4.6 BRAIN MECHANISMS AND LEARNING 



A more specific implication is with respect to delayed response. If 

 extrinsic reinforcement is not necessary for the establishment of a 

 structural trace — if the subject who only hears and tries to repeat a series 

 of digits none the less has some memory of that specific series which can 

 last despite hearing and repeating other series afterwards — then some 

 revision is called for in my interpretation of the phenomenon of the 

 delayed response (Hebb, 1958). A monkey sees a piece of food being put 

 under a cup at the left, none under a cup at the right; his only response at 

 the time is to look at the left cup, since both are out of reach. Both cups 

 are then hidden by lowering a screen, let us say for 20 seconds. When the 

 monkey is permitted to choose he goes at once to the left cup and gets the 

 food. It seemed to me that this could only be accounted for in terms of a 

 perceptual activity held in reverbcratory circuits. In view of the present 

 results, this does not follow. One look may be enough to establish what is, 

 in effect, an S-R connection between the stimulation of the sight of the 

 two cups, and an eye movement to the left. When the monkey is again 

 exposed to the same visual stimulation, he looks left, his hand follows the 

 direction of gaze, and he obtains the food. It is still clear that this 'S-R 

 connection' in the brain of the higher animal can hardly be the kind of 

 direct, one-way route envisaged by earlier theory, and that it must also 

 involve some transmission by re-entrant circuits, but it is still a relatively 

 simple mode of learning. In a lower animal, presumably, the closed-circuit 

 element may not be present, and we can understand better the one-trial 

 learning in the wasp already referred to (Tinbergcn, 195 1). 



GROUP DISCUSSION 



EcCLES. I am very grateful to Dr Hebb for putting up some clear ideas to shoot 

 at. I want to shoot at the synaptic knob story he has given wliich is based on the 

 outmoded theory of electrical synaptic transmission. However, I thought I might 

 leave this synaptic story until tomorrow when I will be talking about the details of 

 what goes on in the synaptic knob during repetitive stimulation and afterwards. 



I would like also to comment on one-trial learning. I was listening very carefully 

 when Dr Hebb spoke his series of digits. As soon as he started off the series I repeated 

 this mentally and I went over the first 3 digits quickly when he got to 3, and so on 

 at the 4 and the 5, so as to develop my memory. It wasn't a single trial for me. 



Hebb. Did you get it right? That is the question. 



EccLES. I don't suppose I did. (Checking) The first ones were right, and also the 

 last two ; there was some muddle in the middle. 



Hebb. I think you will find another method is better. Just to listen and then 

 repeat. 



EccLES. While you are even listening to one digit there will be continued activity 

 in the cortex. In 1949 both Dr Gerard and Dr Hebb published the concept that 

 reverberating circuits played the key role at the early stages o( learning. I have 



