R. HERNANDEZ-PEON AND H. BRUST-CARMONA 403 



inborn and acquired responses {plastic iiihibirioit) (Hernandez-Peon, 1957). 

 Therefore, the total conditioned behaviour varies throughout the different 

 stages of the experimental procedure. It should also be emphasized that 

 the conditioned reaction is not a simple replica of the unconditioned 

 response, but usually involves more complex behavioural changes 

 revealed by the expectant attitude o{ the animal. 



According to the Pavlovian traditional concept, all processes of plastic 

 association and plastic inhibition occur at the cerebral cortex, in the 

 absence of which conditioning is assumed to be impossible. There is 

 plenty of evidence, however, to indicate that acquisition and retention of 

 conditioned responses in dogs and cats do not require the neocortex 

 (Bromiley, 1948; Culler and Mettler, 1934; Girdcn, Mcttlcr, Finch and 

 Culler, 1936; Ten Catc, 1938; Wing, 1946; Wing, 1947; Wing and 

 Smith, 1942; Zeleny and Kadykov, 1938). 



Although conditioning in the decorticate animals provided clear 

 evidence concerning the essential role of subcortical structures in plastic 

 associative phenomena, the relative importance of each ot them for the 

 establishment of conditioning remained to be shown. By making sub- 

 cortical lesions in cats, Hernandez-Peon, ct al. (Hernandez-Peon, Brust- 

 Carmona, Eckhaus, Lopez-Mcndoza and Alcocer-Cuaron, 1958) found 

 that a nociceptive salivary conditioned response was eliminated only by 

 lesions in the mesencephalic reticular formation. These lesions did not 

 interfere with wakefulness, but produced striking behavioural changes to 

 be described later on. 



In so far as the mesencephalic tegmentum is a region of rich afferent 

 convergence, and since the above-mentioned results pointed out its 

 importance in basic associative phenomena, the following questions were 

 raised: does a mesencephalic lesion equally impair conditioned responses 

 to stimuli of different modalities? Does it affect to the same extent both 

 classical alimentary and defensive conditioned responses? In an attempt to 

 answer those questions the following experiments were performed using 

 cats in which two different responses were conditioned: (i) alimentary 

 responses to visual and olfactory stimuli and (2) defensive flexor responses 

 to an acoustic stimulus. 



Cividitioued alimeutary responses in intact cats. The unconditional stimulus 

 consisted of a bit of fish. This was brought near the cat by means of a 

 clamp attached to a thread which was introduced through a tube in the 

 ceiling of the cage. The light of a lamp placed at the ceiling and the sight 

 and smell of the fish were the conditional stimuli. Salivation was measured 

 by counting the drops of saHva flowing through a poly-ethylene tube 



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