104 BRAIN MECHANISMS AND LEARNING 



different light, namely, as a particular case of the general phenomenon of 

 elaboration of a reverse conditioned reflex connection (along with the 

 direct connection) when two stimuli are combined in a stereotyped 

 sequence. The transitory character of this connection, which was pointed 

 out long ago by many workers and was regarded by Pavlov as an enig- 

 matic phenomenon in conditioned reflex activity, may, to a considerable 

 degree, be interpreted as a result of its systematic non-reinforcement 

 during the periodic application of a stimulus for the purpose of testing a 

 conditioned reflex to it. 



// 



Fig. 6 

 Conditioned and unconditioned refle.x local changes in the skin temperature. 



A. Decrease of skin temperature in the dog under the action of food during 60 seconds and 

 subsequent application of cold. I. Record of skin temperature. II. Time — 12 seconds; (a) back- 

 ground; (/>) decrease of temperature during the process of eating and application of cold; 

 (f) recovery of skin temperature after the process of eating and action of cold. i. — mark of 

 presentation of food and beginning of the act of eating; 2. — mark of application of cold; 

 3. — mark of the end of eating and application of cold. 



B. Conditioned reflex decrease of skin temperature during the process of eating, (ci) — back- 

 ground ; (/)) — decrease of skin temperature during the process of eating; (c) — recovery of skin 

 temperature after the process of eating, i. — mark of presentation of food and beginning of 

 the process of eating; 2. — mark of the end of eating. 



IV. The significance of a definite sequence in the combination of stimuli 

 for elaboration of new conditioned reflex connections and for preservation 

 of existing ones is graphically shown in the experimental data obtained by 

 our collaborator Pakovich. 



From the earlier investigations of American psychologists, Schlosberg 

 (1928), Hilgard (1937), Wolflc (1930), Bernstein (1934) and others, which 

 were carried out on human beings, it was known that when the indifferent 

 stimulus precedes the unconditioned stimulus by o.i second and less, the 

 elaboration of a conditioned reflex is impeded, and sometimes even be- 

 comes impossible. In recent years, Pakovich has made a thorough investi- 

 gation of this question in dogs, on motoi reflexes which can be objectively 



