154 BRAIN MECHANISMS AND LEARNING 



usually at a hidden aud unexpressed level, in terms of the association of 

 ideas. A new association in the environment is imprinted on the animal, and 

 we observe the conditioned response not mainly as a measure of the 

 animal's adaptive behaviour but rather as a measure of the degree to 

 which we have impressed the association on the animal. 



Whether there is one basic mechanism with two aspects or two or more 

 basic mechanisms is still almost entirely a matter to be decided by further 

 experiments, although there is considerable argument pro or con associa- 

 tion or response learning. At any rate, our experiments have taught us 

 something rather defniite about the physiological processes underlying 

 response learning. It appears to us that our studies also have relevance 

 to the broader problem of associative learning, but that has not yet been 

 established. Any argument that there is only one basic mechanism with 

 these two different aspects in the learning process will, we believe, make 

 it even more likely that these studies involve one very important basis of 

 learning in general. 



I. SELF-STIMULATION EXPERIMENTS ON LEARNING 



Self-Stimulation tests (Fig. i) described in detail elsewhere (Olds, iQS-S) 

 indicate that electric stimulation in medial-forebrain-bundle regions of 

 the hypothalamus, or in related structures of basal ganglia, paleocortex 

 and tegmentum (Fig. 2), causes positive reinforcement of learning to the 

 degree that animals will spend Ic^ng times turning on the stimulus to the 

 brain in preference to all other pursuits. Animals will learn to run in a 

 runway (Fig. 3) with such stimulation as the only reinforcement; they 

 will cross an electrified grid (Fig. 4) to reach a pedal and stimulate their 

 brains; and they will learn rapidly to perform without errors in a multiple 

 choice maze (Fig. 5) (Olds, 1956) with the electric stimulus to the brain as 

 the only incentive. Both acquisition and extinction curves on all these 

 problems compare favourably with those where the positive reinforce- 

 ment (or unconditioned stimulus) is a food object. It is clear, therefore, 

 that stimulation in these regions promotes repetition of a learned response. 



II. INTERFERENCE-STIMULATION AND LEARNING 



The question we went on to ask was a question that has troubled 

 physiological psychology for some time: is there any sense in saying 

 learning mechanisms are somehow localized in these heldsf Or before we 

 ask that, is learning localized at alh. 



