18/1 DESIGN FOR A BRAIN 



little orderliness as the drops of rain that run, joining and separat- 

 ing, down a window-pane. But though the wanderings seem 

 disorderly, the whole is regular ; 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 reflexes in dogs, removed 



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



the conditioned reflexes. 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 reflex 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 it was 



found that the removal of cerebral cortex from other parts of the 



brain gave vague results. Removal of almost any part caused 



some disturbance, no matter from where it was removed or what 



type of reflex or habit was being tested ; and no part could be 



found whose removal would destroy the reflex 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 multistable system the whole 



reaction will be based on step-functions and 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 scattered elements will tend to have more 



immunity to localised injury than one whose elements are few and 



compact. 



192 



