394 BRAIN MECHANISMS AND LEARNING 



Habituation is not only the primary learning process, but seems to be 

 a common and necessary denominator in the establishment of all other 

 more complex types of learning. Indeed, the acquisition of any specific 

 motor performance is accompanied by the elimination of a great number 

 of originally present irrelevant responses. Therefore, when searching for 

 the neurophysiological basis of learning, the importance of understanding 

 the basic mechanisms of habituation becomes apparent. 



Granting that through specialization of function, plasticity as well as 

 excitability are more developed in certain elements of multicellular 

 organisms, there is no doubt that the nervous system is endowed with the 

 privilege of both properties. There are two fundamental questions about 

 the mechanisms of plastic changes, and therefore of learning, (i) What is 

 the ultimate nature of plasticity ? (2) Where are the neurones in which 

 these properties are more developed; Our total ignorance on the first 

 question cannot be overemphasized. The reason for this, it must be ad- 

 mitted, is that the right method for obtaining direct evidence of plastic 

 changes has not as yet been elaborated and applied to the study of the 

 nervous system. More attention has been devoted to the second question 

 concerning the locus of learning in the nervous system. The outstanding 

 development of the cortical mantle in the human brain, and its upper 

 anatomical location have impressed most scientists in the past who have 

 granted the cerebral cortex the highest rank in the neural hierarchy. Very 

 often, this contention has been upheld to the point of assuming that all the 

 learning processes take place exclusively at the cortical level. There is 

 much evidence, however, to indicate that many types of simple learned 

 behavioural responses do not require the cerebral cortex. Lower verte- 

 brates with no true cortex as well as invertebrates which do not even have 

 a brain, are capable of learning. But although it is evident that habituation 

 in lower vertebrates does not require phylogcnetically new structures such 

 as the cortex, it might be argued that in mammals the greatly developed 

 cortical mantle has taken over the basic mechanisms of learning originally 

 located in lower brain structures of other animals. With the aim of 

 assessing the role and capacity of different levels of the mamnialian 

 neuraxis for the establishment of habituation the following experiments 

 were performed. 



Vestibular liahitiiatioii in decorticated and decerebrated cats. Following a 

 period of controversy, Griffith (1920) established that post-rotatory 

 nystagmus in man decreases with repetition of rotation, and that such 

 decline bears a striking resemblance to the traditional learning curve. 

 The same author observed the cumulative and long-lasting effects of 



