14/7 REPETITIVE STIMULI AND HABITUATION 



through the uniselectors, so the whole is ultrastable. At each D 

 the operator displaced l's magnet through a constant distance. 

 On the first ' stimulation ', 2's response brought the system to 

 its critical states, so the ultrastability found a new terminal field. 

 The second stimulation again evoked the process. But the new 

 terminal field was such that the displacement D no longer caused 

 2 to reach its critical states; so this field was retained. Again 

 under constant stimulation the response had diminished. 



14/7. 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 : 



'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 \°/ 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. 



A variety of special explanations have been put forward to 

 explain its origin, but the almost universal distribution of its 

 occurrence in living organisms should warn us that the basis 

 must be some factor much more widely spread than the neuro- 

 physiological. The argument of this chapter suggests that it is to 

 be expected to some degree in all polystable systems when they 

 are subjected to a repetitive stimulus or disturbance. 



189 



