1562 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



and used also bv others 1 185 1, was able to demonstrate 

 functional connections between the cortex and specific 

 and nonspecific nuclei. 



As early as 1933 Bartley and Bishop (16, 25) were 

 suggesting reverberating circuits between the thalamus 

 and cortex to account for synchronization they ob- 

 served between alpha-like rhythms of the cortex and 

 activity in the pathways and thalamic nuclei of the 

 visual system. Subsequently Jarcho (124) and Chang 

 (44) also postulated reverberating or loop circuits to 

 account for interactions they observed between the thal- 

 amus and cortex. Jarcho believed that the reverberat- 

 ing circuit responsible for repetitive after-potentials 

 was a part of the specific projection system, whereas 

 Chang felt thai there were two independent systems, 

 but capable of influencing one another. Chang demon- 

 strated that the second of two click stimuli was able 

 to abolish the recurrent after-potentials which fol- 

 lowed the specific evoked response to the first click 

 in the auditory cortex, and that it reset or established 

 its own rhythmic aftereffects. Only when the interval 

 between the two clicks was properly adjusted to the 

 duration of the rhythmic after-potentials of the first 

 evoked response would it give an optimal response. 

 Thus the responses of the specific system are influenced 

 by the recurrent rhythms of the nonspecific system, 

 and the rhythms of the latter system can be influenced 

 or reset by the activity in the specific projection sys- 

 tem. This may lie an important step in the process ol 

 attention and consciousness, and will be discussed in 

 a later section. 



Another important neurophysiological clue to tin- 

 control of attention and consciousness derives from 

 the work of Morison & Dcmpsev (59, i;<)> who dis- 

 covered that stimulation of the mid-line, intralaminar 

 and dorsomedial nuclei of the diffuse thalamic pro- 

 jection system .ii frequencies off) to 12 per see. in the 

 cat gives rise 10 a 'recruiting response' in several 

 widespread areas of the cortex. The first three or lour 

 shocks to the thalamus produce a gradually increas- 

 ing magnitude of response in the cortex, hence the 

 term 'recruiting,' implying that more and more units 

 or increments to the field of activity have been brought 

 in 1. 1 synchrony. By about the fifth stimuli is the ampli- 

 tude oi the response stabilizes. If a much higher 



frequency of stimulation is applied to the same elec- 

 trodes, the recruiting response is abolished and a 



1 1, million "i 'activation 1 prevails, much like that 

 from stimulation of the reticular formation of the 

 lowei brain stem which brings the ARAS into play 



Ii is interesting thai stimulation of the mesen- 

 cephalii reticulai formation with shocks ol too to ;<<<< 



per sec. produces clcctrocortical activation, behavioral 

 arousal and alerting of enduring persistence, whereas 

 similar high-frequency stimulation of the recruiting 

 areas of the thalamus produces the same effects but 

 without long-lasting persistence and with distinctly 

 more limited topographic representation. This again 

 may reflect the relatively different roles of the ARAS 

 and DTPS in the matter of the temporal control and 

 flexibility with respect to attention, the former per- 

 haps determining longer-lasting general states of 

 alertness and the latter modulating these ^i,n,s on a 

 shorter and more variable temporal scale. Other 

 differences in the two systems are reflected in the 

 fact that the recruiting mechanism is not affected 

 by barbiturate anesthesia, either in its low-frequency 

 recruiting response or in its higher frequency activa- 

 tion response, whereas the activation mechanism of 

 the ARAS is seriously disrupted. Still another differ- 

 ence is the tendency, reported by Hess (118) and 

 Akimoto el ol. (8), for low-frequency stimulation in 

 certain regions of the DTPS to produce sleep, whereas 

 similar stimulation in the reticular formation of the 

 ARAS does not. Such neurophysiological differences 

 serve to distinguish the ARAS and DTPS as having 

 different functional roles, although there is also 

 reason to believe that they may work integratively 

 and in some instances may have mutually reinforcing 

 effects. 



One very important point seems to be th.u the 

 ARAS, except when it is blocked by anesthesia, lias 

 an activation effect which takes precedence over that 

 of the DTPS. This would seem to have ideological 

 Significance, from a protective viewpoint it would 

 appear to Ik- more important for a general alerting 

 mechanism such as the ARAS (o clear the way for 

 any or all warning messages from a threatening en- 

 vironment, than 10 have the central mechanism, 

 represented bv the thai. nuns and cortex, remain pre- 

 occupied with a specific locus ol attention and ob- 

 livious to danger. 



Further distinctions between the ARAS and DTPS 

 are seen in their reactions 10 drugs and anesthetics. 

 Ronvallet tt al. (29) bv some ingenious experiments 

 demonstrated that epinephrine acts principally on 

 the upper part of the brain-stem reticular formation to 

 produce activ alion of the ARAS, w iih aceoinpanv ing 

 eleCtrOCOrtical activation or arousal. This etlec 1 was 



shown bv these authors, and by Rothballer (205), to 

 be limited 10 the pontomesencephalic n ticular forma- 

 tion, since lesion ol the ARAS at the junction ol the 

 midbrain and diencephalon prevented epinephrine 

 from having this effect. Thus the upper midbrain 



