I5 68 



I1WDBOOK OF PHYSIOLOGY 



M 1 ROI'HYSIOLOGY III 



AFTER 

 IO- 12' 



fig. 7. Limitation of the cortex in a cat in responding to 

 closely spaced brief light flashes, but facilitated by reticular 

 stimulation. Two 20-fisec. Hashes of light separated by 50 msec, 

 and presented once per sec. are responded to as one by the 

 visual cortex 1 VC) until after a 5-sec. period of reticular forma- 

 tion stimulation. For 10 sec. thereafter two evoked potentials 

 appear, and then return to the original single response. This 

 is an example of temporal facilitation. OT, optic tract, LG, 

 lateral geniculate body; VC, visual cortex. [From Lindsley 

 (it,,, 162). 



potentials to optic nerve stimulation when the centro- 

 mcdian nucleus of the DTPS was stimulated. 



Thus we see that ARAS and DTPS interaction 

 with STPS, or STPS cortical effects, is capable of 

 facilitation, hut also in some instances inhibition. 

 Similarly, we have noted that centrifugal discharges, 

 due to cortical or reticular stimulation, produce in- 

 hibition in peripheral sensor) relays and in a few 

 instances give rise to potentiation of specific responses. 

 What is the role of these mechanisms? Do they pro- 

 vide a means of selective control of sensory input 

 such as might seem to be required for restricting 

 attention? 



( ortical Interaction of Specific ami Unspeciflt Influences 



The role of the specific I STPS) and unspecific 

 (ARAS and DTPS) sensorv systems is by no means 

 clear at the present time, and yet it is certain that 

 the) must interact if incoming sensor) messages are 

 to be decoded and integrated with past experience 

 in meaningful ways. One important indication of the 

 necessit) for this is the fact that perceptual discrimina- 

 tion will not occur in the absence of ARAS influence, 

 when blocked b) lesion or anesthesia, or even when 



reduced as in natural sleep, despite the apparent de- 

 liver) of sensor) messages over the specific system as 

 indicated b) unimpaired cortically evoked potentials. 

 Such interaction may be conceived in al least two 

 general ways, one involving individual units and their 

 local relationships, the other involving more general 



relationships ol ret eptive anil association /ones. With 



ikI to the first of these there have been hypotheses 



about the axosomatic terminations of STPS upon 

 cortical units, and axodendritic terminations of the 

 more diffusely arrayed ARAS and DTPS influences. 

 The existence of interneurons between diffuse pro- 

 jections and cortical units has also been proposed 

 (197). Although far from complete, microelectrode 

 studies of cortical unit activity is beginning to supply 

 some information on these questions. 



With regard to the grosser arcal relationships, there 

 is already some interesting evidence. Starzl & Magoun 

 (222) and others have observed that diffuse thalamic 

 projections, as exhibited by the cortical areas in which 

 the most prominent recruiting responses can be 

 elicited, tend to be limited to the associational cortex 

 of the frontal, cingulate, orbital and suprasylvian 

 portions of the hemisphere in the cat, but with o\ er- 

 lapping of the motor fields. Jasper et al. (133 ) demon- 

 strated a more labile topographic organization, and 

 under more specific conditions observed that recruit- 

 ing responses extended also to s en sory receiving 

 areas. Dempsey & Morison (59) originally observed 

 that primary sensory areas, and particularly auditory 

 and visual areas, developed poor recruiting responses. 

 When they applied repetitive low-frequency stimula- 

 tion to specific thalamic nuclei and adjacent regions, 

 they observed another, but perhaps related, phe- 

 nomenon which they called 'augmenting' responses 

 in the cortical zones of specific projection (61, 180). 

 Augmenting responses have a shorter latency than 

 recruiting responses, and can coexist with spontaneous 

 waves and spindle bursts; recruiting responses on the 

 other hand displace spontaneous waves and spindle 

 bursts, suggesting that they involve some of the same 

 cortical elements. 



There is obviously a need to distinguish more clearly 

 between, or to identify common features among, the 

 following : spontaneous waves of the cortex, spindle 

 bursts, recruiting waves, sensory after-discharge (44), 

 secondarv responses (62, 75), and self-sustained and 

 corticalh spreading or corticocortical responses (4, 

 •202). Xot only must these be distinguished in terms 

 of the elements contributing to them, but also the 

 areas from which they arise The functions which 

 these areas subserve must also be more clearly de- 

 lineated 



The significance of these areas, their local functions 

 and their thalamic counterparts might derive some 

 meaning it viewed in the light ol the phv logenetic 



interpretation and approach suggested b) Herrick 

 & Bishop (108) and Bishop (26). From a phylogenetic 



viewpoint Bishop sees the reticular System as a series 



of segmental, level-to-level integrating systems. The 

 reiic ul. 11 formation is one of these, the links between 



