THE EVOKED POTENTIALS 



307 



lus Strength but varies with the state of anesthesia and 

 the area from which the observations are made. The 

 repetitive discharges evoked by afferent impulses 

 differ from the spontaneous waves in many respects, 

 though they may happen to have the same frequency. 

 The evoked repetitive discharges are always surface- 

 positive waves whereas the spontaneous activity of 

 cortical neurons is not necessarily so. The latter 

 appears to be regulated to some extent by the intra- 

 laminar nuclear groups of the thalamus (39); but the 

 evoked periodic after-discharges are not affected by 

 surgical removal or electrical stimulation of the massa 

 intermedia (14). 



The presence of the evoked periodic discharges is 

 dependent upon the integrity of the pathways between 

 the cerebral cortex and the thalamic nucleus con- 

 cerned. The periodic wa\es recorded from the cor- 

 responding thalamic nucleus are similar in pattern to 

 those observed from the sensory cortex and can be 

 abolished by removal of the cortex. Likewise, repeti- 

 tive discharge of the same kind can be evoked by 

 direct stimulation of the cortical surface and abolished 

 jjy the destruction of the corresponding thklamic 

 structure or by interruption of the thalamocortical 

 connections. From these experimental facts it is sug- 

 gested therefore that the specific periodic after- 

 discharges following afferent stimulation represent the 

 activity of the reverberating circuit between the 

 sensory cortex and the thalamus. It is as.sumed that a 

 volley of afferent impulses from the thalamus, after 

 arriving at the cortex, will return to the thalamic 

 nucleus and ascend again to the cortex to start 

 another cycle of activity along a closed chain of 

 neurons. 



It is believed that the general periodic wa\es ob- 

 served in the central nervous system can be due to 

 many causes. The activity of reverberating circuits is 

 only one of many possible mechanisms underlying the 

 periodic waves. It would be a mistake to regard all 

 kinds of rhythmic discharges as being due to the 

 activity of reverberating circuits. The evoked repeti- 

 tive discharge in the sensory cortex must be dis- 

 tinguished from the spontaneous rhythmic waves 

 which sometimes present themselves in such a manner 

 as to confu.se or mislead the obser\er. It is true that in 

 unanesthetized animals or in animals anesthetized 

 with chloralose the rhythmic discharges following a 

 single sound stimulus do occur in the medial genicu- 

 late body after decortication (36). However, as 

 pointed out by Galambos, rhythmic waves al.so occur 

 spontaneously without deliberate stimulation. It is 

 obviously a mechanism entirely different from that 



underlying the specific corticothalamic reverberating 

 waves which, under experimental conditions, can be 

 evoked only by an afferent volley from the thalamus 

 or by direct cortical stimulation. So specific is this 

 response that corticothalamic waves hav-e never been 

 obtained by stimulation of a symmetrical point on the 

 opposite cortex although the callosal response itself 

 may be a very intense one (18, 19). The failure of a 

 callosal volley to initiate the repetitive discharges of 

 the sensory cortex at the same cortical locus where the 

 thalamic volley can do so very well seems to provide 

 strong evidence that the appearance of the repetitive 

 waves is dependent on the presence of a specific 

 neuronal circuit rather than being representative of 

 mere local after-discharge of an unorganized ag- 

 gregate of neurons having autorhythmic properties 



One of the main difficulties in interpretation of the 

 specific periodic after-discharges according to the 

 hypothesis of corticothalamic re\erberation is perhaps 

 the long interval (45 to 1 50 msec.) between the con- 

 secutive waves which appears to be of too great a 

 duration to be accounted for solely by the time neces- 

 sary for the impulses to travel along the neuronal 

 circuit between the thalamus and the cortex. A pos- 

 sible explanation is that the surface-positive re- 

 verberating waves, like the positive component of the 

 primary response, presumably consist of synchronous 

 discharges of cortical neurons triggered by the recur- 

 rent thalamic volleys but not the afferent impulses 

 themselves. The latter may not be detectable from 

 the cortical surface. Because of the \ariation in size of 

 fibers interconnecting the thalamus and the cortex and 

 the variation in number of synapses interposed in the 

 circuit, the degree of temporal dispersion of the 

 circulation of the reverberating impulses along the 

 circuit must result in a continuous train instead of 

 intermittent waves. It is probable that among the 

 returning impulses only those which arri\e at certain 

 phases of the excitability cycle of the cortical neurons 

 are capable of initiating synchronous discharges of 

 these neurons, and the others which arrive at the 

 cortex during the period of postexcitatory depression 

 rendered by the previous wave will not cause excita- 

 tion. Therefore, the interval between the reverberat- 

 ing waves is probably determined by the state of 

 excitability of the cortical neurons at the time of 

 action, rather than by the conduction time and the 

 number of synapses in the circuit. Thus, it becomes 

 evident that the activity of the corticothalamic 

 reverberating circuit cannot l^e taken as a simple 

 circulating of impulses along a closed chain of neurons. 



According to Bremer (9) the initiation and main- 



