E. GRASTYAN 249 



ascending reticular conduction. This nihibition seems to be mediated 

 chiefly through the cntorhynal cortex and stria medullaris system (Adey, 

 Merrillees and Sunderland, 1956; Adey, Sunderland and Dunlop, 1957; 

 Adey, 1957). Similar inhibitory influence has been shown by our labora- 

 tory (Grastyan ct al., 1956, Lissak, ct al. 1957). Unfortunately no direct 

 observations have been made in these experiments on the startle response. 

 As we shall see later the inhibitory role of the hippocampus has to be 

 considered in the inhibition of the orientation reflex too. 



A further inhibitory mechanism which might be responsible for the 

 inhibition of the startle response is offered by the observations of Her- 

 nandez-Peon (195s) and Hernandez-Peon, Scherrer and Jouvet (1956). 

 This inhibition seems to control the effect of incoming impulses in the 

 first sensory relays. At present, however, we are still uncertain about the 

 exact structural origin of this inhibition. Some evidence has been furnished 

 that it originates in the reticular formation. 



An important inhibitory mechanism oi diffuse neocortical origin, 

 controlling reticular influences on peripheral reflex function, has recently 

 been discovered by Hugelin and Bonvallct (1957). This inhibitory mechan- 

 ism has to be considered first as responsible for the brief, phasic character 

 of the startle response. However, its possible role in the extinction of the 

 startle response as well as its relation to the archicortical inhibitory mech- 

 anisms mentioned above, remains a matter of speculation at the present 

 time. 



A definite difference between the mechanisms of the startle response and 

 the orientati(Mi reflex is reflected by the accompanying electrical manifesta- 

 tions of the two responses. These differences are particularly characteristic 

 in the hippocampus. As I already mentioned, the appearance of the 

 startle response is accompanied in the hippocampus by a desynchroniza- 

 tion. At the appearance ot the orientation reflex a marked change occurs; 

 the characteristic thcta rhythm replaces the former desynchronization. In 

 the course of regular reiniorcement of the conditioned stimulus, the 

 latency of the starting of these theta potentials gradually shortens, and 

 finally the stimulus elicits theni instantaneously. 



The hippocampal evoked potential undergoes a similar marked change 

 during the develc>pment oi the orientation reflex. At the time ot the 

 manifestation of the startle response, the changes of the amplitude of 

 the evoked potential seem to be rather inconsistent; at the appearance of the 

 slow waves, however, in the great majority of cases a significant decrease 

 in amplitude or even disappearance of the potentials can be observed. 



These marked electrical changes would suggest that the hippocampus 



