13/16 THE SYSTEM WITH LOCAL STABILITIES 



come from the same line, but the two portions may also come 

 from different lines.) As the example shows, it is not implied 

 that the two sets shall contain no common element, only that the 

 two sets are not identical. 



The importance of dispersion will be indicated in S. 13/17. 

 Here we should notice the essential feature: though the two por- 

 tions may start from points that differ only in one, or a few, 

 variables (as in S. 12/3) the changes that result may distribute 

 the activations (S. 12/18) to different sets of variables, i.e. to 

 different places in the system. Thus, the important phenomenon 

 of different patterns (or values) at one place leading to activations 

 in different places in the system demands no special mechanism : 

 any polystable system tends to show it. 



13/15. If the two places are to have minimal overlap, and if the 

 system is not to be specially designed for the separation of parti- 

 cular patterns of input, then all that is necessary is that the parts 

 should have almost all their states equilibrial. Then the number 

 active will be few; if the fraction of the total number is usually 

 about r, and if the active variables are distributed independently, 

 the fraction that will be common to the two sets (i.e. the overlap) 

 will be about r 2 . This quantity can be as small as one pleases by 

 a sufficient reduction in the value of r, which can be done by 

 making the parts such that the proportion of states equilibrial is 

 almost 1. Thus the polystable system may respond, to two 

 different input states, with two responses on two sets of variables 

 that have only small overlap. 



13/16. It will be proposed later that dispersion is used widely 

 in the nervous system. First we should notice that it is used 

 widely in the sense-organs. 



The fact that the sense-organs are not identical enforces an 

 initial dispersion. Thus if a beam of radiation of wave-length 

 0*5 ii is directed to the face, the eye will be stimulated but not 

 the skin ; so the optic nerve will be excited but not the trigeminal. 

 But if the wave-length is increased beyond 0-8 yu, the excitation 

 changes from the optic nerve to the trigeminal. Dispersion has 

 occurred because a change in the stimulus has moved the excita- 

 tion (activity) from one set of anatomical elements (variables) 

 to another. 



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