6/5 



DESIGN FOR A BRAIN 



characteristic lines of behaviour (two responses to the stimulus) 

 existed, and that one line of behaviour was shown when the 



a. '00 200 100 200 300 



BLOOD GLUCOSE (mg. per 100 ml.) 



Figure 6/4/2 : Fields of the two lines of behaviour, A and B, 

 from Figure 6/4/1. Cross-strokes mark each quarter-hour. 



parameter ' concentration of curare in the tissues ' had a high 

 value, and the other when the parameter had a low value. 



6/5. The physicist, studying systems whose variables are all 

 clearly marked and controllable, seldom confuses change of state 

 with change of field. The psychologist, however, studies systems 

 whose variables, even in the simplest systems, are so numerous 

 that he cannot, in practice, make an exact list of them : his 

 grasp of the situation must be intuitive rather than explicit. 

 In his practical work he seldom fails to distinguish between the 

 variables he is observing and the parameters he is controlling ; 

 it is chiefly in his theoretical work, especially when he discusses 

 cerebral mechanisms, that he is apt to allow the distinction to 

 become blurred. To preserve the distinction between variable 

 and parameter we must discuss, not the real ' machine ', with 

 its infinite richness of variables, but a defined system. The 

 advantage to be gained will become clearer as we proceed. 



Stimuli 



6/6. Many stimuli may be represented adequately as a change 

 of parameter-value, so it is convenient here to relate the physio- 

 logical and psychological concept of a ' stimulus ' to our methods. 

 In all cases the diagram of immediate effects is 



(experimenter) — > stimulator — > animal — > recorders. 



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