474 WALTER R. STAHL 



stimulus, a weak light as the "weak" stimulus, and a sound of the same 

 intensity, but different frequency, as the "differentiating" or inhibitory 

 stimulus. 



In a normal animal with a well developed CR pattern, there is a clear 

 parallelism between the strength of stimula and response, and differentiation 

 is distinct and complete. Possible alterations in CR stereotypes include 

 equalization of reactions to varying stimuli, or a strong reaction to a 

 weak stimulus and none at all to a strong one, failure of differentiation, 

 weakening of all responses, residual inhibition following the differentiation 

 stimulus, or increased errors or variability of response. All of these changes 

 have been stated to occur after radiation, in Soviet studies. 



Pavlov demonstrated that animals differ in their reactions to a stereotype. 

 He classified several higher nervous types (CNS types), namely, strong- 

 balanced, strong-unbalanced, weak, and poorly balanced types, the last 

 with a predominance of excitation or inhibition. In all cases, one is referring 

 to the occurrence of excitation and inhibition, as judged by strength, balance, 

 and mobility. Objective methods for classifying animals are well known in 

 the Soviet Union and involve tests of reactions to strong stimuli, reactions 

 to inhibitory stimuli, responses following caffein, etc. Typing is not always 

 easy or clear-cut, and many animals may not fall into a distinct type, but 

 nonetheless this classification is helpful for assessing reactions. In higher 

 animals the complexities are still greater, but human typing has been 

 attempted. As a simple illustration, persons react differently to a strong, 

 frightening stimulus — some tend to respond with activity (excitation) and 

 others with inactivity (inhibition) ; moreover, the time needed for restoration 

 of normality varies (being a function of CNS mobility and balance). The 

 occurrence of CNS types adds another degree of freedom to CD studies 

 and complicates their interpretation further. 



Pavlov did not attribute any morphologic or biochemical basis to excita- 

 tion and inhibition but felt the phenomena were purely functional. Basically, 

 one starts with the view that salivation is preceded by excitation in the 

 "salivatory center," and that there are antecedent foci of excitation before 

 the latter is stimulated. If salivation fails to occur when expected, then 

 inhibition occurred somewhere in the cerebrum. The quantitative nature of 

 CR experiments places a strong emphasis on the relative balance of excita- 

 tion and inhibition. Possibly the terms excitation and inhibition have been 

 identified too fully with the identical words used in Western psychology, 

 although in the original Pavlovian context they were different and perhaps 

 simpler. 



"Internal inhibition" often enters into the discussion in studies of radi- 

 ation effects on CRs. Pavlov originally distinguished between types of inhi- 

 bition, such as that exemplified by failure of a response due to frequent 



