many informations that has to be handled.
    Hopefully there is also fast paths to stop the car when a kid didn’t look right and left before crossing, Maybe the brain is highly multi cored that can be switched on/off, more than you think.

  11. The brain is great at synthesizing many inputs. For example at high level, you have read three books or papers on a topic in the last week, and all the information is getting synthesized. Clearly, trying to multitask at this time is a mistake.

    At a low level, talking of neurons now (but without any personal qualifications), your working memory of short term storage might be a resonant circuit of linked neurons. The short term memory can be synthesized at synapses in the here-an-now, so keep at the same task to make best synthesis of the short term memory.

    Another post mentioned exercise. As a person with ADHD, I find that important.

  12. To clarify, “brain” in this context really does mean our attention system. This system has a high context switching cost. There’s also a price to be paid by keeping too many obligations “active” in your brain, even if you’re not working on them all simultaneously at the current moment.

    “Multi-threading” is therefore a rough metaphor for having to juggle lots of simultaneous obligations…

  13. Cal, towards the end of the article, you talk about waiting to reach a natural stopping point before moving on to next task. I struggle to define this…is it like 5 minutes, 10 minutes ? what are some of the things that one should one being doing after ending a single threaded deep work session before hitting the next one? …even in the context of breaks b/w deep work sessions, i struggle to define what should i be doing so i don’t lose my focus due to “multi threading”.

  14. Hi Cal,
    I discovered your blog and your books (too) recently, and I am practically saving each one of your posts.
    I think one of the main reasons why we tend to multitasking is to control anxiety. I often start a task1 of Project1,then I get worried about Project2 and start a task1 for Project2, then I see I lost track of task1Project1, and go back and forth, randomly answering to emails etc.
    Which becomes a kind of Procrastination in the end.
    Personally I’ve started to do “Deliberate Practice” every morning and I am finding it extremely useful in order to calm down this anxiety, because after every session I feel like I gained some competence more.
    I am on a sabbatical year now, but I really hope I will maintain this habit as much as I can when courses start again.

  15. Cal, I’ve often thought of the brain working like Intel’s Hyper-Threading in their CPUs. In fact, I like to clarify that people don’t/can’t multi-task. Technically, we hyper-task: We don’t actually work on many tasks at the same time in parallel but rather switch between tasks in series. I totally agree with the task-switching cost you talk about and am glad you described it well. While it’s fun to draw comparisons between human brains and computer CPUs, I guess we must reiterate that we’re not robots.

    Now let’s talk speed. Computers have gotten faster over time. Rather than letting people be done with work sooner, we just pile on extra work. And it seems people are expected to work as fast as computers. Like the task-switch cost, I think this contributes to the overall problem of being too busy yet not productive. The modern slow movement may often unplug to recharge; we’re the opposite of robots.


  16. I had the good fortune to attend meetings of a Stanford faculty search committee (don’t misunderstand, my role was to take notes…). Regardless of the decision, these people, who included David Kelley (IDEO) and other world-renowned engineers, focused their attention laser-like on one thing at a time. In my work I frequently observed how they behaved in other contexts. The notable features of their style were tremendous energy and single-pointed (fierce!) attention.

    I was similarly blessed to be able to observe exactly the same quality in a well-known Eastern spiritual teacher – the same phenomenal energy, focused concentration, and thoroughly engrained habit of placing his attention on one thing at a time.

  17. What about moments when you are thinking about the problem and can’t resolve it? You switch to the next task. And while you are busy with that another task, you suddenly find the solution to the previous, because your brain is still working on it. I think that it can be called multithreading.

  18. Your study and hypothesis leaves out a small, but significant portion of the human species. I am a polymath much like Da Vinci. My left and right brain can almost act on their own. I am ambidextrous, I can carry on conversations if I wish from 2 different points of view, I can simulate multiple constructs and combine data. I am able to truly multitask on 2 different computer monitors and absorb multiple streams of data at once, not just switch between them. Including a hypothesis that covers ALL humans is flawed and should be amended to address outliers such as myself.

  19. Yes because our brains certainly cannot handle walking, breathing, heat beat, metabolism, hormones, emotions, temperature, language and speech, planning, spatial mapping, indendent fingers and limbs moving as if having a mind of their own, seeing, hearing, smelling, tasting, feeling, visualizing, remembering — the brain certainly could not do all of that at the same time. No, it’s single threaded, it has to beat the heart once, then breath, then take a step, then process a trillion photons hitting the retina, then hear sounds and determine their location etc, one by one, one task at a time. That totally sounds like us. Not.

    This argument is just weak. “the brain is not a computer processor”. Excuse me, do you know how the brain produce human cognition, does anybody? If not then what is your evidence for the claim that it isn’t a computer processor – how do you know that? Are 100,000,000,000 neurons not computing and processing information, electrons and photons?

    Whether its wet and messy or dry and neat is not a disqualifier for being a multithreaded computer. You said it yourself – we have threads going on that literally keep us awake at night — and yet you seem to think we’re single threaded? No, we have at least as many threads as there are different specialized organs within the nervous system. Maybe every neuron is its own thread and they are simply connected via an IoT.

  20. I totally believe all of this.
    BUT, for tasks which are less about just chugging thru to the finish and instead which require TANGENTIAL THINKING, such as coming up with something new, creative applications, etc, wouldn’t shifting from different tasks help to create new, unexpected connections and ideas to arise?

    for example, one of our fave creative brainstorming warm ups is called a “mashup”, where we intentionally come up with 2 unrelated things to mash up together, one of which relates to the topic, one which does not. this forces us to bring in new elements to and try to integrate them.

    I can see how switching between tasks might make each of the individual tasks slower, but create many more possible unexpected intersections, some of which might turn out to be quite brilliant.



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