The McCullochPitts Neuron 
Written by Mike James  
Thursday, 04 March 2021  
Page 3 of 3
Regular sequences are important because they are the most basic pattern that a simple computer – a finite state machine – can recognise. What is important about McCullochPitts networks is that it is easy to show that you can build a network that will generate or recognise any regular sequence and so such networks are equal in power to a finite state machine. Why is this important? Well if you agree that McCullochPitts neurons capture the essence of the way biological neurons work then you also have to conclude that biological networks are just finite state machines and as such can only recognise or generate regular sequences. In their original work McCulloch and Pitts extended this observation into deducing a great deal about human brain function. Most of this seems a bit farfetched from today’s standpoint but the basic conclusion that the brain is probably nothing more than a simple computer – i.e. a finite state machine – still seems reasonable. If you know a little about the theory of computation you might well not be happy about this “bottom line” because a finite state machine isn’t even as powerful as a Turing machine. That is, there are lots of things that a Turing machine can compute that in theory we, as finite state machines, can’t. In fact there are three or more complexities of grammar, and hence types of sequence, that finite state machines, and hence presumably us, cannot recognise. This sort of argument is often used to demonstrate that there has to be more to a human brain than mere logic – it has a nonphysical “mind” component or some strange quantum phenomena that are required to explain how we think. All nonsense of course! You shouldn’t get too worried about these conclusions because when you look at them in more detail some interesting facts emerge. For example, all finite sequences are regular and so we are really only worrying about philosophical difficulties that arise because we are willing to allow infinite sequences of symbols. While this seems reasonable when the infinite sequence is just ABAB… it is less reasonable when there is no finite repetitive sequence which generates the chain. If you want to be philosophical about such things perhaps it would be better to distinguish between sequences that have no fixed length limit – i.e. unbounded but finite sequences  and truly infinite sequences. Surprisingly, even in this case things work out in more or less the same way with finite state machines, and hence human brains, lagging behind other types of computer. The reason for this is simply that as soon as you consider a sequence longer than the number of elements in the brain it might as well be infinite! As long as we restrict our attention to finite sequences with some upper limit on length, and assume that the number of computing elements available is much greater than this, then all computers are equal and the human brain is as good as anything! McCulloch and Pitts neural networks are not wellknown or widely studied these days because they grew into or were supersede by another sort of neural net – one that can be trained into generating any logic function or indeed any function you care to name.
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Last Updated ( Thursday, 04 March 2021 ) 