138 



BRAIN MECHANISMS AND LEARNING 



US. For convenience the prefix 'R' or 'L' will be used to designate to 

 which hemisphere the stimulus was applied, e.g. L-CS with R-US means 

 CS applied to left posterior ectosylvian gyrus, US to right post cruciate. 

 For the first forty pairings of R-CS with R-US there was no response to 

 the R-CS. During the next few sessions the R-CS frequently elicited a 

 lowering of the head and flexion of the right foreleg as shown in Fig. 3, 

 i.e. the limb opposite that in which the UR was induced. The first left 



Fig. 2 



Unconditioned response of Dog Gainuui to stnnulation in right 



field Hi of Forel. 



foreleg CR occurred on the eighty-first presentation of R-CS and in the 

 next 113 trials occurred hfty-cight times. These CRs were discrete, 4-20 

 cm. elevations of the left foreleg held for several seconds and frequently 

 returned to the floor prior to the UR. The threshold for CR elicitation was 

 0.5-0.7 niA. Other movements, not so specific, occurred regularly, so that 

 movement now occurred 95 per cent of the time to this CS. 



L-CS was now paired four times with R-US, and except for the first 

 presentation produced flexions of the right foreleg (again opposite to the 

 UR) each time. Two sessions later differentiation was begun and continued 



