ATTENTION, CONSCIOUSNESS, SLEEP AND WAKEFULNESS 



!5 6 9 



thalamus and cortex constitute others, and within 

 the cortex there are still others. Both specific and 

 nonspecific systems get duplicated and reduplicated 

 in the highest forms of evolutionary development of 

 the brain. Thus a sense mode may be represented by 

 three primary receiving areas and two or more asso- 

 ciation areas. The reticular formation, the region of 

 the centromedian nucleus and the older associational 

 cortices are areas in which one might expect con- 

 vergence of sensory influx with multiple sensory 

 representation, as Albe-Fessard et al. (9, 10) have 

 found recently. The intermixture and convergence of 

 the older and newer specific, nonspecific and asso- 

 ciational systems is undoubtedly responsible for some 

 of the difficulty and confusion which exists currently 

 in attempting to identify and separate the fields of 

 primary and secondary evoked responses as well as 

 other patterns of activity which exist on the cortex. 

 According to Bishop, in primates newer association 

 areas act as facilitators or modulators of older asso- 

 ciation areas which he refers to as thalamocortex. 

 It is in this thalamocortex that he believes the two 

 sensor) systems, specific and unspecific, converge. 

 Could it be here that a temporal coincidence and 

 convergence makes possible perceptual identification 

 and discrimination? If the phylogenetic concept of a 

 'reticular system' is one extending from cord to cor- 

 tex, and this integrating network is connected 

 throughout by multisynaptic junctions, but with seg- 

 mental links with some hierarchical ordering, then 

 throughout the neuraxis it becomes a dominating 

 influence for afferent and efferent regulation, as fore- 

 going sections of this chapter have indicated. But 

 wakefulness, attention and discrimination are prob- 

 ably some of the principal functions of major segments 

 of 'the reticular system,' as represented in the reticu- 

 lar formation of the lower brain stem, the unspecific 

 nuclei of the thalamus and the associational areas of 

 the cortex, respectively. 



The recent and important studies of Buser et al. 

 (41, 42) and of Albe-Fessard et al. (.9, 10) suggest the 

 existence of not only dual, but triple thalamocortical 

 systems, and convergences within the thalamus and 

 cortex of multiple sensory representations, just as 

 Bishop's concept would seem to require. Using chlora- 

 lose anesthesia or curarized preparations, Buser et al. 

 (41, 42) have found what they call secondary associa- 

 tive responses in two separate association zones in the 

 auditory and in visual fields in response to auditory 

 and visual stimuli. These responses are enhanced by 

 chloralose, but are blocked by barbiturates. This 

 would suggest that thev originate in or are a part of a 

 reticular system. They do not depend upon their 



corresponding primary projection areas which can 

 be removed or depressed by drugs without loss of 

 response in the well-defined cortical association areas. 

 Destruction of the reticular formation at a mesen- 

 cephalic level did not abolish these secondary re- 

 sponses which suggest that the mechanism is not a 

 part of the ARAS nonspecific system, but does not 

 rule out the DTPS. 



The secondary association responses are confined 

 to the association zones and can be distinguished from 

 primary responses by their longer latency and greater 

 duration. With respect to alerting and wakefulness, 

 it is especially interesting to note that the association 

 irradiation responses are best observed not during 

 marked activation or arousal, but during lower de- 

 grees of vigilance, with only slight activation or alert- 

 ness. Since these responses can be produced by stimu- 

 lating the specific thalamic nuclei (medial and lateral 

 geniculate bodies) for audition and vision, or from 

 the lateral-posterior nucleai group of association 

 nuclei, it has been proposed that thev depend upon 

 'collateral' elements which branch from the primary 

 sensory pathway and enter the thalamic association 

 nuclei. Stimulation of the primary nuclei for audition 

 and vision gives rise to both primary and associative 

 responses in the cortex. The appearance of these 

 responses in a certain level of wakefulness and alert- 

 ness implies thai they are related to the process of 

 attention, and 1n.1v have still further significance so 

 far .is perception is concerned. 



The observations of Albe-Fessard et al. (9, 10) 

 indicate that stimulation of different limbs of the cat 

 and monkey, irrespective of location, causes evoked 

 potentials in the centromedian nucleus and in parietal 

 and frontal association areas, both ipsilaterally and 

 contralateral^. Such nonprimary responses, elicited 

 by somatic stimulations of different origins, have not 

 previously been described. Their latencies and dura- 

 tions are greater than those of primary responses. 

 However, they are individualized with silent zones 

 between. The outstanding characteristic is the con- 

 vergence of stimulations upon a given cortical locus, 

 with all somatic areas having representation in this 

 convergence response. Stimulation of the centro- 

 median nucleus will cause responses in the superior 

 frontal gyrus, but with latencies too long to suggest 

 direct projections. 



MtCROELECTRODE studies. The response of individual 

 cortical units provides a more detailed evaluation of 

 specific-unspecific interaction. Jung et al. (137) have 

 discovered several types of units in the visual cortex. 

 Some show no reaction to light (A neurons); others 



