DESIGN FOR A BRAIN 13/19 



nervous system. The set of variables activated at one moment 

 will usually differ from the set activated at a later moment; 

 and the activity will spread and wander with as little apparent 

 orderliness as the drops of rain that run, joining and separating, 

 down a window-pane. But though the wanderings seem dis- 

 orderly, the whole is reproducible and state-determined ; so that if 

 the same reaction is started again later, the same initial stimuli 

 will meet the same local details, will develop into the same patterns, 

 which will interact with the later stimuli as they did before, and 

 the behaviour will consequently proceed as it did before. 



This type of system would be affected by removals of material 

 in a way not unlike that demonstrated by many workers on the 

 cerebral cortex. The works of Pavlov and of Lashley are typical. 

 Pavlov established various conditioned responses in dogs, removed 

 various parts of the cerebral cortex, and observed the effects on 

 the conditioned responses. Lashley taught rats to run through 

 mazes and to jump to marked holes, and observed the effects of 

 similar operations on their learned habits. The results were 

 complicated, but certain general tendencies showed clearly. 

 Operations involving a sensory organ or a part of the nervous 

 system first traversed by the incoming impulses are usually 

 severely destructive to reactions that use that sensory organ. 

 Thus, a conditioned response to the sound of a bell is usually 

 abolished by destruction of the cochleae, by section of the auditory 

 nerves, or by ablation of the temporal lobes. Equally, reactions 

 involving some type of motor activity are apt to be severely upset 

 if the centre for this type of motor activity is damaged. But 

 removal of cerebral cortex from other parts of the brain gave 

 vague results. Removal of almost any part caused some dis- 

 turbance, no matter from where it was removed or what type of 

 response or habit was being tested; and no part could be found 

 whose removal would destroy the response or habit specifically. 



These results have offered great difficulties to many theories of 

 cerebral mechanisms, but are not incompatible with the theory 

 put forward here. For in a large polystable system the whole 

 reaction will be based on activations that are both numerous and 

 widely scattered. And, while any exact statement would have 

 to be carefully qualified, we can see that, just as England's paper- 

 making industry is not to be stopped by the devastation of any 

 single county, so a reaction based on numerous and widely 



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