1/8 DESIGN FOR A BRAIN 



the neuron of this property shall result in the whole animal's 

 behaviour being improved. 



Even if we allow the neuron all the properties of a living 

 organism, it is still insufficiently provided. For the improvement 

 in the animal's behaviour is often an improvement in relation to 

 entities which have no counterpart in the life of a neuron. Thus 

 when a dog, given food in an experiment on conditioned reflexes, 

 learns to salivate, the behaviour improves because the saliva 

 provides a lubricant for chewing. But in the neuron's existence, 

 since all its food arrives in solution, neither ' chewing ' nor ' lubri- 

 cant ' can have any direct relevance or meaning. Again, a rat 

 learns to run through a maze without mistakes ; yet the learning 

 has involved neurons which are firmly supported in a close mesh 

 of glial fibres and never move in their lives. 



Finally, consider an engine-driver who has just seen a signal 

 and whose hand is on the throttle. If the light is red, the 

 excitation from the retina must be transmitted through the 

 nervous system so that the cells in the motor cortex send impulses 

 down to those muscles whose activity makes the throttle close. 

 If the light is green, the excitation from the retina must be 

 transmitted through the nervous system so that the cells in the 

 motor cortex make the throttle open. And the transmission is 

 to be handled, and the safety of the train guaranteed, by neurons 

 which can form no conception of ' red ', ' green ', ' train ', ' signal ', 

 or ' accident ' ! Yet the system works. 



1/8. In some cases there may be a simple mechanism which 

 uses the method that a red light activates a chain of nerve-cells 

 leading to the muscles which close the throttle while a green light 

 activates another chain of nerve-cells leading to the muscles which 

 make it open. In this way the effect of the colour of the signal 

 might be transmitted through the nervous system in the appro- 

 priate way. 



The simplicity of the arrangement is due to the fact that we 

 are supposing that the two reactions are using two completely 

 separate and independent mechanisms. This separation may well 

 occur in the simpler reactions, but it is insufficient to explain the 

 events of the more complex reactions. In most cases the ' correct ' 

 and the ' incorrect ' neural activities are alike composed of excita- 

 tions, of inhibitions, and of other changes that are all physiological, 



6 



