DESIGN FOE A BRAIN 6/6 



Sometimes a parameter is changed sharply and is immediately 

 returned to its initial value, as when the experimenter applies a 

 a tap on a tendon. The effect of the parameter-change is a brief 

 change of field which, while it lasts, carries the representative 

 point away from its original position. When the parameter is 

 returned to its original value, the original field and state of equili- 

 brium are restored, but the representative point is now away 

 from the state of equilibrium; it therefore moves along a line of 

 behaviour, and the organism ' responds '. (Usually the point 

 returns to the same state of equilibrium : but if there is more than 

 one, it may go to some other state of equilibrium.) Such a 

 stimulus will be called impulsive. 



It will be necessary later to be more precise about what we mean 

 by ' the ' stimulus. Consider, for instance, a dog developing a 

 conditioned reflex to the ringing of an electric bell. What is the 

 stimulus exactly ? Is it the closing of the contact switch ? The 

 intermittent striking of the hammer on the bell ? The vibrations 

 in the air ? The vibrations of the ear-drum, of the ossicles, of 

 the basilar membrane ? The impulses in the acoustic nerve, in the 

 temporal cortex ? If we are to be precise we must recognise that 

 the experimenter controls directly only the contact switch, and 

 that this acts as parameter to the complexly-acting system of 

 electric bell, middle ear, and the rest. 



When the ' stimulus ' becomes more complex we must generalise. 

 One generalisation increases the number of parameters made to 

 alter, as when a conditioned dog is subjected to combinations of a 

 ticking metronome, a smell of camphor, a touch on the back, and 

 a flashing light. Here we should notice that if the parameters 

 are not all independent but change in groups, like the variables 

 in S. 3/3, we can represent each undivided group by a single 

 value and thus avoid using unnecessarily large numbers of 

 parameters. 



Joining dynamic systems 



6/6. We can now make clear what is meant, essentially, by the 

 concept of two (or more) systems being ' joined '. 



This concept is of the highest importance in biology, in which it 

 occurs frequently and prominently. It occurs whenever we think 

 of one system having an effect on another, or communicating with 

 it, or forcing it, or signalling to it. 



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