I (l.l. 



II.WDHoeiK ()K F'HYSIOLOGY 



NEUROPHYSIOLOGY III 



In liis summary of the literature on the function of 

 the parietal lobes, Critchley has cited numerous 

 examples from studies of his own and of other investi- 

 gators (46). Phenomena observed during 'double 

 stimulation,' usually of bilaterally symmetrical parts 

 of the body, bring out in a clear fashion disturbances 

 in sensory discrimination that can be most easily 

 described as losses of capacity to attend. In some pa- 

 tients with a unilateral parietal lobe lesion such that 

 the projection area of one hand is presumably dam- 

 aged or destroyed, tests are made in which the 

 affected and unaffected hands are stimulated simul- 

 taneously. The patient when blindfolded may report 

 feeling an object only in the normal hand if two like 

 objects are simultaneously placed in both hands. If 

 the object is removed from the normal hand, the pa- 

 tient may then report the presence of an obiect in the 

 affected hand. These and similar phenomena suggest 

 that the deficit may be one in control of attention. 



While such observations as those cited in the above 

 examples were reported frequently, researchers in the 

 field of sensory discrimination have been reluctant to 

 deal directly with the problem of attention. This re- 

 luctance is understandable. It is due to a lack of knowl- 

 edge of any neural system which might interact with 

 the main sensor) systems in such manner that ap- 

 propriate control of input channels might occur. The 

 discovery and clarification of the functions of the as- 

 cending reticular system and of the diffuse thalamo- 

 cortical projection systems have not only dissipated 

 the scientists' reluctance to face the problems of 

 attention but has broughl these problems to the fore. 

 It is somewhat premature as yel to propose a neural 

 theory of attention, but is is possible to cite a few 

 experiments which bear upon the nature of the inter- 

 action between the specific and unspecific afferent 

 sy stems. 



It has been show n that in the brain stein the ascend- 

 ing reticular system (as discussed in Chapter I, II of 

 this Handbook) receives collaterals I mm the specific 

 afferent pathways of the visual, auditory, somesthetic 

 vestibular systems (199). When the peripheral 

 sense organs are stimulated, impulses are condui b d 

 therefore nol only to the primary projection areas of 

 the cortex via the specific afferent pathways but, after 

 .1 longer latent ) due to delay in passage through many 

 synapses in the reticular formation, to wide-spread 

 .us ol the cortex 1 198) .is well .is to the projection 



II the specific afferent pathways are 



11 .msec ted, sen I organ stimulation fails to produce the 



ivpic.il short latency evoked responses in ihe primary 

 projei Hon areas; activation of the BEG does occur in 



unanesthetized preparations (60, 62, 129, 130). In 

 the anesthetized animal, the unspecific route via the 

 reticular system appears to be blocked while the 

 specific afferent pathways remain open; at the cortex, 

 good evoked responses can be recorded from the 

 primary projection areas during sense organ stimula- 

 tion, activation of the EEG does not occur (61 ). These 

 and other results suggest the hypothesis that interac- 

 tion of the activity set off in the specific and unspecific 

 systems must occur during sensory stimulation if the 

 sensory stimulus is to be attended to and perceived 

 clearly. That the cortex plays an important role in 

 this interaction might be inferred from some of the 

 evidence cited above. Lindsley, for example, has sug- 

 gested that during sensory stimulation the activity 

 arriving at the cortex via the unspecific system "sets 

 the stage for the spreading and elaboration of the 

 effects which reach the primary receiving areas. Its 

 relative nonspecificity would favor this. If it acted bv 

 resetting excitability cycles of many neuronal aggre- 

 gates so that a greater statistical availability of cellular 

 units or patterns of units could at once be made possi- 

 ble, it should facilitate the chances of the primary 

 message being received and transmitted in secondary 

 or association areas and eventually leading to a re- 

 sponse through the motor system" (127, p. 329 



A number of investigators (35, 99, 114, 127, 128, 

 147) have outlined in broad terms a theory which 

 attempts to account, in ncurophvsiologic.il terms, for 

 a general arousal mechanism and a specific attention 

 mechanism. The results of studies of experimental abla- 

 tions in animals and of cortical damage in man sup- 

 port this notion of two systems, one having to do with 

 maintenance of a general state of alertness, the other 

 with control of direction ol attention. In relation to 

 the general alertness system, note should be taken of 

 the proposal of Granit as to one possible significance ol 

 the spontaneous activity which has been recorded in 

 peripheral sensory nerves, lie suggests that the sense 

 organs may serve as 'energizers'; the spontaneous 

 activity in peripheral nerves feeds into the ascending 

 reticular system via the collaterals from the- main 

 afferent pathways and helps to maintain a general 

 st. He of wakefulness or alertness (82 ). This, .is ( h.init 

 points out, is only one possible role of the spontaneous 

 .ictivitv. As we have noted above (p. 1454) inhibition 

 .is well .is increase ol spontaneous .utiviiv m.iv occur 



during sense-Organ stimulation. This kind of mecha- 

 nism has greater information-handling capacitv than 

 one which is silent except during stimulation. 



Another important recent development in neuro- 

 physiology which must be considered in relation to 



