136 



BRAIN MECHANISMS AND LEARNING 



CS. Movement of the leg or tail was never elicited by the CS. This CR 

 subsequently occurred up to lOO per cent of the time in some sessions and 

 had a threshold of about 0.3 mA. It could also be elicited by 0.4 niA. 

 applied to a second pair of electrodes about 2 mm. distant from the 

 original CS pair. 



Fig. I 

 Unconditioned response o( Do^ Alpha to stimulation just posterior to left postcrucial sulcus. 



The same CR was evoked by a CS of 3 seconds clicks after one session 

 (eight trials) combining this CS with the UR. A second UR was coupled 

 with 9/seconds clicks as CS. This at first produced the previous CR which 

 gradually became modified to a sidewise oscillation of the head with nose 

 pointing down. This new 'CR', however, had nothing in common with 

 the second UR. 



It was very difficult to teach this dog to eat in the experimental situation. 

 Once trained, however, the animal pressed the lever repeatedly despite 

 accompanying CS or US stimulation which in the case of the US produced 

 violent movements. In contrast, if the side of the cage was tapped gently 

 each time the lever was pressed, two or three taps abolished all pressing 

 for the rest of the session. 



