140 BRAIN MECHANISMS AND LEARNING 



might possibly arise from some 'irritative' process consequent to the 

 repeated presentation of the US, but the following experiments show this 

 is not the case. Using the usual timing and parameters, the L-US alone 

 was presented lOO times in twelve sessions producing a forceful right fore- 

 leg flexion each time. No spontaneous lifts of this limb occurred in this 

 period. Another 150 presentations were then given in which L-US 

 preceded L-CS by irregular intervals. Initially L-CS still gave some left 

 foreleg CRs or other movements, but these reactions were extinguished by 

 this procedure since for the last seventy presentations there was no response 

 whatever to L-CS. Orthodox temporal relations for L-CS and L-US were 

 then used. At first this restored the nonspecific movements, then left 

 foreleg CRs and ultimately right foreleg CRs occurred about 50 per cent 

 of the time to L-CS at a threshold of 0.7 mA. 



The R-CS was then presented for the first time in 79 days. The left 

 foreleg was lifted 15 cm., flexing at the wrist, and was held so for about 

 5 seconds. Ultimately it was possible to obtain right leg flexions to L-CS 

 and left leg flexions to R-CS. 



Coupling L-CS, L-US or R-US with the animal's lever-pressing had 

 no effect, whereas auditory stimuli or R-CS completely abolished pressing. 

 The effect of R-CS had been predictable on the basis of behaviour changes 

 seen as soon as the use of R-CS was resumed (after the hiatus described 

 above). Since more than 5 months had then elapsed from time of electrode 

 implantation it seems likely trigeminal fibres had grown into this medial 

 electrode location. 



Monkey 1. The CS of i.o mA. applied to the left occipital pole con- 

 sistently produced movement of the eyes, and sometimes the head down 

 and to the right. At any time during this stimulation, however, the eyes 

 might be moved elsewhere if attention was so directed. The 0.2 mA. US 

 (300/second) in the left precentral cortex produced a smooth, vigorous 

 flexion of the right forearm and contraction of the muscles on the right side 

 of the neck and face causing the mouth to open (Fig. 4). Coupling of the 

 CS and US was begun 2 weeks after surgery and continued for 4 weeks, 

 200 trials in twenty-four sessions. In these 200 trials the right arm made 

 random movements during the CS eleven times. Obviously there was no 

 conditioning. During the next 25 weeks the animal was used sporadically 

 for testing various lever-pressing procedures with fruit juice rewards. 



Coupling of CS and US was then resumed. The effects of these stimuli 

 were still exactly as they had been 7 months earlier. LJsing a two-minute 

 intcrtrial interval for the first fifty-one trials in five sessions there was no 

 sign of conditioning. A four-minute interval was then used for trials 



