ATTENTION, CONSCIOUSNESS, SLEEP AND WAKEFULNESS 



•567 



the pattern from one characteristic of sleep with 

 large slow waves and spindle bursts to one of low- 

 amplitude fast activity or, in the case of wakefulness, 

 further differentiating the pattern by desynchroniza- 

 tion and activation. If its activity is reduced by anes- 

 thesia, or if its upward extensions are cut off from the 

 lower brain-stem portion of the reticular formation, a 

 state of somnolence and unconsciousness ensues, 

 despite the fact that sensory messages may still traverse 

 the classical pathways to the thalamus and cortex. 

 Even under light to moderate barbiturate anesthesia 

 such messages are ineffective so far as perceptual 

 discriminations are concerned without the influence 

 of the ARAS. 



The routes of the upward extensions of the ARAS 

 are as yet uncertain, but it is believed that one takes 

 an extrathalamic course, possibly by way of the sub- 

 thalamus and internal capsule, and that another 

 passes by way of the thalamus, presumably terminat- 

 ing in the mid-line and intrathalamic nuclei, in the 

 reticular nucleus or in both. Whatever its influences 

 may be in the thalamus, upon DTPS or SIPS, its 

 ultimate effects upon the cortex arc not to be denied. 

 Its extrathalamic influences are believed to be rela- 

 tively rapid ones and concerned with general arousal, 

 its thalamic influences are more likely concerned 

 with gradations of alerting to attention and may be 

 related to scanning or modulating influences atlei 1 

 ing the STPS, or the integration of STPS information 

 by the DTPS. 



The functions and relations of the DTPS are siill 

 less clear than those of the ARAS. Some of the nuclei 

 composing this system apparently serve intrathalamic 

 association functions, others interact with the cortex, 

 basal ganglia and rhinencephalic structures. Those in 

 direct or indirect connection with the neocortex ap- 

 pear to be capable of electrocortical activation or 

 desynchronization, after the manner of ARAS func- 

 tion. Another function appears to be the regulation 

 of so-called spontaneous cortical rhythms, control of 

 after-discharge and after-potentials following STPS 

 action, and the regulation of temporal synchroniza- 

 tion between thalamus and cortex in a fashion per- 

 mitting the development of cortical recruiting waves, 

 sleep spindles and other rhythmic phenomena of the 

 cortex, perhaps including the alpha rhythm of the 

 normal resting EEC As such it may control waxing 

 and waning cycles of excitability in the cortex which 

 could regulate rapid shifts of attention (159). Evi- 

 dence of such excitability cycles has been put forward 

 by Bishop (24), Bartley & Bishop (17), Chang (44-46), 

 Lansing (153) and others. 



INHIBITION AND FACILITATION VTA THE RETICULAR 



formation. Cortical or reticular stimulation has been 

 shown in several experiments to be capable of pro- 

 ducing inhibition (63, 84, 89, 92, 94, 100, 141) or 

 facilitation (36, 66, 161) in one or another of the 

 several sensory systems. Inhibition has been demon- 

 strated from the first synaptic relay to the final relay 

 in the thalamus. To take one example, Hernandez- 

 Peon et al. (107I have demonstrated that stimulation 

 of the mesencephalic reticular formation depressed 

 or abolished the secondary wave of the evoked poten- 

 tial in the nucleus gracilis induced by stimulation of 

 the dorsal column. They also demonstrated that the 

 second component of the evoked potential in the 

 trigeminal nucleus, elicited by stimulation of the 

 infraorbital nerve, could be enhanced by destruc- 

 tion of the midbrain tegmentum, thus demonstrating 

 release of tonic inhibitor) influence from the reticular 

 formation. Finally, and important in relation to other 

 recent experiments dealing with the visual system, 

 they show that photicallv evoked potentials in the 

 optic tract, lateral geniculate bod) and visual cortex 

 could be significantly modified by reticular stimula- 

 tion. In both the lateral geniculate bod) and in the 

 visual cortex the second component of the evoked 

 complex was depressed during and following brief 

 reticular formation stimulation. The optic tract 

 showed both depression and potentiation, believed to 

 result from two antagonistic effects of centrifugal dis- 

 charges upon retinal synapses. 



In contrast to the above results, recent studies 

 |6, ;;, 66, Mii I have shown facilitatory effects in the 

 visual cortex and in the lateral geniculate bodies as 

 a result of reticular stimulation concurrently with 

 excitation of the visual pathways bv photic or optic 

 nerve stimulation. Improved resolution or temporal 

 facilitation in the visual cortex of the cat has been 

 reported by Linclslev (161 1 following reticular stimu- 

 lation. Figure 7 shows a single evoked potential in the 

 visual cortex (VC) to a pair of light flashes 50 msec, 

 apart, but alter reticular formation stimulation the 

 same pair of flashes resulted in two evoked potentials. 

 The dual response continued for about 10 sec. and 

 then the response became single again. Another kind 

 of facilitation has been described by Bremer & 

 Stoupel (36, 37) and by Dumont & Dell (66). These 

 investigators report increased magnitude of evoked 

 potentials in the lateral geniculate bodies and visual 

 cortex to optic nerve stimulation which followed 

 stimulation of the mesencephalic reticular formation. 

 The former also observed enhancement of cortical 



