DESIGN FOR A BRAIN 14/4 



definite state in the field and there to release it. The possibilities 

 sketched in Figure 14/3/1 will illustrate the process sufficiently. 



Suppose the system is in equilibrium at A. S is applied; its 

 effect is to move the representative point to the end of the arrow, 

 in this example moving it into another confluent. The system is 

 now, by hypothesis, left alone until it has settled : this means that 

 the basic field operates, carrying it, in this example, to the state 

 of equilibrium B. Here it will remain until the next application 

 of S, which in this example, again moves it to a new confluent ; 

 here the basic field takes it to the state of equilibrium C. So does 

 the alternation of S and the basic field take it from equilibrium to 

 equilibrium till it arrives at E. From this state, S moves it only 

 to within the same confluent and the ' leaving alone ' results in its 

 coming back to E. S (having by hypothesis a unique effect) now 

 takes it to the arrow head, and again it comes back to E. This 

 state of affairs is now terminal, and the representative point is 

 trapped within the i?-confluent. 



It can now be seen that the process is selective; the representa- 

 tive point ends in a confluent such that the ^-displacement carries 

 it to some point within the confluent. Confluents such as A, C, 

 and D, with the S- displacement going outside, cannot hold the 

 representative point under the process considered; confluents 

 such as E, J, and L can trap it. 



14/4. The diagram shows two complications that must be con- 

 sidered for completeness' sake. 



The first is that the events of the right-hand side may occur, 

 where the process considered takes the representative point 

 cyclically through confluents P, Q, R, P, Q, R, P, . . . 



The second is shown on the left at X, where the confluent comes 

 to a cycle. From this cycle a variety of displacements may be 

 caused by S, depending on the precise moment at which S is 

 applied (i.e. on just where the representative point happens to be). 



Whether such cycles (between or within confluents) are com- 

 mon in the nervous system is a question to be settled by experi- 

 ment. The cycle within the confluent, as at X, will hardly dis- 

 turb the conclusions below; for either all ^-displacements fall 

 within the confluent (in which case it is trapped as stated above) 

 or it will sooner or later leave the confluent (and we are no longer 

 concerned with the cycle). Thus in the Figure, unless the period 



186 



