ATTENTION, CONSCIOUSNESS, SLEEP AND WAKEFULNESS 



'559 



fig. I. Ascending reticular activating system (ARAS) 

 schematically projected on monkey brain. The reticular forma- 

 tion, consisting of the multineuronal, multisynaptic central 

 core of the region from medulla to hypothalamus, receives 

 collaterals from specific or classical sensory pathways and 

 projects diffusely upon the cortex Impulses via specific sensory 

 pathways are brief, discrete, direct and of short latency in 

 contrast to those via the unspecitic ARAS which are persistent, 

 diffuse and of long latency. 



seemed to imply primarily the elimination of trans- 

 mission in the classical sensory pathways. Later it 

 was shown by Lindsley el al. ( 163, 1641, in both acute 

 and chronic preparations, that the lemniscal path- 

 ways would not suffice to maintain the brain and 

 behavior in a waking state when the midbrain teg- 

 mentum was transected. Under the reverse condition, 

 with the lemniscal pathways severed and the reticular 

 formation intact, the EEG and behavior were like 

 those in a normal waking animal (see fig. 4). This 

 was subsequently confirmed in the monkey by French 

 & Magoun (78). 



One further development should be cited. Al- 

 though Haenel (93) and Taussig (224) were the first 

 to propose that sleep is essentially a passive state and 

 that wakefulness and consciousness depend upon af- 

 ferent impulses, Kleitman & Camille (145, 14b! 

 were the first to develop, on the basis of experimental 

 evidence, the notion that the waking state depends 

 upon afferent impulses. Kleitman (145) proposed 

 what he refers to as an evolutionary theory of wake- 

 fulness which corresponds closely to the neurophysio- 

 logical results upon which the concept of the ARAS 

 is based. Kleitman held that wakefulness is an active 

 process supported by afferent influx from visceral 

 and somatic sources which keeps the brain awake. 

 This he called 'wakefulness of necessity." Developing 

 tensions through the course of a night's sleep, as well 



as intrusions of daytime stimuli, would inevitably 

 awaken the sleeper through impulses imposed upon 

 a waking center which Kleitman presumed to be in 

 the hypothalamus. His second concept was that there 

 is also a 'wakefulness of choice.' This concept left 

 room for habit, learning, conditioning, thought and 

 the like to influence from higher centers the waking 

 center in the diencephalon. Such an arrangement 

 seems now to be entirely possible, since the ARAS has 

 been shown to be activated not only by sensory 

 influx but by corticifugal connections to the reticular 

 formation. 



Thus we see that several developments combined 

 to set the stage for the crucial experiment of Moruzzi 

 & Magoun (181). Knowledge of the EEG provided 

 by Berger (19) and the utilization of it by Bremer (33) 

 in connection with his cerveau isole preparation sug- 

 gested important implications for the lower brain 

 stem with respect to sleep and wakefulness. The ex- 

 tensive search throughout the diencephalon and brain 

 stem l>\ Ranson, Hess and others for autonomic and 

 sleep and wakefulness centers was beginning to focus 

 attention upon new mechanisms. The insightful 

 observations of Ranson & Magoun (200) concerning 

 the descending and ascending influences resulting 

 from stimulation in the brain stem and hypothalamus 

 were also contributory to new ideas. Finally, Magoun 

 and collaborators (165, 173, 209, 221) had studied 

 the descending effects of stimulation and lesions in the 

 brain-stem reticular formation upon spinal reflexes 

 and motor behavior, and had been able to dem- 

 onstrate a rostral excitatory area and a caudal 

 inhibitor) region. With the importance of the reticu- 

 lar formation demonstrated for downstream effects 

 upon spinal reflexes and muscular contractions, there 

 remained the exploration of possible ascending in- 

 fluences suggested by Ranson & Magoun (200). It 

 was precisely these that Moruzzi & Magoun were 

 able to demonstrate. 



Figure 1 illustrates schematically the ascending 

 reticular activating system (ARAS) projected upon 

 the monkey brain. Shown here is the pathway of a 

 somesthetic afferent, relaying in the thalamus and 

 proceeding to its destination in the sensory cortex. 

 This represents the specific, primary or classical 

 sensorv system. The unspecific or secondary sensorv 

 system is represented by the ARAS with origins in 

 the reticular formation of the lower brain stem. This 

 is shown by the darker arrow in the central core of 

 the brain stem with multisynaptic relays schematized. 

 The upward extensions of the ARAS in a diffuse 



