470 BRAIN MECHANISMS AND LF.ARNINf; 



V — Dcs hypotheses sont cniin cniiscs pour cxpliqucr Ic mode d'action 

 du systcmc mhibitcur tclcnccphahquc au niveau du S.R.A.A., ct le role 

 probable des dendrites de I'ccorcc est cvoque a ce sujet. 



Research on the Neurophysiological Mechanisms of Sleep and on 

 Some Types of Negative Learning 



SUMMARY 



Negative learning (habituation, extinction ot the orientating reflex) is 

 one of the simplest types of learning since it involves usually excitation 

 from only one receptor. It may be defined, after Thorpe, as an activity of 

 the central nervous system whereby innate responses to certain stimuli 

 w^aiic as the stimuli are repeated for long periods without unfavourable 

 result. It has been shown that habituation cannot be explained by fatigue 

 or 'adaptation' at the receptor level and, since Pavlov, the relationship 

 between habituation, extinction, 'internal inhibition' and sleep are well 

 known. 



Since the problem oi filling asleep is intimately related with negative 

 learning, we shall consider first some experimental facts which suggest 

 that two different mechanisms at least act during sleep. Then, we shall 

 describe some evidence concerning the intervention of one of these 

 mechanisms during two types of negative learning: the habituation of the 

 arousal reaction and the habituation of the orientation reaction. 



I. observations postulating the existence of two inhibitory systems, 



TELENCEPHALIC AND RHOMBENCEPHALIC, ACTING DURING PHYSIOLOGICAL 



SLEEP 



Since sleep involves the total behaviour ot the organism, its mechanisnr 

 has to be studied in relation to environment. Thus, in brain transsection 

 techniques, we must mainly analyse that part of the nervous system which 

 is in relation with the external world through its main afferent and efferent 

 pathways. 



It is well known that a neodecorticated animal can sleep. A sleep-waking 

 rhythm has also been shown to occur in chronic mesencephalic cats, but 

 there is no such pcrit>dicity at the isolated spinal level. 



We recorded the electrical correlates of such sleep-waking rhythm in 

 three types of chronic preparations: normal, neodecorticated and mesen- 

 cephalic cats bearing chronicallv implanted multipolar electrodes in various 

 cortical and subcortical locations. 



