650 BRAIN MECHANISMS AND LEARNING 



discharge and so on. I do not know enough about it to suggest an explanation but 

 I think this is really the problem. 



Hernandez-Peon. In reply to Dr Thorpe's question as to whether or not 

 sensory receptors govern the discharge of nerve cells in the central nervous system 

 I woulci like to point out that in the adult brain there is a considerable amount of 

 tonic inhibition acting upon the spontaneous activity of nerve cells in the central 

 nervous svstem. In this respect, recent experiments of Soderberg and Arden showed 

 that in rabbits a lesion ot the optic tract enhanced the electrical activity of the 

 lateral geniculate body. Brain stem lesions also increased tlais activity. These results 

 indicate that there is certain degree of ascending inhibition coming from the 

 retina and tonic inhibition from the brain stem, both acting upon the lateral 

 geniculate body. 



Gerard. The major advance in neurophysiology in this century was the recogni- 

 tion that we did not have, either in nerve or neurone or whole nervous system, an 

 inert passive instrument waiting for something to arouse it. There is continuing 

 activity. This ultimately reduces to processes at the molecular level — wliich is 

 metabolism — and it is well recognized in physical chemistry that regular rhytlunic 

 changes result only from continuous processes. Comparably, only a constant wind 

 produces a regularly flapping flag. I think the really basic questions are at this level. 

 There is a constant active metabolism and trom this rhythms develop. It is perhaps 

 only secondary, although ot the greatest interest, as to whether these rhythms are 

 primarily within a single cell — self-stimulated if you want to put it that way — 

 whether there are microcircuits, or whether there are macrocircuits. As a matter of 

 fact, the rhythms we found long ago in the olfictory bulb were recently shown by 

 a couple of my students to depend in part at least on microcircuits between mitral 

 and granule cells — in the olfictory cortex. 



The patterns of behaviour must always involve patterns of structure and patterns 

 of time. Certain cells are connected with certain other cells by definite paths ; and 

 these connections arc either inborn or formed as a result of learning. And there are 

 also patterns in time, which mav be continuous activities or may vary with the 

 physiological properties ot the responding units — double tiring, summation, etc. 

 So that I would say we cannot now give an explicit answer but we can indicate 

 the factors that enter into the answer. 



Thorpe. The answers which have been given have carried us much turther 

 towards an explanation. I have learnt much from them. I think that consideration 

 of the actual physical or biochemical mechanism inside a cell which can be giving 

 rise to rhythmic behaviour is of extreme interest and I wish we had time to discuss 

 it much further. Perhaps, however, it is rather too much off the main line of 

 interest at the moment. But I am very gratetul to the members of the symposium 

 for the comments thev have made. To close the discussion with one tiiial sugges- 

 tion it seems to me that the sort of rhythmic mechanism which appears to be acting 

 within a single cell might also be the kind of mechanism which is acting between 

 cells — to inhibit or control or establish a rhvthm. 



