BRAIN MECHANISMS AND LEARNING 



neural mechanisms. The startle response is characterized essentially by a 

 tendency to generalized flexion, and whereas the orientation reflex is not 

 exactly the contrary of this, its more complex nature indicates a basically 

 different biological character. The essential characteristic of the startle 

 response is protective, it has a tendency to exclude influences coming from 

 the environment, but in the orientation reflex on the contrary, the readi- 

 ness to accept the influence is the most typical. 



From these absolutely contrasting features it is evident that the two 

 phenomena cannot be regarded simply as quantitatively different repre- 

 sentatives of an essentially identical mechanism. Supporting evidence is 

 offered for this assumption in the older literature, according to which 

 the intensity of the startle response is not in a causal relationship with the 

 appearance of the orientation reflex. (Secondary manifestations of the 

 startle (Strauss, 1938.)) During the continuous reinforcement of the con- 

 ditioned stimulus the startle response progressively decreases, or disappears. 

 It could not be excluded that it is even this inhibitory process which makes 

 possible the appearance of the orientation reflex. 



According to earlier observations (Forbes and Sherrington, 1914) typical 

 startle responses could be observed on decerebrated animals. This observa- 

 tion clearly shows that the basic mechanism of the startle response is 

 located somewhere in the lower brain stem. It docs not mean, however, 

 that in an intact animal higher processes do not play a role in the integra- 

 tion of the startle response. This is supported by the observation of 

 Gastaut and Hunter (1950) that startle responses ehcited by flicker are 

 modified by the removal of the occipital cortex, further by the observa- 

 tions of Haas ct al. (1953) who established that the motor cortex plays a 

 similarly important role in the integration of experimental myoclonus. 



Considering that every kind of stimuli can be used for the conditioning 

 of the startle response, its organization must occur in a structure which has 

 the capacity to integrate these stimuli. On the basis of this requirement the 

 reticular formation offers itself as a suitable region in the brain stem 

 (Moruzzi and Magoun, 1949; Starzl, Taylor and Magoun, 195 1; French, 

 Amerongen and Magoun, 1952). This supposition finds strong support in 

 the earlier observations of Muskens (1926) that lesions of the medial 

 reticular formation alone result in a permanent disappearance of the startle 

 response. 



The origin and nature of the inhibition of the startle response represent 

 a more difficult problem than its primary integration. Adey, Segundo 

 and Livingston (1957) established the important fact that descending 

 hippocampo-cortical impulses exert a profound inliibitory influence on 



