REVIEW OF RESEARCH IN U.S.S.R. 475 



monotonous repetition of the stimulus and that due to a sudden distracting 

 stimulus, as a door slamming. Later, however, he expressed doubts about a 

 precise classification of inhibitory efTects. Internal inhibition is typically in- 

 nate inhibition which develops in the nonnal course of training, as with im- 

 proving differentiation. It is said to be often the first function to show 

 damage following any noxious influence, including radiation exposure. In 

 my view, internal inhibition can be compared with a multiplicity of cyber- 

 netic functions in the CNS, functions which refine the accuracy of CNS re- 

 actions and control them. 



Finally, statistical analysis of CR studies is rather complex. Theoretically, 

 it typically involves the interpretation of a 8-dimensional vector (the stere- 

 otype) in all its possible alterations, a problem which does not lend itself 

 to simple treatment. One may try to isolate certain elements, such as the 

 strength of the strong positi\e response or the degree of difTerentiation, 

 and test them statistically, but this would not necessarily prove or disprove 

 that some subtle change has occurred in the entire stereotype. Under these 

 conditions, it is easier to understand that the opinion of the experimenter, 

 who has typically done hundreds of studies on each of a small group of 

 animals, is accepted at face value by most Soviet physiology laboratories. 

 However, more objective methods of evaluation will clearly have to be 

 developed, and their design represents a considerable challenge for proba- 

 bility theory. 



In studies of radiation effects on CRs, Soviet physiologists have found 

 differing reactions, and no final opinion concerning the radiosensitivity of 

 the CR mechanism ought to be reached at this time. 



Some workers have observed subtle changes in CR patterns following 

 very low exposure. It was reported to the UNSCEAR that 50 mr had some 

 effect on an inhibitory CR, interpreted as being due to a change in internal 

 inhibition. Several papers by Piontkovsky (1959) and Khozak (1958) state 

 that 0.5-20 r single exposures have a stimulatory effect on conditioned 

 responses lasting days or even months. Cherkasov ( 1960) asserts that a single 

 dose of 30 r or chronic exposure at the rate of 0.1 r per day may cause 

 disappearance of all CR responses in certain experiments. Others state 

 repeatedly that CRs show changes after only low radiation exposures. In 

 rats having a motor-feeding CR to successsive 50 r doses, with interv-als of 

 1-3 weeks for a total of 350 r, Khozak (1958) found marked variability in 

 the particular animal following only 50-100 r total dose (Fig. 1). He inter- 

 prets this result as primarily a stimulatory and disinhibitory action on higher 

 nervous activity. It is unfortunate that only one control graph is shown, 

 since the variability of the animal response prior to irradiation is critically 

 important. 



Another group of experiments deals with near-lethal exposures in rodents 



