474 BRAIN MECHANISMS AND LEARNING 



(i) During the habituation of the orientating reflex in intact cats, a 

 marked enhancement of the secondary negative and positive phase of the 

 cortical (Figs. 13-15) evoked potentials of the indifferent stimuli was 

 observed, and a similar enhancement of a secondary wave could be 

 recorded at the reticular level (Fig. 15). This phenomenon precedes the 

 appearance of induced bursts of spindles and slow waves at the cortical, 

 diencephalic and fmally at the reticular level, and it leads to the appearance 

 of stage I of sleep (Figs. 13-14). 



(2) Habituation of the orientating reflex did not occur, or at least was 

 irregular, in neodecorticated cats. Repetition of stimuli did not induce 

 changes of the subcortical evoked potentials and did not induce sleep. 



(3) Repetition of indifferent stimuli failed to induce the 'archeo-sleep' 

 in a mesencephalic cat. 



(4) It is suggested that, during habituation of the orientating reflex, 

 indifferent stimuli may trigger the inhibitory telencephalic system which 

 acts upon the reticular activating system. 



CONCLUSION 



(i) There exists a rostral mhibitory system (in which the neocortex 

 plays a foremost role) which acts upon the reticular activating system 

 during the first stage of sleep. Its electrical manifestation is the appearance 

 of spindles and slow waves first at the corticodiencephalic and later at the 

 mesencephalic levels. This system is plastic and can be triggered by 

 repetition of indifferent stimuli during habituation of the arousal and of 

 the orientating reflex. 



It is suggested that the late component of the cortical evoked response 

 and the triggered slow waves which are recorded during habituation 

 (and which may represent a cortical dendritic activity) are the electrical 

 manifestation of the corticofugal inhibitory influence. 



(2) There exists also a caudal inhibitory system (in which the reticular 

 formation of the pons plays a major role) which is responsible for the 

 second stage of sleep (paradoxical stage). This system acts caudally on the 

 motor outflow, inhibiting the muscular tone and triggering an 'arousal- 

 like' activity in the structures rostral to the pons. When this system is 

 active, a 'spindling' activity is recorded at the pontine reticular level in 

 intact, decorticated and mesencephalic cats (in which this activity may 

 represent the entry in action of some archeo-sleep mechanism). This 

 activity cannot be induced by repetition of stimuli and there is no evidence 



