THE EVOKED POTENTIALS 



311 



and can be aroused only by stimulation of some sense 

 organs. Experimental evidence shows that afferent 

 impulses necessary for keeping the cerebral cortex in a 

 state of vigilance reach it from the reticular formation. 

 Impulses in the main ascending sensory pathways ap- 

 parently travel in collaterals entering the reticular 

 formation, since they cannot maintain wakefulness 

 after destruction of the reticular formation (45, 63, 

 64). 



Perhaps one of the more illustrative examples show- 

 ing the effect of constant afferent inflow on cortical 

 excitability is the phenomenon of photic potentiation 

 (16). The electrical response to stimulation of the 

 lateral geniculate body was found to be manv times 

 greater when the retina was illuminated than when 

 it was in the dark. Not only was the size of the cortical 

 response to geniculate stimulation greater but also the 

 threshold of the response was lower during retinal 

 illumination than in the dark. The effect of photic 

 potentiation on cortical responses dc\elopcd pro- 

 gressively after the onset of retinal illumination and 

 reached its maximum in about 5 sec. Once the maxi- 

 mal effect was attained, it was sustained at that level 

 as long as the retinal illumination persisted. The 

 enhanced cortical response was promptly reduced to 

 the preillumination magnitude as soon as the light 

 stimulus was withdrawn. The mechanism underhing 

 this potentiation phenomenon is believed to lie mainly 

 in the lateral geniculate body. As is well known, some 

 retinal elements discharge steadilv at low frecjuencv' in 

 the absence of light which is apparently not strong 

 enough to set up a discharge of the postsynaptic neu- 

 rons in the lateral geniculate body. Nevertheless, the 

 summated effect of the incessant bombardment of 

 these subliminal impulses will raise the excitability 

 of the geniculate neurons. If meanwhile an electric 

 shock is applied directly to the geniculate Ijody, the 

 neurons which are normally in the subliminal fringe 

 will discharge because of the summation of the existing 

 presynaptic impulses induced by retinal illumination 

 and the electric stimulus directly applied to the 

 geniculate body. The gradual development of the 

 photic potentiation effect in the case of repetitive 

 stimulation of the geniculate body seems to constitute 

 a good example of recruitment in the truly physio- 

 logical sense of the term. 



A similar process is present, though much less pro- 

 nounced, in the auditory system. Rosenblith et al. 

 (61) described the enhancement, by exposure to 

 continuous tones, of potentials evoked in the cortex by 

 a click. Their results suggest that tones of certain 

 frequencies, especially tones at frequencies between 



100 and 500 cycles per sec, show a very effective 

 potentiation effect following the initial period of 

 depression, while those of high frequencies are not as 

 effective. 



Preliminary observations ha\e been reported on the 

 potentiation effect of continuous retinal illumination 

 on the cortical response to stimulation of the auditory 

 pathway C'6). The interaction between the visual and 

 auditory impulses appears to take place not only in 

 the cerebral cortex through the association neurons 

 but also in subcortical structures, since the removal of 

 the visual cortex cannot completely abolish the 

 photic potentiation effect on the auditory response. 

 The explanation tor the interaction between the 

 visual and auditory impulses at the subcortical level 

 is rendered diflicult by the lack of direct fiber connec- 

 tions between the medial and the lateral geniculate 

 ijodies. The pulvinar is considered a possible site of 

 subcortical correlation for the two great special sen- 

 .sory systems. Since the importance of the core struc- 

 tures of the brain stem in the actixation of the cerebral 

 cortex has been recognized (50, 51, 56), it is pos.sible 

 that the interaction at the cortical level between the 

 afferent impulses from different .sources mav be exe- 

 cuted indirectly through the system of the diencephalic 

 and mesencephalic reticular formation where collat- 

 erals of ascending fibers from different kinds of sense 

 organs converge. The centrally located area in the 

 brain stem comprising the midbrain's tegmentum, the 

 subthalamus, the hypothalamus and the intralaminar 

 portion of the thalamus, has been found to be able to 

 desynchronize the electrocortical activity when it is 

 stimulated repetitively at a rapid rate. It can also 

 exert various effects on the cortical potentials evoked 

 by single shock stimulation of a particular sensorv 

 system when it is stimulated singly in close approxi- 

 mation to the eliciting shock. Stimulation of this area 

 in the brainstem has also ijeen described as having a 

 catholic activating effect on the entire cerebral cortex 

 and especially the frontal lobe of the brain. This 

 activating system receives afferent impulses from 

 various sensory sources through the collaterals of 

 ascending tract fibers and transmits them to the 

 cerebral cortex, probably both by the thalamic and 

 the capsular corticipetal routes. 



SUMM.'^RY 



An evoked potential may be defined as the detect- 

 able electrical change in the brain in response to de- 

 liberate stimulation of any part of the nervous system. 



