CENTRAL CONTROL OF RECEPTORS AND SENSORY TRANSMISSION SYSTEMS 



749 



20 40 60 80 100 120 140 160 i80 200 



msec. 



FIG. 7. Time course of blocking effect of callosal potential on positive and negative components 

 of primary auditory response. Abscissae: Time after delivery of stimulus on contralateral cortex 

 for callosal potential. Ordinatcs: Magnitude of auditory response expressed as percentage of control. 

 Dots represent positive component and circles, negative component. Note that stimulation of the 

 contralateral homotopic cortical locus modifies, by its callosal connections, the sensory-evoked 

 response in a primary receiving area. [From Chang (13).] 



the Other stations along the sensory pathways, the 

 character and e.xtent of sensory-evoked responses are 

 subject to intervention by activities taking place else- 

 where within widespread regions of the brain. 



Corticifugal Influences on Brain-stem Mechanisms 



Bremer & Terzuolo (9) showed that stimulation of 

 the cortex in cats without central anesthesia will in- 

 duce electrocorticographic evidence of arousal. Jasper 

 and co-workers (41), working with monkeys, had 

 earlier shown a spread of localized cortically-induced 

 after-discharges into the brain stem. It has subse- 

 quently been observed, in monkeys without central 

 anesthesia, that single shock stimuli delivered to spe- 

 cific regions of the cortex will yield evoked potentials 

 throughout a wide zone of the cephalic brain stem 

 (1-3, 21). This zone is generally coextensive with the 

 brain-stem region within which sensory responses from 

 different sensory systems appear to converge. Blocking 

 and facilitating interaction takes place in this general 

 region among the various combinations of cortically 

 and peripherally initiated signals (8, 21, 36, 70). 



Examples appear in figures 8 and 9. The corticifugal 

 projections not only interact with other signals con- 

 verging upon the brain stem but also with signals 

 intrinsic to the reticular formation, i.e. signals gen- 

 erated within and recorded from the brain-stem retic- 

 ular formation itself (2), as may be seen in figure 10. 

 The same corticifugal systems are known to be capable 

 of initiating electrocorticographic (73) and behav- 

 ioral C72) arousal, presumably by virtue of their con- 

 nections with the cephalic brain-stem reticular forma- 

 tion. 



It can be seen that not only are input and output 

 patterns modifiable within the cortex, but the cortex 

 itself can also modify activity taking place within the 

 ijrain stem and thereby possibly have an indirect in- 

 fluence back upon sensory patterns as these are ini- 

 tiated and relayed at lower levels. 



Organization of Centrifugal Sensory Control Mechanisms 



Up to this point we have described mainly how 

 activity in each of the sensory neurons linking to- 

 gether a given classical ascending sensory pathway 



