MAN 195 



or moves them anatomically from place to place; the in- 

 born pattern of the nervous system is the most rigid con- 

 ceivable: a system of fixed point-to-point connections. 

 And yet the nervous system of the higher animals is the 

 most plastic and adaptable organ in the body. 



In the rat, for example, the optic nerves from the right 

 eye are connected with the visual areas in the left lobe 

 of the cerebral cortex, so that a spot of light pinpointed 

 on the right retina is projected' (as the neurophysiologist 

 would say) on the left side of the brain with a spread of 

 no more than two or three cell diameters. Nevertheless 

 all but 2 per cent of the visual area in the rat's cortex 

 can be destroyed without afiEecting visual integrations, 

 as judged by learning reactions, and it need not be the 

 same 2 per cent that must be left for the animal to in- 

 tegrate effectively. Locahzation of certain functions is 

 much more evident in man than in the rat, but in both 

 species the network of nerve cells that makes up the brain 

 has properties of organization and responsivity that re- 

 side in activities of the system as a whole (or what is 

 left of it), rather than in its specific point-to-point con- 

 nections. In learning, it is probably the pattern of activity 

 in the brain as a whole, or some large part of it, that 

 changes, and not the anatomical pathways. 



In a crude analogy, the processes of mental activity 

 may be likened to messages spelled out on an electric 

 billboard: each bulb (meaning each nerve cell) flashes 

 on and off in a more or less invariant manner, but the 

 message is never twice the same and can be spelled out 

 with equal intelligibility by different bulbs and circuits 

 (that is, by different neurons in different areas of the 

 brain). The human cortex is estimated to contain ten 

 billion separate neurons coupled in three dimensions into 

 many, many, many times this number of potential cir- 

 cuits—taking even a fraction of the possibilities, the po- 

 tential number of permutations and combinations is in- 

 comprehensible. 



There is no difficulty in accounting for the plasticity 



