48 BRAIN MECHANISMS AND LEARNING 



to do with the mass of the brain or the number of neurones. The minute one goes 

 from these simple things to complex ones, there arc correlations of performance 

 with the total mass of the brain. Indeed I find myself in most exciting conflict with 

 Dr Hebb on the role of large numbers ot neurones. I have used the term 'physio- 

 logical neurone reserve', distinguishing the potentially activable neurones of the 

 brain, in contradiction to the anatomical population. For various reasons of in- 

 creased thresholds or previous activation, many neurones may not at a given time 

 be functionally available. The functional neurone reserve does seem to parallel 

 nicely the ability to 'master a situation', a broader and more behaviourally interest- 

 ing term than simple learning, hi other words, while one may not do better on a 

 prescribed simple stimulus response relation, when it is necessary to find the correct 

 concatenation oi possible behaviours out of an almost infinite variety, then the 

 larger neurone complement is certainly an advantage and allows greater richness. 

 Such an endowed brain can learn things that the other cannot. Maybe in this last 

 point I am violating what you meant to say, but that is the way it struck me. 



Hebb. I don't believe the synapse is irrelevant (what has happened is that I have 

 been restricted to a 15-page document by Dr Delafresnaye and to a 30-minute 

 presentation by Dr Gerard). Behind this statement of mine is an extremely long 

 attempt to see if there was any other way of dealing with the problem. The fact of 

 the matter is that the theory may be bad. I have no doubt that the position suggested 

 by Dr Galambos, by Dr Olds and by others is logically sound. But the flict also 

 is that people do think of a simple theoretical dichotomy — structural modification 

 or a reverberation; 



Gerard. It need not be one or the other, but either or both — or neither. 



Hebb. I understand that. My only point is that I didn't mean to suggest that it is 

 only structural, and not reverberatory. I had thought before, as you did with the 

 telephone book, that I could look up a telephone number, dial it, and forget it 

 completely. That I could listen to a set of digits, repeat them and forget them 

 completely. I suggest to you that your statement about completely forgetting the 

 telephone number is almost certainly wrong, on the basis of the experimental 

 evidence that I have reported. I would have agreed with you 4 years ago. My idea 

 now is that it is a little more complicated. 



One last point on rate of learning in different species. I assumed that we were 

 dealing with simple learning in all cases. Man certainly learns some complex tasks 

 quicker than a monkey does, but the complex task may be regarded as the equiva- 

 lent of a number of simpler ones. But there is a point that I think needs to be made. 

 It has happened that Baerend's and Tinbergen's data have not been believed because 

 the wasp is too low in the scale to have as fast learning as we have. But I propose 

 that learning for which an organism is suited is likely to be faster, rather than 

 slower, at the lower levels. 



KoNORSKi. According to my opinion any neurophysiologist concerned with the 

 problems of conditioned reflexes is entitled, and even obliged, to explain the facts 

 he deals with in the terms of synaptic transmission. As a matter of fact our first- 

 hand knowledge about the principles of this transmission comes out from the most 

 simple two-neurone reflex arcs studied by the most refined electrophysiological 

 techniques. We extrapolate these findings to interpret the more complicated spinal 

 reflexes, and we have also a right to utilize them to understand the mechanisms of 

 conditioned reflexes, although they are much more complicated. 



