'-|7" 



II WDIlooK OF 1'IIYSloI i ii.-i 



M I ROPHYSIOLOGY III 



show reciprocal inhibition and activation (B, C, D 

 and 1. neurons). Of particular significance to the 

 problem ol interaction being considered hen- is the 



fact that there is a convergence of specific and uii- 

 specihe systems upon the same cortical neurons. 

 Jung's group (7, 51, 137) has shown that the mode 

 of discharge, and the number of light-responsive 

 neurons is altered by stimulation of the nonspecific 

 system. They have found aNo that mosl neurons are 

 subject to joint influence in the form of facilitation, 

 inhibition and occlusion. Furthermore, thalamo- 

 reticular stimulation increases the maximum fre- 

 quency with which visual cortical neurons can re- 

 spond to flickering light. The flicker-fusion rate of 

 individual cortical neurons can be raised considerably 

 by thalamic or reticular stimulation. In contrast to 

 these results, Jasper ( 1 29) and Li et al. (157) ha\ e not 

 I >< en able to delect cortical unit spike discharges, ex- 

 cept by stimulation of a specific afferent system. 

 However, they do record an intracortical slow- nega- 

 tive wave upon which the spike response of a single 

 unit may be superimposed; and during repetitive 

 stimulation of the nucleus centralis lateralis, multiple 

 discharges of the same unit may be superimposed 

 upon the resulting slow-wave recruiting responses. 

 This undoubtedly means that facilitation of the unit 

 has taken place due to stimulation of the unspecific 

 system. Thus their results, though not strictly in ac- 

 cord with those of Jung's group, may nevertheless 

 indicate some of the same kinds of influences. 



Jung et al. (137) point to a parallel between the 

 response of individual conical neurons to flickering 

 light at fusion level and above, when accompanied 

 bv stimulation of the ARAS, and the fact that human 

 fusion level mav be elevated by increased attention 

 and alertness. They state that: "Activation and in- 

 creased frequency of cortical neurons in response to 

 peripheral stimulation after excitation through the 

 nonspecific system thus finds a parallel in subjective 

 experience and uggests a possible neurophysiological 

 mechanism for the regulation of attention." 



These workers believe thai the nonresponding A 



neurons constitute about hall' of the cortical neurons 

 in ihc optic cortex. The A neurons are thought to 

 receive thru activation not from the specific visual 



i' la-, nut lei, but fi assoi iation fibers or the medial 



thalamic nuclei. They look upon them as serving a 

 Stabilizing background function which is capable of 



ind adjusting the excitation level in the 

 cortex. This mav verve ., protective function in the 



of massive, vei/ute-like discharges, or il might 



serve .\]) attention-regulating function under lesser 



and more differential stimulation. Thus interaction 

 of ARAS and DTPS with STPS at the level of the 

 cortex might play a significant role in specific alerting 

 and attention. What controls and regulates the 

 specific shifts of attention still remains a problem. Is 

 this a mass influence imposed bv ARAS or DTPS, 

 or is there a certain degree of topographical represen- 

 tation, if so, what particular portions of the reticular 

 formation and what particular nuclei of the DTPS 

 are involved? Jung and fellow workers speak of A 

 neurons acting like inierneurons which might be 

 both inhibitory and f'acilitatorv in their influence 

 upon other neurons subserving STPS functions. 



Jasper ( 129) observes that a single cortical unit will 

 respond at a higher frequency if preceded by a con- 

 ditioning shock to the unspecific svstem, but reports 

 also that from extracellular recordings he has found 

 certain units which will manifest inhibition or facili- 

 tation by either unspecific or specific intervention. He 

 calls attention also to the possibility that unspecific 

 influences mav affect the excitability of the cells 

 possibly by changes induced upon the dendrites. This 

 might suggest that the unspecific system (ARAS or 

 DTPS) could have a modulating influence upon the 

 excitability of specific cortical synapses. Again the 

 question may be raised, which synapses and by what 

 control? At the moment the possibilities are manifold. 

 More and more specific mechanisms are being dem- 

 onstrated which add flexibility to the means ol con- 

 trol and at several different levels of sensory influx 

 from receptors to cortex, but the manner in which the 

 control is exerted still remains elusive. Is there suffi 

 cienl topographic representation in the reticular for- 

 mation or the nonspecific nuclei of the ih.il.nnus to 

 accomplish this? <.'.n\ the degree of differentiation of 

 individual units already demonstrated there ui, 12, 

 207) be modified and regulated sufficiently bv con- 

 ditioning, learning and habituation? The next section 

 provides some partial answers to such a question. 



'Habituation' and Attention 



As was pointed out above in connection with the 

 work of Jung's group, approximately 50 per rent of 

 the cortical units studied bv microelectrodes in the 



visual cortex of the cal were unresponsive to light 

 stimulation upon die retina. This group of neurons 

 they looked upon as having a stabilizing influence 

 upon aelivitv in the visual cortex, and perhaps plav- 

 ing a role in visual response mainly under certain 

 kind- ot conditions in which arous.il, alerting .m(\ 

 attention were involved. Ii is interesting thai Erulkar 



