FUNCTIONAL ROLE OF SUBCORTICAL STRUCTURES 



IN HABITUATION AND CONDITIONING 



R. Hernandez-Peon and H. Brust-Carmona 



Until a few years ago it was generally assumed that at least in mammals 

 the cortex was necessary for learning and that learning was therefore a 

 cortical process. Furthermore, it was assumed that the neural association 

 of stimuli during positive learning takes place intracortically by means of 

 two-dimensional cortico-cortical connections. However, it has been 

 established ni dogs and cats that decorticated animals can learn and can 

 retain some simple responses learned before decortication. Therefore, it is 

 evident that subcortical structures play an important role in basic learning 

 processes. In the present paper this role will be discussed on the basis of 

 the experimental evidence obtained by the authors. 



HABITUATION 



If a non-significant stimulus is repeated even at intervals ot hours or 

 days, a decrement of response is observed which may last for hours, days 

 and years. Recognized as having a nature different from fatigue and 

 adaptation at the receptor (Griffith, 1924) this long-lasting decline of 

 behavioural responses is usually referred to as habituation. As it has been 

 rightly pointed out by Thorpe (1950), habituation represents the simplest 

 type of learning. It involves stimulation of a single modality, and it is 

 found through all the animal scale, from protozoa to man. Habituation in 

 the latter can be observed both in effector responses (somatic and autono- 

 mic) as well as in sensory experiences. Because of the ubiquity ot habitua- 

 tion in organisms of every grade of evolution, it seems warranted to 

 conclude that learning derives from some fundamental property of living 

 matter, and that, therefore, learning does not necessarily require special 

 complex neuronal circuits. We consider it convenient to use the term 

 plasticity proposed by Konorski (194H) in order to distinguish that property 

 from another fundamental phenomenology of living cells, excitability, 

 which is related to very transient changes produced by the stimulus. In 

 animals with nervous systems in which 'all or none' signals are trans- 

 mitted, plasticity permits the storage of information delivered by those 

 signals, whereas excitability is concerned with their generation and 

 transmission. 



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