284 Information Storage and Neural Control 



only by the state in which it is and by the input to that state. 

 Consequently, we do not care how it came to be in that state. 



For our problem of the moment, perception, we shall deal only 

 with essentially synchronous signals to a layer of neurons whose 

 axons end on the succeeding layer, for as many layers in depth as 

 we choose. Such a net can be designed to compute in any layer 

 at any one time as many Boolean functions of its simultaneous 

 inputs as there are neurons in that layer, and no others. We shall 

 not consider any other nets. We shall suppose that our nets have 

 been designed so that functions computed by the output neurons 

 lead to those responses that are most useful to the organism. This 

 assumption simplifies our problem. 



Some twenty years ago, when Walter Pitts and I began our 

 study of a logical calculus for ideas that are immanent in nervous 

 activity, there was good evidence that a neuron had a threshold 

 in the sense that it would fire if adequately excited; that impulses 

 from separate sources, severally subthreshold, could add to exceed 

 the threshold; and that the neuron could be inhibited. For sim- 

 plicity, we took inhibition as being absolute. These few properties 

 served our purpose, which was to prove that a net of such neurons 

 could compute any number that a Turing machine could compute 

 with a finite tape. Some five years later, these properties sufficed 

 for a theory of how we can perceive universals, such as a chord, 

 regardless of key, or a shape, regardless of size. These two papers 

 were crucial in the development of Automata Theory. 



But, ten years ago the inadequacy of these assumptions came to 

 light, theoretically, in von Neumann's paper on probabilistic logic 

 concerned with building reliable computers from less reliable 

 components. 



By that time spontaneously active neurons had been demon- 

 strated in most parts of the mammalian nervous system. Inhibitions, 

 like excitations, had been found to sum, and we had come to grips 

 with those interactions of axons that are afferent to a cell and by 

 which signals in one prevent signals in another from reaching the 

 recipient neuron. 



We could demonstrate this interaction as peripherally as the 

 primary bifurcation of afferent peripheral neurons. In Nature 



