648 BRAIN MECHANISMS AND LEARNING 



the rcinforccnicnt ot smglc-unit discharges, and from the apparent phenomenon 

 that we sometimes ehcit the unit discharge, say by the pressure ot introduction of 

 the wire, and then reinforce it, to hope, I will not say to suppose, only to guess that 

 the unit itself, the spontaneous unit discharge, is something which can be rein- 

 forced. That is, that there is a spontaneous discharge pattern which has a certain 

 probabilistic rate and that this can be augmented by stimulating the hypothalamus 

 after the response has occurred in a fashion that you would not get it you stmiu- 

 lated the hypothalamus at some different time. 



Anokhin. In connection with the problem of the so-called 'spontaneous be- 

 haviour', I should like to draw the attention to those forms of behaviour which 

 depend on the presence of nervous cells with a specialized metabolism. At the 

 present time it can be agreed that certain cells of the hypothalamus and ot the 

 midbrain have a metabolic process which trom the chemical point ot view is quite 

 accurate and specific, hideed, the slightest departure from this specitic metabolism 

 leads to the release of a 'trigger' and to the outburst of nervous impulses which are 

 already spreading within the limits of given functional systems. It will suttice to 

 mention the cells of the respirator)- centre or the receptor formations of the sino- 

 carotid region, which tire as a result ot a slight increase ot CO., in the blood. 



Even more surprising is that nervous mechanism which is 'tuned' to the level ot 

 the osmotic pressure ot the blood. 



As it has been shown, one finds in the hypothalamus nervous cells which by all 

 their construction are adjusted to perceive the slightest changes in the osmotic 

 pressure of the blood. Special vesicles, representing part of these nervous cells, 

 react to the variations of the osmotic pressure oi the blood by diminishing or 

 expanding their volume; these variations in volume bring about an emergence 

 and dissemination ot nervous impulses. We know that these impulses often belong 

 to one or two cells and can have a rather widespread inffuence leadhig for instance 

 to a feeling of thirst and to setting in action important mechanisms of the body, 

 which lead to t]uenching this thirst. These cells, which are rather few in number, 

 act as 'servo-mechanisms' in the true sense of the word, transmitting small energetic 

 variations in the effort oi the whole organism with all its muscular power. 



Numerous behavioural acts are constructed according to this pattern, i.e. b) 

 setting in action nervous elements having a rather specialized metabolism. Un- 

 fortunatelv we, as neurophvsiologists, do not pay sufficient attention to this 

 chemical specialization in the metabolic process ot individual nervous cells, 

 whereas many torms ot the so-called 'spontaneous behaviour' are connected to this 

 characteristic ot the neuro-humoral transmitting mechanisms. 



Hebb. I will speak directly to the point o( self-stimulation. I think we have a 

 tendency to assume that selt-stimulation is the same as excitation. I noticed that the 

 two words are used synonymously ver\- otten. And this is relevant to the third 

 point, as a matter of fact, in asking about the spontaneous discharge, and whether 

 the cell is inhibited or excited in many of these conditions of learning. You will 

 remember perhaps that in reply to an earlier question Dr Olds said that he thought 



