246 BRAIN MECHANISMS AND LEARNING 



recent observations concerning this problem made in Li-)ng Beach in 

 collaboration with Drs Lesse and Eidelberg. 



The first question arising here, too, is whether the startle response is an 

 unconditioned or a conditioned reaction, and also what is the relationship 

 between the startle response and the orientation reflex. I think that the 

 solution of this problem is beyond the scope of a simple terminologic 

 question. Thus, if it becomes apparent that the organization of these two 

 reactions is not an inborn property of the nervous system, then we are 

 facing the two simplest and most basic forms of the learning process, the 

 understanding of which must be an indispensable condition for the 

 approach to more complex conditioned patterns. 



Prosser and Hunter in 1936 and later Landis and Hunt (1939) established 

 that the startle response could easily be conditioned. According to our 

 own observations in a well-isolated, sound-proof experimental situation, 

 a single reinforcement of a click stimulus (which in itself does not evoke 

 the startle response) results in a long-lasting and stable conditioned 

 startle response. The extremely fast development of the conditioned startle 

 response makes it highly probable that a great majority of the startle 

 responses elicited by moderate stimuli through the telereceptors represent 

 conditioned phenomena. The question arises, however, whether they 

 really represent true conditioned reflexes or only sensitization phenomena, 

 i.e. a pseudoconditioning. It is known that repetition of a strong stimulus 

 eliciting startle response in itseli might increase the general excitability 

 for every kind of stimuli without any pairing of them (Grether, 1938). 

 In addition it is an everyday experience that in hyperexcited states every 

 stimulus may elicit startle responses. The arguments of Larssen (1956), 

 however, seem to contradict this supposition. 



He has shown that the unconditioned stimulus could not bring about an 

 increase of the response to the conditioned stimulus unless the two stimuli 

 were paired with a definite interval. Moreover, the conditioned reactions 

 could be inhibited differentially. On the basis of these observations he 

 rejects the explanation of sensitization for his own experiments. Our own 

 observations also throw doubt on the hypothesis of simple sensitization; 

 because no electrical or behavioural signs of increased excitation could be 

 observed before the critical second application of the conditioned stimulus. 

 The fact that many stimuli, similar to the one reinforced, also elicit the 

 startle response is a consequence rather of generalization and so does not 

 exclude the true conditioned nature of these reactions. 



For the correct estimation of the general excitatory level of the CNS or, 

 better, the excitatorv level of the unconditioned mechanisms and drives, 



