DESIGN FOR A BRAIN 



6/4 



In a working clock, the single variable defined by the reading 

 of the minute-hand on the face is state-determined as a one- 

 variable system; for after some observations of its behaviour, we 

 can predict the line of behaviour which will follow any given state. 

 If now the regulator (the parameter) is moved to a new position, 

 so that the clock runs at a different rate, and the system is re- 

 examined, it will be found to be still state-determined but to have 

 a different field. 



If a healthy person drinks 100 g. of glucose dissolved in water, 

 the amount of glucose in his blood usually rises and falls as A 

 in Figure 6/4/1. The single variable ' blood-glucose ' is not state- 



Figure 6/4/1 : Changes in blood-glucose after the ingestion of 100 g. 

 of glucose : (A) in the normal person, (B) in the diabetic. 



determined, for a given state (e.g. 120 mg./lOO ml.) does not 

 define the subsequent behaviour, for the blood-glucose may rise or 

 fall. By adding a second variable, however, such as 4 rate of 

 change of blood-glucose ', which may be positive or negative, we 

 obtain a two-variable system which is sufficiently state-determined 

 for illustration. The field of this two-variable system will re- 

 semble that of A in Figure 6/4/2. But if the subject is diabetic, 

 the curve of the blood-glucose, even if it starts at the same initial 

 value, rises much higher, as B in Figure 6/4/1. When the field 

 of this behaviour is drawn (B, Figure 6/4/2), it is seen to be not the 

 same as that of the normal subject. The change of value of the 

 parameter ' degree of diabetes present ' has thus changed the 

 field. 



Girden and Culler developed a conditioned response in a dog 

 which was under the influence of curare (a paralysing drug)- 



74 



