9/6 DESIGN FOR A BRAIN 



the external rectus would cause the eyeball to turn not outwards 

 but inwards. When the wound had healed, he was surprised 

 to discover that the two eyeballs still moved together, so that 

 binocular vision was preserved. 



More recently Sperry severed the nerves supplying the flexor 

 and extensor muscles in the arm of the spider monkey, and re- 

 joined them in crossed position. After the nerves had regenerated, 

 the animal's arm movements were at first grossly inco-ordinated 

 but improved until an essentially normal mode of progression 

 was re-established. The two examples are typical of a great 

 number of experiments, and will suffice for the discussion. 



In S. 3/12 it was decided that the anatomical criterion for 

 dividing the system into ' animal ' and ' environment ' is not 

 the only possible : a functional criterion is also possible. Suppose 

 a monkey, to get food from a box, has to pull a lever towards 

 itself ; if we sever the flexor and extensor muscles of the arm 

 and re-attach them in crossed position then, so far as the cerebral 

 cortex is concerned, the change is not essentially different from 

 that of dismantling the box and re-assembling it so that the 

 lever has to be pushed instead of pulled. Spinal cord, peripheral 

 nerves, muscles, bones, lever, and box — all are 'environment' 

 to the cerebral cortex. A reversal in the cerebral cortex will 

 compensate for a reversal in its environment whether in spinal 

 cord, muscles, or lever. It seems reasonable, therefore, to expect 

 that the cerebral cortex will use the same compensatory process 

 whatever the site of reversal. 



I have already shown, in S. 8/10 and in Figure 8/10/1, that 

 the ultrastable system arrives at a stability in which the values 

 of the step-functions are related to those of the parameters of 

 the system, i.e. to the surrounding fixed conditions, and that 

 the relation will be achieved whether the parameters have values 

 which are * normal ' or are experimentally altered from those 

 values. If these conclusions are applied to the experiments of 

 Marina and Sperry, the facts receive an explanation, at least in 

 outline. To apply the principle of ultrastability we must add 

 an assumption that ' binocular vision ' and ' normal progression ' 

 have neural correlates such that deviations from binocular vision 

 or from normal progression cause an excitation sufficient to cause 

 changes of step-function in those cerebral mechanisms that 

 determine the actions. (The plausibility of this assumption will 



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