Machines Like Us – Science News

By Peter Hankins

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 in one of my older posts I 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.

Peter Hankins is author of the Conscious Entities weblog.