250 BRAIN MECHANISMS AND LEARNING 



played an active role in the integration of the orientation reflex. Un- 

 fortunately this cannot be stated with certainty because the functional 

 interpretation of the otherwise significant electrical happenings is still 

 extremely uncertain. The only thing which can be established with 

 certainty is that something important happens in the hippocampus at the 

 time of appearance of the orientation reflex. Let us consider what sugges- 

 tions are offered by the stimulation experiments. 



An orientation reflex most similar to the natural one can be elicited in 

 the freely moving animal by stimulation of the hypothalamus and reticular 

 formation (Grastyan, Lissak and Kckcsi, 1957). The same stimulation is 

 one of the most effective in eliciting the theta rhythm from the hippocam- 

 pus (Green and Arduini, 1945). Stimulation of the hippocampus, on the 

 other hand, effectively inhibits the orientation reflex and conditioned 

 reflexes (Grastyan ct al, 1957). This stimulation is accompanied in the 

 contralateral hippocampus by a clear-cut desynchronization. From these 

 observations it would follow that during the appearance of the rhythmic 

 slow waves the function of the hippocampus is inhibited. These observa- 

 tions, together with the fact that the orientation reflex disappears at the 

 stabilization of the conditioned reflex and at the same time a desyn- 

 chronization (presumably representing a more active state) takes place in 

 the hippocampus, would suggest that the hippocampus inhibits the 

 orientation reflex. Since the inhibition of the orientation reflex is a 

 necessary condition for the final stabilization of the conditioned reflex, 

 this inhibitory function would provide the hippocampus with a role in 

 proportion to its morphological importance. 



This supposition is, however, valid only if the interpretation of the 

 electrical events and the behavioural effects elicited by stimulation of the 

 hippocampus are correct. 



An inhibitory function oi the hippocampus is suggested by some interes- 

 ting observations of John and Killam. These authors observed that during 

 the presentation of a negative conditioned stimulus (repetitive flicker) 

 definite following can be observed with a predominance in the hippo- 

 campus ; while during the presentation of the positive stimulus (which is 

 of the same modality as the negative one, differing only in frequency) no 

 following can be recorded in the hippocampus. 



Our own observations concerning the disappearance or decrease of the 

 hippocampal evoked potentials during the presence of the slow waves 

 seem to offer adciitional supporting evidence. The decrease in the evoked 

 potential might be an expression of the inhibited state of the elements 

 responsible for the evoked activity. 



