PARAMETERS 6/6 



In some cases the animal, at some resting state, is subjected to 

 a sudden change in the value of the stimulator, and the second 

 value is sustained throughout the observation. Thus, the pupil- 

 lary reaction to light is demonstrated by first accustoming the 

 eye to a low intensity of illumination, and then suddenly raising 

 the illumination to a high level which is maintained while the 

 reaction proceeds. In such cases the stimulator is parameter to 

 the system ' animal and recorders ' ; and the physiologist's 

 comparison of the previous control-behaviour with the behaviour 

 after stimulation is equivalent, in our method, to a comparison of 

 the two lines of behaviour that, starting from the same initial 

 state, run in the two fields provided by the two values of the 

 stimulator. 



Sometimes a parameter is changed sharply and is immediately 

 returned to its initial value, as when the experimenter applies a 

 single electric shock, a tap on a tendon, or a flash of light. The 

 effect of the parameter-change is a brief change of field which, 

 while it lasts, carries the representative point away from its 

 original position. When the parameter is returned to its original 

 value, the original field and resting state are restored, and the 

 representative point returns to the resting state. Such a stimulus 

 reveals a line of behaviour leading to the resting state. 



It will be necessary later to be more precise about what we mean 

 by ' the ' stimulus. Consider, for instance, a dog developing a 

 conditioned reflex to the ringing of an electric bell. What is the 

 stimulus exactly ? Is it the closing of the contact switch ? The 

 intermittent striking of the hammer on the bell ? The vibrations 

 in the air ? The vibrations of the ear-drum, of the ossicles, of 

 the basilar membrane ? The impulses in the acoustic nerve, in the 

 temporal cortex ? If we are to be precise we must recognise that the 

 experimenter controls directly only the contact switch, and that 

 this acts as parameter to the complexly-acting system of electric 

 bell, middle ear, and the rest. 



When the 4 stimulus ' becomes more complex we must generalise. 

 One generalisation increases the number of parameters made to 

 alter, as when a conditioned dog is subjected to combinations of a 

 ticking metronome, a smell of camphor, a touch on the back, and 

 a flashing light. Here we should notice that if the parameters 

 are not all independent but change in groups, like the variables 

 in S. 3/3, we can represent each undivided group by a single 



77 



