J. OLDS AND M. E. OLDS l6l 



SO as to reinforce learning; and (4) in a very small number of cases, an 

 escape test to discover negative reinforcing effects of stimulation. 



After learning experiments were complete, each animal was sacrificed, 

 and his brain was sectioned and stained to determine the precise location 

 of stimulation. 



RESULTS 



(r) Differences. — The results indicate a dctmite division of the brain into 



(1) a larger set of regions where electric stimulation has very little effect 

 on this learning behaviour, (2) a smaller (but very extensive) region where 

 electric stimulation has very devastating effects and (3) a region where 

 effects are ambiguous. 



First, when electrodes are implanted in the neocortex and in many 

 parts of the adjacent cingulatc cortex, stimulation from 10 to 50 namp. 

 does not cause any major deficit in behaviour (Fig. 7). Similar effects are 

 often seen with electrodes in the thalamus (left of Fig. 9). In all these cases, 

 there is learning, with, of course, some day-to-day differences. Stimu- 

 lation may cause some increment in errors, but it does not prohibit 

 perforniance or learning. Animals run well, eat well, and learn well 

 whether the stimulus is going or not. 



Seconc^, when electrodes stimulate in other places, quite different results 

 are obtained (Fig. 8). It appears totally impossible for animals to reach 

 criterion with the stimulus going in these areas. Except for confusion of 

 learning, the animals perform well. They run almost as fast as usual; that 

 is, the total number of runs is about equal for the stimulated and control 

 series. The animals eat food well when the correct response activates the 

 food magazine. But they cannot learn to eliminate errors on days when 

 the stimulus is going. Because the animals continue to run and to eat, it is 

 assumed that there is no interference with behaviour or drives. The inter- 

 ference has to do only with the ability to eliminate errors. The stimulus in 

 these cases is very small, interfering usually at levels of 20 laamp. or less. 

 Finally, it should be noted that the stimulus does not merely cause or 

 enhance a position preference, for errors on right or left levers are both 

 caused to increase greatly by the electric stimulus. 



Third, there are sometimes effects which make it impossible to make a 

 satisfactory test: (i) animals stop running when the stimulus is introduced: 



(2) the stimulus produces seizures so that no test can be made; (3) the 

 animal stops eating so that it is no longer possible to assume motivation 

 for learning ; and (4) the stimulus causes some definite confusion which is 



