ATTENTION, CONSCIOUSNESS, SLEEP AND WAKEFULNESS 



'555 



electrical potentials recorded indirectly and diffusely 

 from the brain, or directly and focally in certain 

 regions of the brain. 



The electrophysiological changes in the brain, 

 when coupled with behavioral observations and 

 measurements and, in the case of humans, with sub- 

 jective reports, show remarkable correspondences 

 among these diverse criteria. The electroencephalo- 

 gram (EEG) has proved to be one of the more re- 

 liable and less disrupting methods of studying the 

 transition from wakefulness to sleep, and the reverse. 

 It reveals several distinct stages in the process of going 

 from wakefulness to deep sleep. It permits continu- 

 ous monitoring of sleep without disturbance to the 

 sleeper, as may be the case with test stimuli applied 

 to determine the threshold for behavioral or subjec- 

 tive response. Insofar as they are applicable, the 

 criteria of depth of sleep revealed by the latter meth- 

 ods correspond well with those of the EEG. Further- 

 more by combined use of EEG, behavioral and sub- 

 jective methods, it has become possible to view the 

 differential and gradational stages of attention and 

 consciousness upon the same continuum with those 

 of wakefulness and sleep (see fig. i i and table i I. 



The neural mechanisms which underlie these 

 changes are gradually being revealed, and with this 

 increased understanding have come new concepts of 

 the functional organization of the brain. Awake or 

 asleep, the characteristic rhythms and patterns of 

 electrical activity in the brain appear to regulate not 

 only the discharge patterns of individual neurons and 

 of the aggregates of which they are a part, but of 

 other neurons and aggregates more widely dispersed. 

 Whether such interactions within the brain are ac- 

 complished by direct connections, by resonance ef- 

 fects, by Held effects or by other means yet unknown 

 remains to be determined. Regardless of how the 

 interactions take place, it is known that certain dif- 

 ferential electrical patterns exist at any moment in 

 widely dispersed regions of the brain, and that a 

 certain amount of correlation has already been 

 demonstrated between these electrophysiological pat- 

 terns and the behavioral and subjective indices of 

 attention, consciousness, wakefulness and sleep. 



NELIROPHYSIOLOOICAL MECHANISMS 



During the past 30 years electroencephalographic 

 and neurophysiological studies have provided evi- 

 dence that attention, consciousness and sleep depend 

 upon a common neurophysiological mechanism. In 



broad functional outline this mechanism has been 

 described as the ascending reticular activating system 

 (ARAS), with origins in the reticular formation of 

 the lower brain stem and with upward extensions 

 including parts of the hypothalamus, subthalamus 

 and thalamus. (This system is the subject of Chapter 

 LII of this Handbook.) Integral with, or closely related 

 to, the ARAS is the diffuse thalamocortical projection 

 system (DTPS), with origins in the nonspecific nuclei 

 of the thalamus (discussed in chapter LIII). These 

 neuroanatomically and functionally related neuronal 

 systems of the lower brain stem and diencephalon 

 provide not only a basis for understanding sleep and 

 wakefulness, but also make possible some meaningful 

 correlations between neurophvsiological, behavioral 

 and psychological events which help to define the 

 limits of the various states or gradations of attention 

 and consciousness. The manner in which neuro- 

 anatomical, neurophysiological, electroencephalo- 

 graphic, behavioral and psychological data conjoin 

 10 support the above statements will be presented in 

 subsequent seetions of this chapter. 



It is difficult to trace the origin of the ideas which 

 have led lo .1 new concept or theory, or to decide at 

 what critical juncture the accumulated experimental 

 evidence established confirmation of the theory. 

 However, with respect to the functional concept of 

 the ARAS and the role it plavs in sleep and wakeful- 

 ness, as well as in clcctrocortic.il activation and be- 

 havioral alerting, there is little doubt that Moruzzi & 

 M.inoun (1K1), and subsequently Magoun and his 

 collaborators (78-80, 163, 164, 172), clearly and 

 firmly laid the groundwork lor an important new 

 concept of brain organization in relation to behavior. 



Similarly, following upon the pioneering work of 

 Morison & Dempsey (59, 60, 179, 180) in which 

 they described the verv significant 'recruiting' and 

 'augmenting' responses elicited in the cortex by 

 stimulation of certain regions ot the thalamus, Jasper 

 and collaborators (95, 96, 126, 130-132) in a series 

 of experiments have outlined some of the functional 

 relationships of the diffuse thalamocortical projec- 

 tion system (DTPS). Jasper (127, 128) has further 

 proposed some important functions which this system, 

 in conjunction with the ARAS, may play in relation 

 to consciousness and attention. Tissot & Monnier 

 (225) and Monnier el al. (176) have recently pro- 

 vided additional information of importance to the 

 understanding of the roles of the ARAS and DTPS. 



The pictures of both the ARAS and the DTPS have 

 been painted in broad and bold strokes, but the out- 

 lines of the figures are unmistakable. These concepts 



