ATTENTION, CONSCIOUSNESS, SLEEP AND WAKEFULNESS 



. 5 6l 



HYPOTHALAMIC-CORTICAL DISCHARGE CONCEPT. The 



point of view of Gellhorn (87) concerning the im- 

 portance of the hypothalamus in relation to sleep 

 and wakefulness, consciousness and attention should 

 also be considered. He has argued on the basis of a 

 variety of experiments that "direct and reflex ex- 

 citation of the posterior hypothalamus is associated 

 with a diffuse excitation of the cerebral cortex. The 

 intensity of this hypothalamic-cortical discharge is 

 directly related to the excitability of the posterior 

 hypothalamus." He states further: '"The hypotha- 

 lamic-cortical discharge is associated with the state of 

 wakefulness. Conditions which interfere with this 

 discharge cause somnolence and coma." Finally 

 Gellhorn places emphasis upon the role of proprio- 

 ceptive and nociceptive influences, hypothalamic 

 imbalance, and corticofugal discharges in the main- 

 tenance of hypothalamic excitability. The excitability 

 and activation of the hypothalamic-cortical system, 

 maintained from external and internal sources, he 

 believes to be responsible for the maintenance ul 

 wakefulness, consciousness and the state of awareness 

 exhibited by perceptual discriminations. 



It will be recognized that Gellhorn places his main 

 emphasis upon the hypothalamus and particularly 

 the posterior hypothalamus for the regulation and 

 control of states which many have attributed to the 

 diffuse reticular system, including the ARAS and 

 DTPS. There is no real inconsistency here, however, 

 since the reticular substance extends into the posterior 

 hypothalamus and the upward efferent projections 

 from it extend to and beyond the hypothalamus (204). 

 It seems likely that many of the results reported l>\ 

 Gellhorn and his collaborators might well be ac- 

 counted for in terms of the ARAS, inclusive of parts 

 of the hypothalamus. However, the importance of 

 many of the autonomic factors considered by Gell- 

 horn should not be overlooked either from the point 

 of their direct influences upon the cortex or their 

 indirect homeostatic influences. 



Origins and Characteristic!! of the Diffuse 

 Thalamic Projection System 



Before beginning this topic it may be well to clarify 

 the terminology to be used. Because of the great 

 complexity of interrelationships among diencephalic 

 structures, and particularly those of the thalamus, 

 it will be necessary to deal with a simplified concept 

 of this organization. Thus, following the terminology 

 used by Jasper (130), we will speak of specific and 

 diffuse thalamic projection systems. The topically 



organized projections from classical sensory relay 

 nuclei upon somewhat delimited cortical receiving 

 areas will be called specific thalamic projection 

 systems (SPTS), while those shown to have a wide- 

 spread effect upon electrocortical activity, either in 

 terms of activation or recruiting responses, will be 

 referred to as diffuse thalamic projection systems 

 (DTPS). This distinction might be made on the basis 

 of neuroanatomical considerations as well as neuro- 

 physiological, but with many more qualifications and 

 much less simplicity. The dorsally placed association 

 nuclei will be included under the diffuse system, al- 

 though like the specific nuclei they tend to be more 

 topically organized and delimited in their cortical 

 influence. 



To use the label 'diffuse thalamic projection sys- 

 tem' rather than 'diffuse thalamocortical projection 

 system' takes cognizance of the fact, as pointed out 

 by Droogleever-Fortuyn (65) and Nauta & Whitlock 

 (183), that there is uncertainty that more than a few 

 of the nuclei lumped under the terms diffuse or non- 

 specific actually project directly to the neocortex. 

 Instead, there is indication that many of them project 

 cither to the caudate, striatum or rhinencephalon, 

 or form connections with the dorsal association nuclei, 

 nucleus ventralis anterior and the rostral pole of the 

 reticular nucleus. Among the so-called diffusely 

 projecting nuclei, the reticular complex, according to 

 Rose (201), shows degenerative changes following 

 neocortical ablations, but the temporal character of 

 these is such as to suggest that they may be secondary 

 to degeneration in the dorsal thalamic nuclei. Thus 

 the over-all picture in the thalamus is complex neuro- 

 anatomically, and neurophysiologically it is not as 

 easy to categorize the effects produced in the cortex 

 by stimulation as first supposed since there appears 

 to lie considerable overlapping and interaction of 

 specific and nonspecific systems. Further details of 

 the relationships of the DTPS or 'unspecific thalamo- 

 cortical projection system' are taken up by Jasper in 

 Chapter LI 1 1 in this Handbook. 



Ramon y Cajal (198) described two-way connec- 

 tions between thalamus and cortex, and saw in this 

 arrangement a possible means of control by the cortex 

 over the sensory influx via the thalamus, thus afford- 

 ing a possible mechanism of attention. Head & 

 Holmes (97), on the basis of clinical neuropathological 

 data, postulated an inhibitory control of the cortex 

 upon the thalamus which was believed to regulate 

 attention and affect. It was some time later, however, 

 before the method of strychnine neuronography, as 

 developed by Dusser de Barenne & McCulloch (67) 



