350 BRAIN MECHANISMS AND LEARNING 



experiments. We nuist block the afferent pathways without cutting them, 

 which is probably a matter of improving our technique of cold blockage. 



Gerard. This is the same kind of answer I gave to Dr Grastyan's point, that one 

 single stimulus could leave a trace. Secondly there are many experiments to show 

 that memories mostly (not always) do not remain unchanged over time; pictures 

 are recalled with directional distortions. 



EsTABLE. I would like to make some comments: in the first place, the problems 

 of learning are usually approached taking for granted that the neurone is static, 

 that it is not modified during life. Neurones are like trees that modify their finer 

 branches, for example, the neurones in the skin, which change the direction of 

 their terminal branch Vv^hen the cell they were ennervating dies and has peeled off. 

 I wonder whether something like this might not occur in the C.N.S. and provide 

 an explanation for the changes in its performance. In the second place, small 

 changes in synapses may give large functional changes — for example in certain 

 diseases and after some drugs — and in the embryo, where the synapses are identical 

 to those of the adult but do not yet operate. 



EccLES. I do agree that there are all sorts of growth processes going on in synapses. 

 Anyone who has seen the cine-photography of Pomerat in tissue culture will agree 

 that the outgrowing nerve terminal is vividly alive. It is growing, retracting and 

 moving all the time. 



Olds. Dr Eccles has shown us a good example of a long run adaptive change 

 through use, and with such few examples of this type, it cannot be ignored. 

 However, I wish to sound a word of warning to my fellow psychologists. What 

 Dr Eccles has suggested is that by sending an impulse down an axon we cause more 

 transmitters to be available in the terminal, and note that this is true whether or not 

 the impulse 'crosses' the synapse. This data is often used by theorists to show how 

 the successful synapse gains power at the expense of the unsuccessful synapse — 

 and my impression is that both these synapses arc equalh' reinforced and that 

 makes this mechanism unusable for theories like those of Hebb, Konorski and 

 Gerard. 



Eccles. I think Dr Olds had misunderstood the problem. I am not interested in 

 whether a particular synapse generated an impulse or not. All I am interested in 

 showing is that every time you activate synapses the)' become more efficacious. 

 Every time a signal comes in from some receptor it fires more cells and thus 

 operates along channels that become progressively more powerful with more 

 impulses in parallel, more convergence on the next stage and so on ... Thus there 

 is a progressively more effective channelling. Now this would only occur if the 

 'unsuccessful' synapses shared in the potentiation. 



AsRATYAN. I want to congratulate Dr Eccles on his valuable new data which is a 

 great contribution to neurophysiology. I want to draw attention to the extreme 

 importance of new data provided by Dr Eccles concerning the stable results of 

 excessive use and disuse of synaptic apparatus on their structural and functional 

 properties. Recently we have obtained in our laboratory some facts which are in 

 full accordance with this data, although it was not obtained under such precise 

 conditions. We worked with dogs whose spinal cord has been transected at the 

 level of the lower thoracic segments. If the animals are strictly limited in their 

 mobility it leads to an evident muscular atrophy of these limbs and to a lessening of 

 their reflex activity. If, however, these limbs are systematically treated by special 



