TRANSMISSION Of VARIETY 8/5 



Ex. 10: The concentrations "high" or "low" of sex-hormone in the blood of a 

 certain animal determines whether it will, or will not, go through a ritual 

 of courtship. If the sex-hormone is very complicated chemically and the 

 ritual very complicated ethnologically, and if the variable "behaviour" is 

 regarded as a coded form of the variable "concentration", how much 

 variety is there in the set of messages? 



8/5. Coding by machine. Next we can consider what happens 

 when a message becomes coded by being passed through a machine. 



That such questions are of importance in the study of the brain 

 needs no elaboration. Among their other appUcations are those 

 pertaining to "instrumentation" — the science of getting information 

 from some more or less inaccessible variable or place, such as the 

 interior of a furnace or of a working heart, to the observer. The 

 transmission of such information almost always involves some 

 intermediate stage of coding, and this must be selected suitably. 

 Until recently, each such instrument was designed simply on the 

 principles peculiar to the particular branch of science; today, 

 however, it is known, after the pioneer work of Shannon and Wiener, 

 that certain general laws hold over all such instruments. What they 

 are will be described below. 



A "machine" was defined in S.3/4 as any set of states whose 

 changes in time corresponded to a closed single-valued transforma- 

 tion. This definition applies to the machine that is totally isolated, 

 i.e. in constant conditions; it is identical with the absolute system 

 defined in Design. ... In S.4/1 the machine with input was defined 

 as a system that has a closed single-valued transformation for each 

 one of the possible states of a set of parameters. This is identical 

 with the "transducer" of Shannon, which is defined as a system whose 

 next state is determined by its present state and the present values 

 of its parameters. (He also assumes that it can have a finite internal 

 memory, but we shall ignore this for the moment, returning to it in 

 S.9/8.) 



Assume then that we have before us a transducer M that can be 

 in some one of the states 5*1, S2, ■ ■ ., S„, which will be assumed here 

 to be finite in number. It has one or more parameters that can 

 take, at each moment, some one of a set of values P^, Pi, . . ., Pi^. 

 Each of these values will define a transformation of the 5"s. We 

 now find that such a system can accept a message, can code it, and 

 can emit the coded form. By "message" I shall mean simply some 

 succession of states that is, by the coupling between two systems, at 

 once the output of one system and the input of the other. Often 

 the state will be a vector. I shall omit consideration of any "mean- 



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