Picture: brain not computer. Chris Chatham has a nice summary of ten key differences between brains and computers which is well worth a read. Briefly, the list is:

  • Brains are analogue; computers are digital
  • The brain uses content-addressable memory
  • The brain is a massively parallel machine; computers are modular and serial
  • Processing speed is not fixed in the brain; there is no system clock
  • Short-term memory is not like RAM
  • No hardware/software distinction can be made with respect to the brain or mind
  • Synapses are far more complex than electrical logic gates
  • Unlike computers, processing and memory are performed by the same components in the brain
  • The brain is a self-organizing system
  • Brains have bodies
  • The brain is much, much bigger than any [current] computer

Actually eleven differences – there’s a bonus one in there.

Hard to argue with most of these, and there are some points among them which are well worth making. There is always a danger, when comparing the capacities of brains and computers, of assuming a similarity even when trying to point up the contrast. There have, for example, been many attempts to estimate the storage capacity of the brain, or the memory, over the years for example, always concluding that it is huge; but a figure in megabytes doesn’t really make much sense. Asking how many bytes there are in the memory is like asking how many pixels Leonardo needed to do the Mona Lisa: it’s not like that. Chris generally steers just clear of this danger, although I’d be more inclined to say, for example, that the concept of processing speed has no useful application in the brain rather than that it isn’t fixed.

I wonder a bit about some of the positive assertions he makes. Are brains analogue? Granted they’re not digital, at least not in the straightforward way that a digital computer is, but unless we take ‘analogue’ as a synonym for ‘non-digital’ it’s not really clear to me. I take digital and analogue to be two different ways of representing real-world quantities; I don’t think we really know exactly how the brain represents things at the moment. It’s possible that when we do know, the digital/analogue distinction may seem to be beside the point.
And are brains ‘massively parallel’? It’s a popular phrase, but one of the older bits of this site long ago looked at a few reasons why the resemblance to massively parallel computing seems slight. In fairness, when people make this assertion they aren’t really saying that the brain is like a parallel processing set-up; they’re trying to describe a quality of the brain for which there is no good word; ie that things seem to be going on all over it at the same time. Chris is really warning against an excessively modular view. Once again we can agree that the brain is unlike computers – this time in the way they funnel data and instructions together in one or more processors; but the positive comparison is more problematic.

There’s some underlying scope for confusion, too, about what we mean when we assert that brains are not computers. We could just intend the trivial point that there isn’t actually a physical PC in our heads. More plausibly, we could mean that the brain doesn’t have the same general architecture and functional features as a computer (which I think is about what Chris means to do). We could mean that the brain doesn’t do stuff that we could easily recognise as computation, although it might be functionally similar in the sense of producing the same output to input relationships as a computed process. We might mean it doesn’t do stuff that we could easily recognise as computation, and that there appears to be no non-trivial way of deriving algorithms which would do the same thing. We might go one further and assert, as Roger Penrose does, that some of what the brain is doing is non-computable in the same sort of way as the tiling problem (though here again we have to ask is it really like that since the question of computability seems to assume the brain is typically solving problems and seeking proofs). Finally, we could be saying that the brain has altogether mysterious properties of free will and phenomenal experience which go beyond anything in our current understanding of the physical world, and ergo far beyond anything a mere computer might possess.

A good thought-provoking discussion, in any case.


  1. 1. Shankar says:

    If the Big Blue project is scaled up, except for the first point (analog vs digital), the others would no longer hold true.

    And even if not, the differences are mundane, not in any philosophical sense.

  2. 2. Peter says:

    I don’t think that’s true. Blue Brain would add another layer, a further computational substrate which real brains don’t have, but the properties of the substrate would not be those of the simulated brain. The computer underneath might be using conventional memory, but you’d have implemented content-addressable memory in the program, at a higher level, if you see what I mean.

  3. 3. Jacob Russell says:

    The novelist, Richard Powers, who has a degree in physics and working experience as a programmer, deals many of these questions of distinctions between computers and brains in Galatea 2.2. The focus here is on language acquisition.

  4. 4. Peter says:

    Thanks, Jacob – Galatea 2.2 is well worth reading. I believe it’s recommended by David Lodge, who has also written a novel which touches on these issues (‘Thinks’) though it doesn’t engage with them quite so fully, I think.

  5. 5. Christophe Menant says:

    There is also another difference between a brain and a computer. It is evolution.
    A human brain is the result of billions years of evolution with memorized representations and experiences. A computer does not have such a history. And the question becomes “how can billions of years of data processing for survival expertise generate human consciousness ?”
    We can look for evolutionary scenarios that could introduce such a performance.
    A possible one is about primates accessing a form of auto-representation that becomes existing in the environment thanks to identification with conspecifics. The auto-representation then becomes a self-representation that introduces self and phenomenal consciousness. (More on this at http://cogprints.org/4957/)

  6. 6. Alex G. says:

    It is true that the functionality of a human brain is not like that for a conventional personal computer. However, a powerful point that can be made by comparing computers to brains is the fact that the physical system of a brain can be simulated completely by a computer. Assuming the simulation goes no lower than the molecular level – which is justified because quantum effects that would render this simulation invalid are neglegable in human brains for the vast majority of the time – the simulation is representing a computable and deterministic system. The analogue functions of the brain can certainly be represented digitally if the simulation is on the molecular scale. What I mean is that in the same way that colors exist on an analogue spectrum of experience in the brain, they can still be represented and reproduced exactly using the digital foundation of a computer, since the ‘analogue’ representation in the brain is still reducable to a matter of individual molecules with descrete and computable effects; every color that we can see on our ‘analogue’ color scale can be reproduced by a digital system of sufficient complexity.

    Computers can comprehensively and completely simulate the physical system of the brain, and in thinking about consciousness, the implications of this are profound.

  7. 7. Alex G. says:

    In response to comment #2:

    On the issue of futher/deeper ‘computational substrates;’ this is not a meaningful issue. The brain certainly has computational substrates beyond neural pathways: individual neurons. It also certainly has computational substrates beyond individual neurons: the molecular make-up of the neurons and the neurotransmitters. Beyond that there is without a doubt the computational substrate of atoms and atomic interaction…and so on and so forth, infinitely.

    The Blue Brain project does not add any computational substrate, in that there are more computational substrates in Blue Brain than in human brains. Blue Brain replaces the molecular level computational substrates of the human brain with a digital simulation of the molecules involved. Because those molecules obey physical laws in a computable and deterministic way, there is no reason to suspect that the digital simulation is not functionally identical. So that when examining the system on the level of individual neurons or neuronal pathways, the simulation is complete and comprehensive. There are no extra ‘computational substrates’ that are relevent because the simulation and the actual system and functionally identical on all relevent levels.

    This is not as clear as I would like it to be, but this is what I mean: at the molecular level of complexity, the Blue Brain simulation is exact to the extent that the substitution of that simulation at the molecular level has no effect on functionality at any higher levels. Furthermore, any difference in functionality at lower levels is currently believed to be irrelevant to the problem at hand; and if this belief turns out to incorrect(i.e. Blue Brain does not generate consciousness), well, then at least we would have a better idea about where to look.

  8. 8. Alex G. says:

    In response to comment #5:

    When talking about evolution in this sense – that you mean in your comment – we can define it as the system by which the particular information processing system of the human brain has been formed. To site the fact that computers dont have that same system behind them as a relevent point in this discussion I think has two possible responses.

    1: It’s irrelevant because our brain can be comprehensively understood at some level as a system of physical objects(molecules) obeying physical laws and behaving in computable and deterministic ways. If we replace those molucules in a simulation, with objects that behave identically as defined by relevent laws – in a computable and deteministic way – that we can see are functionally identical, then, we have a meaningful and comprehensive simulation on all scales of complexity beyond and including that which we started at. Meaning that for all functional matter above the atomic scale we have a complete and comprehensive simulation, regardless of the fact that the simulation does not have evolution behind it.

    2: This idea is slightly more abstract and considerably more fun, but computers do have evolution behind them. They certainly wouldn’t exist without it. Human brains are directly resposible for the computers in question, so if evolution is defined as the system by which human brains were formed, and human brains then went on to form computers, isn’t evolution directly responsible for them, too?

    One way to accurately describe evolution is as an increasingly complex organization of information… so long as the information is able to be sustained.

    i.e. Randomness after the big bang –> solid objects –> stars –> stars with planets(solar systems) = galaxies(collections of solar systems) –>(in our case) planets with complex organizations of complex molecular structures –> life –> more complex life –> humans(essentially the human brain, we are not otherwise impressive in our complexity) –> computers, cities, hospitals, the internet, the information network which we are almost all part of… such is the progress of reality.

    This evolution of the universe and information that exists at least as an observable trend, implies that a similar force is behind our brain and computers…

    Only argue with me about the first point…the second is only an interesting thing to think about.

  9. 9. Ginger Campbell, MD says:

    I want to thank both you and Chris Chatman for your contributions to what I consider to be an important question. I generally come down on the “brains are not computers”side of the argument. Through out history humans seem to have tried to describe the mind by using the technology of their day. (Consider how thinking of the brain in terms of a steam engine led to Freud’s misguided idea that suppressed ideas caused a dangerous pressure build-up. This mistaken hydraulic metaphor still drives the sometimes mistaken idea that talking about a traumatic event is the best therapeutic option.)

    The computer metaphor of the mind has its uses, but it has also been an obstacle to understanding, and appreciating what our brains really do. On the other hand, people working in the fields of embodied artificial intelligence and embodied cognition are beginning to show how recognizing the differences can actually inform both the disign of computers and our understanding of how the brain works.

    Ginger Campbell, MD
    Host of the Brain Science Podcast

  10. 10. Christophe Menant says:

    In response to comment #8:
    I’m afraid we cannot consider that a simulation of a brain can reproduce all the performances of the brain.
    The physical system of the brain is far from being all the brain, and the best simulation will be missing a least one key point: the simulation of life.
    As we do not know the nature of life, we cannot simulate it nor reproduce it.
    Assembling together the best we can all the atoms, molecules and proteins of an elementary living cell will not make us build up a living cell.
    The simulation of a system cannot simulate what we don’t know of the system.
    Same for the performance of memory. How could a simulation of a brain simulate the memorized information as we don’t really know how organisms store information ?
    So I still beleive that a computer cannot simulate the billions years of evolution of a human brain with the memorized representations and experiences.

  11. 11. Alex G. says:

    Dr. Campbell,

    Perhaps believing that the function of the human brain can be completely and comprehensively translated to the functioning of a computer is flawed in some way. However, to say that because scientists and thinkers of the past have equated the functioning of their technology to the functioning of the brain, and that since we are also in a sense doing the same thing, our equation/metaphore/comparison is necessarily flawed… I think that that is not enough. How is it flawed? If this is a common mistake, in what way, in this instance, is it incorrect? I’m open to the possibility that I’m repeating the mistake of past thinkers, but just to say that these methods are similar does not necessarily imply the same faulty reasoning.

    On the contrary, I believe that to say that a non-quantum scale physical system cannot be represented completely by a conventional mathematical function is to say that modern physics is flawed in a profound and definite way. Because of this, I think that the only reasonable assumption at this point is that the brain can be completely and comprehensively simulated at the molecular level by a sufficiently complex computer(that is no different in principal from a conventional super computer, only larger and more powerful). To believe otherwise is to directly reject the essential paradigms of physics, which has a tremendous body of evidence compared to the evidence on which one might base their rejection.

  12. 12. Alex G. says:

    Mr. Menant,

    I think that your argument is dependant on something of an elan vital…or a spirit of life. My response is that life is nothing more than an incredibly complex arrangement of the same physical substances that make up non-living things. The only thing special is the complexity involved, there is no magic or spirit or mystical force involved, the chemical reactions that dictate the behavior of an individual cell abide by the same physical laws that every other chemical reaction is subject to. The issue here is only that life is these same chemicals and same laws that make up everything else, in such an incredibly complex(in fact so complex that it is almost impossible to conceptualize that this is the case) structure, that it seems like there is something special going on. The only thing special is the degree of complexity. Scientists in the field will defend this idea; in all of their study they have never found a spirit of life, or something miraculous, only a complex organization of the same substances that make up everything else, which generates complex behavior.

    My original argument (comment 8 ) was that since this is the case, every law and the behavoir of every relevant substance under these laws can be simulated by a computer. If there is a concept of something outside the realm of conventional science(other than consciousness, the greatest mystery of all) there is certainly no reason to believe it, because there is no evidence for it. The fact that it is hard to conceptualize how life can just be a complex arrangement of the same stuff that makes up non-living things is not evidence.

  13. 13. Alex G. says:

    The smiley face in the above is meant to be the number eight followed by a comma, I don’t know what happened. If I ever use one of those intentionally in a post I make, please, kill me.

  14. 14. Peter says:

    Sorry about the inadvertent smiley, Alex – I’ve removed it.

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