13/11 DESIGN FOR A BRAIN 



conditions that made them large, while small responses do not 

 destroy the conditions that made them small. 



13/11. In animal behaviour the phenomenon of ' habituation ' 

 is met with frequently : if an animal is subjected to repeated 

 stimuli, the response evoked tends to diminish. The change has 

 been considered by some to be the simplest form of learning. 

 Neuronic mechanisms are not necessary, for the Protozoa show it 

 clearly : 



4 Amoebae react negatively to tap water or to water from a 

 foreign culture, but after transference to such water they 

 behave normally.' 



4 If Paramecium is dropped into J% sodium chloride it 

 at once gives the avoiding reaction ... If the stimulating 

 agent is not so powerful as to be directly destructive, the 

 reaction ceases after a time, and the Paramecia swim about 

 within the solution as they did before in water.' (Jennings.) 



Fatigue has sometimes been suggested as the cause of the 

 phenomenon, but in Humphrey's experiments it could be excluded. 

 He worked with the snail, and used the fact that if its support is 

 tapped the snail withdraws into its shell. If the taps are repeated 

 at short intervals the snail no longer reacts. He found that when 

 the taps were light, habituation appeared early ; but when they 

 were heavy, it was postponed indefinitely. This is the opposite 

 of what would be expected from fatigue, which should follow more 

 rapidly when the heavier taps caused more vigorous withdrawals. 



The nature of habituation has been obscure, and no explanation 

 has yet received general approval. The results of this chapter 

 suggest that it is simply a consequence of the organism's ultra- 

 stability, a by-product of its method of adaptation. 



Reference 

 Humphrey, G. The nature of learning. London, 1933. 



152 



