3/11 DESIGN FOR A BRAIN 



in the Pavlovian experiments with conditioned reflexes : the 

 stimulus may evoke salivation, but the salivation has no effect 

 on the nature or duration of the stimulus. 



Such an absence of feedback is, of course, useful or even essen- 

 tial in the analytic study of the behaviour of a mechanism, 

 whether animate or inanimate. But its usefulness in the labora- 

 tory should not obscure the fact that the free-living animal is not 

 subject to these constraints. 



Sometimes systems which seem at first sight to be one-way 

 prove on closer examination to have feedback. Walking on a 

 smooth pavement, for instance, seems to involve so little reference 

 to the structures outside the body that the nervous system might 

 seem to be producing its actions without reference to their effects. 

 Tabes dorsalis, however, prevents incoming sensory impulses from 

 reaching the brain while leaving the outgoing motor impulses un- 

 affected. If walking were due simply to the outgoing motor 

 impulses, the disease would cause no disturbance to walking. In 

 fact, it upsets the action severely, and demonstrates that the 

 incoming sensory impulses are really playing an essential, though 

 hidden, part in the normal action. 



Sometimes the feedback can be demonstrated only with diffi- 

 culty. Thus, Lloyd Morgan raised some ducklings in an incubator. 



4 The ducklings thoroughly enjoyed a dip. Each morning, 

 at nine o'clock, a large black tray was placed in their pen, 

 and on it a flat tin containing water. To this they eagerly 

 ran, drinking and washing in it. On the sixth morning the 

 tray and tin were given them in the usual way, but without any 

 water. They ran to it, scooped at the bottom and made all 

 the motions of the beak as if drinking. They squatted in it, 

 dipping their heads, and waggling their tails as usual. For 

 some ten minutes they continued to wash in non-existent 

 water . . . ' 



Their behaviour might suggest that the stimuli of tray and tin 

 were compelling the production of certain activities and that the 

 results of these activities were having no back-effect. But further 

 experiment showed that some effect was occurring : 



' The next day the experiment was repeated with the dry tin. 

 Again they ran to it, shovelling along the bottom with their 

 beaks, and squatting down in it. But they soon gave up. 

 On the third morning they waddled up to the dry tin, and 

 departed.' 



38 



