1 82 BRAIN MECHANISMS AND LEARNING 



Supposing for a moment that the unit itself is the conditionablc entity in 

 these experiments, we would then be provided with some sort of a trace 

 system. We could suggest that instrumental conditioning techniques 

 leave a changed pattern of repetitive discharge in these units, a suggestion 

 which someone more given to theorizing than we might inflate into a 

 model for a brain. 



SUMMARY 



Three sets of experiments have been reviewed here, suggesting first 

 obliquely and then more directly that the hypothalamic-rhinencephalic 

 system is more intimately related to instrumental conditioning than the 

 other parts of the brain. 



First, self-stimulation experiments which show that stimulation in this 

 area can serve as a strong positive-reinforcement or unconditioned stimulus 

 in Skinner box, runway, obstruction box, and maze experiments were 

 cited. These suggested that somehow a good deal of activity emanating 

 from this system might foster at least the repetition of preceding responses, 

 if not the learning of responses themselves. 



Second, interference-stimulation experiments were conducted to find 

 where electric stimulation might interfere with performance in a T-maze 

 type of test. It was found that precisely the same system was implicated. 

 Stimulation in positive-reinforcement points appeared to cause confusion 

 because of the prepotency of the electric brain-stimulus 'reward' over the 

 food reward to the hungry rats. But other points in the hippocampus 

 proper caused confusion without any accompanying positive-reinforcing 

 effects. These points were also distinct from positive-reinforcement 

 points in that interference occurred only during learning but not during 

 later performance. 



Third, instrumental-conditioning experiments were carried out on 

 single-unit responses in cortical, palaeocortical and subcortical regions. 

 Units in subcortical and palaeocortical structures were often readily 

 conditioned. Only rarely and with great difficulty were cortical units 

 conditioned. Conditioning in lower centres often appeared to suggest 

 basic mechanisms, as stimulation after a unit discharge would have 

 different effects from those it would have against a background of silence, 

 or, several stimulations after a unit discharge, would cause seizure-like 

 repetitive discharge. 



The data from all three experimental programmes might be summarized 

 by the hypothesis that the quantitative measure of activity in the hypotha- 

 lamic-palaeocortical system is a measure of the positive reinforcement, the 



