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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



FIG. 6. Cortical zones showing reinforcement of responses 

 between two heterogeneous sensory stimuh. Cats curarized, 

 without central anesthesia or with a light dose of chloralose. 

 When two heterogeneous stimuli occur nearly simultaneously, 

 certain cortical loci outside of the primary receiving areas 

 show an interaction between the two stimuli (potentially the 

 effect is either facilitatory or inhibitory but is most readily ident- 

 ified if facilitatory). Auditory and somesthetic reinforcement in 

 ,-1, visual and somesthetic in B; auditory and visual in C. The 

 principal primary receiving areas are outlined by dolled lines. 

 GSP, posterior sigmoid gyrus; C, coronal gyrus; SSA and SSM, 

 anterior and middle suprasylvian gyri; LAT, lateral gyrus; 

 ESM, middle ectosyl\ian gyrus. [From Buser & Borenstein 

 (")•] 



and secondary response systems arc differendated by 

 being a) independent in pathway (lemniscus and 

 medial brain-stem reticular formation), i) different 

 in susceptibility to anesthetic agents (primary re- 

 sponses are highly resistant, secondary responses are 

 more vulnerable to barbiturate anesthesia), <) differ- 

 ent in latency (for primary somesthetic responses, 

 approximately 8 to lo msec, as compared with those 

 for secondary responses, approximately 40 to 80 msec.) 

 and d) different in areal extent on cortex (primary 

 localized to classical sensory projection area, second- 

 ary extending widely into association cortex where 

 the modalities belonging to the different sensory 

 systems overlap each other). 



There is an important functional distinction be- 

 tween the classical sensory pathways and the ascend- 



continuity of secondary responses during the application of 

 increasing increments of barbiturate anesthesia. An early 

 secondary response which can be discerned when conditions 

 are favorable in the completely unanesthetized cat disappears 

 with light stages of anesthesia (pentobarbital, 15 mg/kg), and 

 a much larger secondary response appears at a deeper stage of 

 anesthesia (30 mg/kg) and after a substantially longer latency. 



ing brain-stein reticular system. French & Magoun 

 (22) found that monkeys with bilateral destruction of 

 the classical lemniscal pathways in the midbrain are 

 still aroused from sleep by sound and touch stimuli. 

 When the reticular formation in the midbrain is 

 destroyed, however, leaving the classical ascending 

 sensory pathways intact, the monkeys remain in coma, 

 even though sensory evoked potentials can be re- 

 corded in the auditory and somesthetic receiving 

 cortices. Central anesthetics block conduction in 

 certain extralemniscal pathways, and this undoubt- 

 edly represents an important basis for their action as 

 anesthetics (23). These facts underline the importance 

 to sensory evoked arousal, and presumably to sensa- 

 tion in general, of the extralemniscal pathways. 



Cortical Intrrarlion Sysli'ms 



High frequency stimulation of the brain-stem retic- 

 ular formation yields a generalized reduction in de- 

 gree of synchronization among cortical neurons (63). 

 The effect on the electrocorticographic patterns imi- 

 tates the desynchronization that takes place during 

 natural arousal. It has been shown that brain-stem 

 activation is accompanied by an increase in the rate 

 of discharge of neurons throughout the cephalic brain- 

 stem, including the diffusely projecting thalamic 

 system (57). As is well known, almost all individual 

 cortical loci are reciprocally related to points that are 

 symmetrically placed on the opposite hemisphere, as 

 though in mirror image of each other. Chang dis- 

 covered, as shown in figure 7, that when one records 

 evoked potentials from a given cortical locus, an 

 intervening stimulation of the homotopically related 

 point on the opposite hemisphere will modify the 

 evoked response (12, 13). von Euler & Ricci (81) 

 have analyzed this capacity for interference with pri- 

 mary cortical sensory-evoked responses on the part of 

 separate cortical inputs. By stimulating the classical 

 thalamic relay nuclei and recording the primary 

 evoked cortical responses, these investigators could 

 then add conditioning stimuli to the contralateral 

 homotopic cortical point. They find, as did C^hang, 

 that these systems converge and interact within the 

 sensory cortex (81). Afferent impulses arriving in the 

 sensory cortex are known to interact there with non- 

 specific impulses from the thalamic recruiting system 

 (43). Moreover, recruiting responses recorded from 

 the cortex are found to be altered during behavioral 

 alerting to sound stimuli (18). 



All of these facts sulxstantiate the general principle 

 that within the cortical receiving areas, as at each of 



