CONDITIONING 81 



<£<, < h' < </> + e , (1) 



</>,(, < h" < </>. + </>. 0tt > (2) 



</>o« < ^ , (3) 



where £ is the value of the excitation at s c due to the circuit when in 

 steady-state activity, and where the </>'s refer to the afferent neurons. 



If the unconditioned stimulus S u exceeds the threshold of /„ suf- 

 ficiently, the response R may be elicited. But, as we assume that the 

 circuit C is not active initially, the stimulus S c cannot produce the 

 response R because 4> < h' . Furthermore, neither S u nor S c can 

 bring C into activity when there is too long a time between their oc- 

 currence. Thus S u alone can produce R but S c cannot. Now suppose 

 that S u and S c are applied together for a sufficiently long time. Though 

 simultaneous presentation is not a necessary condition, it will be con- 

 sidered here to simplify matters. Because of condition (2) , the thresh- 

 old of the circuit will be exceeded and the circuit will pass over into 

 a state of steady activity. If, now, a large enough S c is applied alone 

 for a long enough time, the threshold h' will be exceeded because of 

 condition (1). Thus the response R may now be produced by the 

 hitherto inadequate stimulus S c alone, and the structure exhibits one 

 of the principal features of conditioning. 



If we add now the inhibitory neurons III' and III, the resulting 

 structure will exhibit another feature important in the phenomenon 

 of conditioning. Whenever S u is applied, the effect of neuron III is 

 blocked by III'. But if S u is not applied, then the continuous or re- 

 peated application of S c may cause III to produce enough inhibition 

 at s' to block the action of S c if conditioning has previously taken 

 place. This corresponds to the loss in effectiveness of the conditioned 

 stimulus which occurs when it is applied repeatedly without rein- 

 forcement by the unconditioned stimulus. 



If, instead of a single circuit C , we assume that there are a num- 

 ber of them having different thresholds, we should be able to show 

 that a more intense S u and S c would tend to produce a more intense 

 response. Furthermore, by considering repeated applications of S u 

 and S , it is possible to determine the effect of the number of repeti- 

 tions on the conditioning. By combining these extensions of the struc- 

 ture, N. Rashevsky (1938, chap, xxv, equation 44) obtains an expres- 

 sion 



e B = A( i l-e°») (4) 



for e R , the excitation tending to produce the response R when S is ap- 

 plied as a function of the number, n , of repetitions. The constant A in- 

 creases with the intensity of the conditioned stimulus, while the con- 



