ATTENTION, CONSCIOUSNESS, SLEEP AND WAKEFULNESS 



•557 



sleep is due to anabolic depression of sympathetic 

 activity on the one hand and to reduction of psychic 

 activation on the other. The modern neurophysio- 

 logical conception recognizes both a peripheral sen- 

 sory and a central influence upon the activation of 

 the ARAS. Several other views emphasize that in 

 the physiological accompaniments of sleep there is a 

 predominance of parasympathetic influence, in con- 

 trast to sympathetic dominance during wakefulness. 

 Marinesco et al. (174) believe on the other hand that 

 parasympathicotonia is a cause and not a result of 

 sleep, and that endocrine secretions act as sensitizers 

 of vegetative centers. Their view holds that sleep is 

 a conditioned process and that the circumstances 

 favoring sleep (reduction of stimulation) constitute a 

 conditioning situation which gives rise to the un- 

 conditioned humoral and vegetative responses. 



Tromner (227) and Spiegel (219) have advocated 

 a sleep-regulating center in the thalamus. Spiegel 

 proposed a primitive center of consciousness in the 

 thalamus, and believes that inhibitory and excitatory 

 interaction between thalamus and cortex may in- 

 fluence consciousness as well as sleep and wakefulness. 



Pavlov (189, 190) has described a variety of condi- 

 tions under which local, and subsequently generalized, 

 inhibition of the cortex develops and leads to sleep 

 Among these are conditions of extinction of a con- 

 ditioned response in which the conditioned stimulus 

 is repeatedly presented without adequate reinforce- 

 ment, or in which delayed reinforcement i> unduly 

 prolonged. Some of these conditions of stimulation 

 are reminiscent of the modern concept of 'habituation 1 

 described by Hernandez-Peon and collaborators (105, 

 106) but in which the explanation of the effects oi 

 repeated stimulation seems to reside in the brain- 

 stem reticular formation rather than the cortex as 

 Pavlov maintained. There remains much to be learned 

 about the conditioning phenomena described by 

 Pavlov and the 'habituation' effects studied by 

 Hernandez-Peon and others, particularly in terms of 

 their neurophysiological basis. 



Hess (ill, 112) has described the experimental 

 production of sleep in cats by electrical stimulation in 

 the posterior hypothalamus and along the walls of 

 the third ventricle. These effects were produced by 

 d.c. stimulation which may have produced polariza- 

 tion effects capable of blocking transmission in an 

 area now known to lie part of the ARAS and im- 

 portant to the maintenance of arousal and wakeful- 

 ness. Ranson et al. (199), stimulating with faradic 

 current in the same regions of the hypothalamus, were 

 not able to produce sleep. On the contrary, as would 

 now be expected, thev produced activity and excite- 



ment, apparently due to excitation of portions of the 

 ARAS traversing the posterior hypothalamic region. 



The more recent work of W. R. Hess (113—117) 

 and of his son, R. Hess, and collaborators (109, no) 

 has been summarized in Brain Mechanisms and Con- 

 sciousness (118). They now distinguish two systems 

 represented in different areas or fields which give 

 quite different results upon electrical stimulation. 

 The ergotrophic or 'dynamogenic' field is located in 

 the posterior and mesial part of the hypothalamus, 

 and extends into the mesencephalon. Stimulation in 

 this region, which overlaps with the ARAS, is said 

 to produce excitement and arousal which Hess de- 

 scribes as mobilization and preparation for defense. 

 Others from the time of Karplus & Kreidl ( 1 39) have, 

 of course, found this general region favorable to the 

 production of sympathetic nervous system effects 

 associated with arousal and mobilization (18, 138, 

 200). In contrast to this field of general excitability 

 and alertness, Hess describes another which he calls 

 the trophotropic field or 'hypnogenic' center. This 

 region of the diencephalon lies lateral to the massa 

 intermedia, and extends eaudally to the habenulo- 

 peduncular tract and rostrally to the mammillotha- 

 lamic bundle. Medially il is 1.5 to 2 mm from the 

 mid-line. Its lateral boundaries have not been de- 

 termined due to technical difficulties. Stimulation of 

 (hi-, trophotropic region with slow Iv rising d.c. pulses 

 of 1 to a v and a frequcy of 4 to 12 per sec. causes a 

 generalized depression of activity and sleep, capable of 

 reversal by stronger or higher frequency shocks. The 

 electrocortical activity and behavior induced by 

 low-frequency stimulation in this zone is said to be 

 indistinguishable from natural sleep. As its name 

 implies, its significance is believed to be the preserva- 

 tion of energetic resources, and the reparation and 

 protection of tissues from overstrain. It is thought to 

 function as an antagonist to the arousal system. 



The trophotropic system of Hess overlaps with the 

 region in which Morison & Dempsey (59, 179) and 

 others (222, 223) have consistently produced 're- 

 cruiting responses' in the cortex at similar frequencies 

 of stimulation, but from which higher frequency of 

 stimulation (50 to 300 per sec.) produces instead 

 cortical desynchrony or activation and behavioral 

 arousal. The fact that both the ergotropic and tropho- 

 tropic systems overlap to some extent with the ARAS, 

 and also the DTPS, would easilv account for the 

 activation and arousal responses with high-frequency 

 stimulation. The results of low-frequency stimulation 

 in the trophotropic area are less easily accounted for. 

 They undoubtedly produce recruiting responses simi- 

 lar to those of Morison & Dempsey with corre- 



