Il6 BRAIN MECHANISMS AND LEARNING 



of the work in this field. These workers estabhshed that if a given 'in- 

 different' stinaukis is reinforced not during its action but some time — of a 

 range of seconds or minutes — after its cessation it is possible to establish in 

 a dog the CR not to the stimulus itself but to a short period of time just 

 preceding the moment of reinforcement. Thus, if for example a dog 

 repeatedly receives food 3 minutes after the cessation of the conditioned 

 stimulus (CS), he learns to salivate not earlier than about half a minute 

 before the moment of feeding. In a variation of such experiments the 

 animal receives food simply at constant intervals and learns to expect it at 

 the proper moment — 'the CR to time'. The end of the act of eating 

 plays in this experiment a role of a trace CS. 



Facts of this kind mean that (i) the CS produces some changes in the 

 CNS which persist some time after its cessation, and (2) that the animal 

 is able to 'measure' somehow the time elapsed after it. 



The process of elaboration of a trace CR is such that tu'st both the CS 

 itself and the whole period between the stimulus and the reinforcement 

 evokes salivation and then gradually the conditioned reaction is more 

 and more postponed. This shows that first there is a generalization of the 

 CR to all moments following the CS and then differentiation of these 

 moments occurs. 



A peculiar property of trace CRs is that they arc very widely generalized : 

 even the application of new stimuli, much different from the original CS, 

 produces salivation in the appropriate moment after their cessation; for 

 example, when the original trace CR was elaborated to a tactile stimulus 

 applied to one spot of the skin, it is produced by tactile stimuli applied to 

 quite different spots and also in response to auditory stimuli. This property 

 of trace CRs seems to suggest that they are formed not to the traces of the 

 given exteroceptive stimulus itself, but rather to some of its consequences 

 which are common for various sorts of stimuli. As any external stimulus 

 elicits an orientation reaction it may be that the proprioceptive stimuli 

 generated by this reaction form the true basis for elaboration and occur- 

 rence of trace CRs. We shall return to this question in a later section. 



2. DELAYED RESPONSES 



Nearly at the same time Hunter (19 13), according to the suggestion of 

 Carr, introduced into behaviouristic psychology another method of 

 investigation of recent memory based upon the so-called delayed responses. 

 The general principles of this method are roughly these: the animal is 

 taught to receive food in two or more different places (or run to the food 



