412 BRAIN MECHANISMS AND LEARNING 



responses in different manners. Moreover, we must admit the possibility that an 

 animal confronted with a certain training routine mav have a 'choice' as to the 

 neural structures it will utilize in that particular learning process: this could be one 

 explanation tor the apparently confficting results of lesions. 



Hernandez-Peon. I think that different parts of the brain do have different 

 roles in different types of learning and even for a given type of learning at different 

 stages of learning. We must always point out the type of learning and the stage of 

 learning and whether we are dealing with positive or negative learning, hi other 

 words, wc have to take into consideration the effects produced by plastic associa- 

 tion and by plastic inhibition, because certainly different parts of the brain have 

 more importance for one or another type of learning. 



RosvoLD. I should like to emphasize the point already made. If comparisons are 

 going to be made, the methods used must be the same. We found that very slight 

 changes in the testing methods can elicit entirely different behaviour. 



I should like also to say that in monkeys, at least, the effect of lesions in medialis 

 dorsalis are not those one would expect from its connections in the frontal lobe. 

 For example, one does not find any of the effects that one fmds from frontal lobe 

 lesions. On the contrary, one finds that lesions in the head of the caudate nucleus 

 do give the effects found after frontal lobe lesions. 



Thorpe. I also would like to emphasize the importance ot standardizing the 

 circumstances in which these responses are obtained. Having in mind the biological 

 importance of habituation, I like (see Thorpe, 1956) to distinguish it from various 

 other types of response waning known to the ethologist and to regard it, from 

 the biological point of view, as Joiio-tcrin stiiiiiiliis-spccific waiiiiio. It thus becomes 

 very important to know the relation ot the degree of habituation or dishabituation 

 to the intensity of the stimulus. I was, ther'efore, particularly interested to know of 

 Dr Segundo's results for it seems that information about the habituation effect of 

 different tones may be a very important piece of evidence in understanding what is 

 really going on. If in the case of spinal habituation wc are uncovering something 

 of the fundamental physiological mechanism of the habituation process, we might 

 expect a very considerable difference in the detail of correlation between the 

 intensity and the duration ot the stimulus and of the response as between intact 

 and decorticate animals. It is important that these should be standardized. It also 

 bears on the similarity between habituation and internal inhibition. Pavlov, I 

 think, at one time thought of internal inhibition as in some respect connected 

 with the absence of the unconditioned stimulus. At other times he thought of it 

 more as the result of some kind of competition. The first view seems much more 

 natural to the biologist and that is why the ethologist thinks ot habituation {loini- 

 tcnii stiiiiiihis-spccific u'aiiiii(<) as characteristic of the absence of the consummatory 

 act or consummatory stimulus situation. If this innately determined system is 

 present we should expect the habituation to be reduced or absent. On the other 

 hand, if some similar, but non-identical stimulus is present we should expect the 

 habituation to be affected in corresponding degree. So again it becomes highly 

 important, in experimental studies on habituation, to have specific information 

 about the details of the stimulus employed and the relation between intensity of 

 the stimulus and the intensity and duration of the effect. 



