402 BRAIN MECHANISMS AND LEARNING 



Stimulus. Again, the effects of repetition of the tactile stimuli in the spinal 

 cats were long lasting, and appeared to be cumulative. 



These experiments indicate that plastic inhibition can develop in the 

 isolated spinal cord and confirm the results obtained by Prosser and 

 Hunter (1936) in the rat. By electromyographic and mechanical record- 

 ings they observed in the spinal rat fading of spinal flexor reflexes with 

 repetition of the corresponding stimuli. Because of the features which 

 prevent the identification of this diminution of responsiveness with fatigue 

 and sensory adaptation, Prosser and Hunter, rightly interpreted the 

 phenomenon as habituation and assumed that it involved spinal inter- 

 nuncial neurones rather than the first sensory elements or the final moto- 

 neurones. 



From the foregoing considerations it is evident that in aeidition to the 

 supraspinal influences acting upon sensory impulses there is an intraspinal 

 mechanism which regulates the excitability of lower order sensory neu- 

 rones, and consequently the entrance of afferent impulses into the spinal 

 cord itself Such a mechanism implies a local feed-back circuit in which 

 spinal interncurones are likely to be involved. The interneurones are 

 activated by collaterals from spino-thalamic fibres, and in turn they would 

 inhibit the corresponding sensory neurones. This hypothetical sensory 

 circuit would be comparable to the ventral horn circuit formed by the 

 recurrent collaterals of motor axonS and the Rcnshaw cells which have 

 been found to inhibit the motoneurones (Eccles, 1957). 



CONDITIONING 



Conditioning results in the acquisition of a response (conditioned 

 response) to a stimulus (conditional stimulus) which previously was not 

 effective in eliciting that response. Conditioning may be considered to 

 represent one of the simplest types of positive learning, just as habituation 

 represents the simplest type of negative learning. From the neurophysio- 

 logical point of view, conditioning must necessarily involve plastic 

 associatii'c pliciioiiwiia which must take place in some central region of 

 convergence of sensory impulses. Pavlov (Pavlov, 1928) used the term 

 tonporary couucctioiis to designate the process of neural association, but we 

 prefer to call it simply plastic association, since its manifestations can be so 

 permanent as to endure for the life of the individual. 



In studying conditioning it must be borne in mind that the development 

 of the conditioned behaviour involves the emergence of new responses to 

 the conditional stimulus {plastic association), as well as the disappearance of 



