DISTURBED SYSTEMS AND HABITUATION 13/7 



ment by fields of ' normal ' stability (S. 20/2) that look like C 

 of Figure 13/4/1 and act simply to keep the representative point 

 well away from the critical states. 



Effects of repeated stimuli 



13/6. So far we have studied only the effects of irregular dis- 

 turbances : what of the regular ? By the argument of S. 6/6, all 

 such can be considered as * stimuli ' and are of two types : a 

 sudden change of parameter-value, and a sudden jump of the 

 representative point. The two types will be considered separately. 



The effect on an ultrastable system of an alternation of a 

 parameter between two values has already been described in S. 

 11/4, where Figure 11/4/1 showed how the ultrastable system is 

 automatically selective for any set of step-function values which 

 gives stability with both the parameter- values. 



The facts can also be seen from another point of view. If we 

 start alternating the parameter and observe the response of the step- 

 functions we shall find that at first they change, and that after a 

 time they stop changing. The responses, in other words, diminish. 



13/7. Next consider the effect of repetitions of the other type of 

 stimulus — the displacement of 

 the representative point. Its 

 effect can readily be found by 



asking what sort of field can be • ' 



terminal. Suppose, for instance, ' 



that the displacement was a A ' Q 



movement to the left through » 



the distance shown by the arrow 

 below Figure 1 3/7/1 . It is easy 

 to see that a field, to be terminal 

 in spite of this displacement, 

 must have its resting state 

 within B. If the constant dis- 

 placement is applied from time *"* • 

 to time to an ultrastable system FlGU Rf 13/7/1 : Region of an ultra- 

 , r. i t , . stable system. The representative 

 whose fields have resting states point must stay in B if the field is 

 distributed over both A and B, to b f imm " n e to a displacement 

 ., • i /> i i • , . equal and parallel to that shown 

 then terminal fields with resting by the arrow. 



147 





