EVOKED

POTENTIALS

CORTEX

ON

PAIRED

F. N. Serkov, E. and I. I. Shelest

OF

THE

AUDITORY

STIMULI F.

Leonova,

UDC 612.858° 72:612.014.421

Changes in the r e s p o n s i v e n e s s of the auditory cortex to an acoustic click and to d i r e c t stimulation of the medial geniculate body were studied by the method of evoked potentials in an extended e x p e r i m e n t on cats with implanted e l e c t r o d e s . It is shown that the m i n i mum interval between two stimuli for which a second click produces an E P in the audito r y c o r t e x is f r o m 30 to 50 m s e c . The relative r e f r a c t o r y period consists of two parts. The f i r s t (50-100 msec) is c h a r a c t e r i z e d by a rapid r e c o v e r y , and the second (about 500 msec) by a slow r e c o v e r y . In c o n t r a s t with a click, d i r e c t stimulation of the geniculate body does not produce a r e f r a c t o r y condition but one of facilitation. The effects of Nembutal and chloralose anesthesia and the state of a l e r t n e s s on the r e c o v e r y of audito r y c o r t e x r e s p o n s i v e n e s s were investigated. The r e a s o n for the absence and the r e duction of an E P f r o m the auditory c o r t e x to a testing click during absolute and relative r e f r a c t o r y periods is not a passive d e c r e a s e of excitability of the usual r e f r a c t o r y kind, but an active interplay of e x c i t a t o r y and inhibitory p r o c e s s e s in the c e r e b r a l cortex, geniculate bodies, and r e t i c u l a r formation of the brain stem. Studies of the evoked potentials ~ P) at various sections of the auditory a n a l y z e r in r e s p o n s e to two acoustic clicks separated by small, a c c u r a t e l y determined time intervals have been c a r r i e d out by a number of investigators [1-4, 11, 13, 17]. The main objective of these investigations was to determine the excitability r e c o v e r y cycle at various sections of the analyzer when an afferent discharge a r r i v e s . The dynamics of the changes in excitability foilowing the f i r s t (conditioning) click have usually been e x p r e s s e d in a curve whose a b s c i s s a s a r e laid off in the time intervals between clicks and whose ordinates are the value of the E P f o r the testing click in percent of the E P for the conditioning click. Although such a curve is an excitability r e c o v e r y curve in the full meaning of the phrase, nonetheless it effectively a c counts for both the facilitating and inhibiting effects of an afferent discharge. These authors showed that the second click is effective only after a c e r t a i n absolute r e f r a c t o r y period. Then a relative r e f r a c t o r y period follows during which there is a gradual r e c o v e r y of the E P for a testing click f r o m zero to the value of the E P for the conditioning click. It was shown that the duration of these periods is not the same at the various sections of the auditory s y s t e m [2-4]. The effect on them of length of sleep, anesthesia, active wakefulness [2, 11, 13], intensity of the conditioning click [1], and the point of the E P pickup [16] were studied. The r e s u l t s obtained by various authors are in many ways inconsistent. There are considerable d i s c r e p a n c i e s in the data on the durations of the r e f r a c t o r y periods, on the general behavior of the r e c o v e r y curve, the effect of anesthesia, of active wakefulness, etc. We have undertaken the task of explaining some of these questions.

A. A. Bogomolets' Institute of Physiology, Academy of Sciences, Ukrainian SSR, Kiev. Translated f r o m Neirofiziologiya, Vol. 1, No. 1, pp. 54:-64, July-August, 1969. Original article submitted April 14, 1969. I

©1970 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street~ New York, ?,!. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $I5.00.

42

] 1

t

METHODS Experiments were made on eight cats with implanted electrodes. The evoked potentials were picked up in monopolar fashion from the cortex of the middle section of the eetosylvian gyrus (first auditory area). The inert electrode was introduced into the nasal bone. l)aired clicks were obtained from two identical dynamic loudspeakers which were supplied with rectangular current pulses 0.2 msee in duration and at a voltage of i00 V. Specific measurements indicated that the duration of the acoustic click under these conditions ran between 3 and 4 msec and its intensity was 30-40 dB. The loudspeakers were connected to the outputs of a type ESU-I two-channel stimulator which made it possible to obtain either single or paired clicks with various intervals. The experiments began from 7-10 the cat was in a recumbent position in a em from the animal's head. The evoked "Orion" electroencephalograph and on a

days after the electrodes were implanted. During an experiment shielded room. The loudspeakers were located at distances of 30 potentials were recorded on the four-channel pen register of an type SI-19 Dufour oscillograph.

The responsiveness recovery curves of the auditory cortex were plotted as follows. Ten responses to paired clicks with various intervals from 20 to 600 msec were recorded. The EP amplitudes for the first and second clicks were measured from the peak of the initial electropositive to the peak of the electronegative excursion. For each interval the amplitudes were averaged and the average value of the E1 ) for the second click was determined as a percentage of the average value of the E1 ) for the first click. These values were plotted on the coordinate axes. A recovery curve was measured repeatedly on each animal over a period of several months. RESULTS The minimum interval between clicks for which the second stimulus cortex of the various cats in our experiments ranged from 30 to 50 msec. constant.

produces an E P in the auditory For any one cat it was fairly

Following the "absolute refractory" period a recovery period for the auditory cortex's responsiveness began. At first the recovery proceeded uniformly and rather rapidly so that in an interval of 70-80 msee the amplitude of the EP for the testing click would reach 50-70% of the E1 ) amplitude for the first (conditioning) click. Thereafter the recovery process was disturbed and a further increase of the interval between the clicks did not produce an increase but rather a decrease in the E P amplitude for the second click. Such a secondary decrease of the El) amplitude for the second click was observed in the majority of the experiments. For different animals it might occur for intervals of 70, 80, I00, and even 150 msec. A second msee. the E1 )

further increase of the interval between clicks led to an increase of the El) amplitude for the click. Complete recovery of the auditory cortex's responsiveness did not take place even after 600 The average amplitude of the second E1 ) for this interval usually came to between 90 and 95% of amplitude for the conditioning click.

It must be noted that the EP amplitude for the second click in separate pairs at intervals of 600, 500, 400, and 300 msec might be both equal to the amplitude Of the first EP, larger, or smaller. The average EP amplitude for the first EP was usually lower the smaller the interval between clicks. The curves shown in Fig. 1 for the recovery of the auditory cortex's responsiveness were obtained from such averaged data. As is well known, the EP produced in the auditory cortex of an intact eat in response to a click consists of a series of electropositive and electronegative components [2, 5, 15]. When the time characteristics of these EP components are compared with the changes in the responsiveness of the auditory cortex that take place after a conditioning click, a certain connection is distinguishable between them. Thus, an El ) does not occur in response to a testing click during the initial electropositive excursion and the ascending phase of the initial electronegative excursion of the E1 ) for the first click. If the testing stimulus acts at the end of the aseending phase of the initial electronegative excursion, then the response to it is sometimes an El ) in the form of a small cleavage in the peak of the eleetronegative component in the first El). The responses to a testing click became regular only during the descending phase of the eleetronegative excursion in the first E1 ). Such a relationship means that the shorter the duration of the initial E1 ) components for one cat or another, the shorter would be the interval for producing an E1 ) from the testing click.

43

20+8 80100

20O

.]00

+Oa

500

600 msec

Fig. 1. R e c o v e r y of r e s p o n s i v e n e s s in the auditory c o r t e x of three cats. Along the a b s c i s s a is the interval between clicks in m s e c ; along the ordinate is the average E P amplitude for a testing click in p e r c e n t of the E P amplitude for the conditioning click. Above is the d i a g r a m of the auditory cortex.

Fig. 2. Responses evoked by paired clicks in the auditory cortex of cats. I) Ink recordings; 2) photo taken from the screen of a Dufour oscillograph; 3) the same for a fivefold superposition. The figures at the left are the intervals between clicks. If the second E P falls in the descending phase of the subsequent electropositive E P excursion in r e sponse to the f i r s t click, then its initial electropositive excursion m e r g e s with the latter, and the total amplitude turns out to be less. This, apparently, is one of the r e a s o n s for the amplitude reduction of the second E P described above when the intervals between clicks are 70, 80, and 100 m s e c . In this c a s e , where the E P produced in response to the second click falls at tile beginning or in the ascending slope of the subsequent electronegative excursion for the f i r s t E P, its initial electropositive exc u r s i o n is sharply reduced while the amplitude of the initial eleetronegative excursion can be even g r e a t e r than for the E P produced by the conditioning click.

44

/00 8Q

4Q :0 20 406080100 200

Jo0 ~00 ~00

rnsec

Fig. 3. R e c o v e r y of r e s p o n s i v e n e s s (lower curve) and of excitability (upper curve) in a c a t ' s auditory c o r t e x after a conditioning click.

_

i.

=l

it~

.

.

.

.

I|_

Fig. 4. Responses evoked in the auditory c o r t e x of cats by paired clicks under Nembutal anesthesia. The figures at the left are the i n t e r vals between clicks, in m s e c .

The complex five-component f o r m of the auditory c o r t e x ' s E P, and also the information r e g a r d i n g the e l e c t r i c a l reaction of the neurons in the medial geniculate body in r e s p o n s e to a click [9], provide a basis for a s s u m i n g that under the action of a click s e v e r a l afferent pulse t r a i n s enter the auditory cortex. The f i r s t a r r i v e s at the cerebra1 c o r t e x some 8-10 m s e c after the click, the second after 15-20 m s e c , and possibly a third after 80-100 m s e c . The duration of the absolute r e f r a c t o r y period for these pulse t r a i n s is not the same. The s h o r t e s t was for the f i r s t train. When the intervals between the clicks were small, the E P produced by the second click was just a small electropositive excursion. As the interval was increased up to 60 or 70 m s e c , E P s appeared that had even a s m a l l electronegative component (Fig. 2). In those c a s e s where the absolute r e f r a c t o r y period was tong (50 msec), the E P produced by the testing click had at f i r s t three components: two electropositive and one electronegative. F o r still g r e a t e r intervals the E P consisted of five components. In a n u m b e r of e x p e r i m e n t s the degree of the excitability r e c o v e r y of the auditory analyzer after a conditioning click was determined by two methods: f r o m the value of the E P in the auditory c o r t e x for a testing pulse and f r o m the change in the stimulation threshold. The stimulation threshold was a s s u m e d to be the m i n i m u m click intensity that produced an E P in the auditory c o r t e x of a p a r t i c u l a r animal. In Fig. 3, r e c o v e r y c u r v e s are shown which were obtained on one cat by these two methods. It is evident that they differ substantially f r o m each other. This difference is e x p r e s s e d in the length of the r e c o v e r y period. When determined f r o m the E P amplitude, it is 600 m s e c , and f r o m the stimulation threshold it is only 150 m s e c . In other e x p e r i m e n t s this difference was g r e a t e r . The complete r e c o v e r y of excitability as determined f r o m the stimulation threshold took place after 80-100 m s e c , and the r e c o v e r y of the E P amplitude in r e s p o n s e to a testing click o c c u r r e d only after 600 m s e c . The duration of the absolute r e f r a c t o r y period was the same by both methods. These data were obtained on intact cats in a state of quiet wakefulness or drowsiness. We studied the effect of Nembutal and c h l o r a l o s e anesthesia as well as the state of active wakefulness (alertness) on the dynamics of r e s p o n s i v e n e s s r e c o v e r y of the auditory c o r t e x f o r these cats.

The Nembutal was given intra-abdominally in a 25-40 m g / k g dose. The evoked potentials were picked up at the s a m e points of the auditory c o r t e x through the same implanted e l e c trodes. The r e c o v e r y curve was determined only f r o m the E P amplitude for the testing click before and after the Nembutal injection. In the dosages indicated Nembutal produced anesthesia of medium depth accompanied by a substantial i n c r e a s e of the slow e l e c t r i c a l activity in the c e r e b r a l cortex. The E P s in this background were lower and v e r y d i v e r s e in both magnitude and shape. Along with the E Ps which consisted of electropositive and electronegative components there were frequently E Ps having only one e l e c tropositive component. These various E P shapes were accompanied by nonuniform variations in the r e s p o n s i v e n e s s of the auditory c o r t e x which were p a r t i c u l a r l y manifest when the intervals between stimuli were small. Thus, if an E P for a conditioning click had only an e l e c t r o p o s i t i v e excursion, then the period of absolute unexcitability was s h o r t e r than when an E 1o had electropositive and electronegative components.

45

2O

10 O 20 ~0 60 ,~0 100

,~00

300

OOO

5OO

msec

Fig. 5. R e c o v e r y of the auditory cortex r e s p o n s i v e n e s s in a cat. 1, 2) In states of quiet and active wakefulness, respectively; 3) under Nembutal anesthesia of medium depth. The designations a r e the same as in Fig. 1. When the intervals between clicks ranged f r o m 40-150 m s e c , the E P s appearing in response to the testing clicks following electropositive E Ps were not only equal to but often c o n s i d e r a b l y g r e a t e r in m a g nitude than the E P s of the conditioning clocks (Fig. 4). Besides, the E P s for the testing clicks in these c a s e s had both electropositive and electronegative components. All this indicates that the conditioning click does not produce a d e c r e a s e in the r e s p o n s i v e n e s s of the auditory c o r t e x but r a t h e r an i n c r e a s e , i.e., it facilitates the p e r f o r m a n c e . If a conditioning click produces an E P having positive and negative c o m ponents, a condition of reduced r e s p o n s i v e n e s s develops in the auditory cortex. The facilitating and r e d u c ing effects m a y appear in one and the same s e r i e s of paired clicks all having the same interval (50 msec) between clicks (Fig° 4). These data indicate that a prolonged d e c r e a s e in the auditory c o r t e x ' s r e s p o n s i v e n e s s after a conditioning click o c c u r s only when a valid E P develops in r e s p o n s e to it in the auditory cortex. They also confirm the assumption r e g a r d i n g the connection between the absolute r e f r a c t o r y period and the development period for the electronegative component of the E P produced by the action of a conditioning click. The next m o s t important feature of r e s p o n s e r e c o v e r y in the auditory c o r t e x under Nembutal anesthesia is the n e a r l y complete disappearance of the second phase of the relative r e f r a c t o r y period,with the r e s u l t that its duration is shortened to 200 or 300 m s e c (Fig. 5). The r e s p o n s i v e n e s s r e c o v e r y p r o c e s s of the auditory c o r t e x under the influence of c h l o r a l o s e (15-20 m g / k g intra-abdominally) has completely different c h a r a c t e r i s t i c s . With this chloralose dosage a p r o nounced increase in the amplitude of both the electropositive and the electronegative E P components in the auditory c o r t e x was produced. The duration of the absolute r e f r a c t o r y period was i n c r e a s e d here to 60 or 80 m s e c . In c o n t r a s t to Nembutal, the chloralose did not appreciably change the c o u r s e of r e s p o n siveness r e c o v e r y in the auditory c o r t e x during the second phase of the relative r e f r a c t o r y period. In o r d e r to study the dynamics of r e s p o n s i v e n e s s r e c o v e r y in the auditory cortex when in a state of alertness or active wakefulness, experiments were c a r r i e d out in the following manner. Recordings were made of five pairs of auditory c o r t e x r e s p o n s e s to two clicks with a p a r t i c u l a r interval between them using a cat in a drowsy or quietly wakeful state. Then, without interrupting the r e c o r d i n g , between the fifth and sixth p a i r s of clicks a s h o r t additional stimulation (a loud r a p or the cat call " p u s s - p u s s " ) was applied. Such an added stimulation produced a p r e l i m i n a r y r e a c t i o n and a m a r k e d desynchronization of the c o r t i c a l e l e c t r i c a l activity for 10-12 sec. This made it possible to r e c o r d during this time five or six paired r e sponses. The average values of the five paired r e s p o n s e s obtained before the additional stimulation were c h a r a c t e r i z e d by the r e c o v e r y level of auditory cortex r e s p o n s i v e n e s s for quiet wakefulness, and the

46

a v e r a g e values of the five succeeding paired r e s p o n s e s by the r e c o v e r y level f o r active wakefulness. I n a s m u c h as this was d e t e r m i n e d for e v e r y interval, it is possible to obtain f r o m the data of one e x p e r i m e n t two c u r v e s that c h a r a c t e r i z e the dynamics of r e c o v e r y for the quiet and active states of wakefulness. The e x p e r i m e n t s indicated that a state of a l e r t n e s s is accompanied by a reduction of the E P amplitude in the auditory cortex both for the conditioning and the testing clicks. However, the E P produced by the f i r s t click is r e duced m o r e substantially than the E P due to the second stimulus. This means that the degree of r e c o v e r y as d e t e r mined by the E P amplitude for the second click is for alm o s t all intervals g r e a t e r during a l e r t n e s s than in a state of quiet wakefulness. This difference is obliterated when the intervals between clicks are short. The duration of the absolute r e f r a c t o r y period for quiet and active wakefulness is the same. But the relative r e f r a c t o r y period is less in the latter case and runs to 200 or 300 m s e c (Fig. 5).

'qwv' toogv I

If~O I

sec

~VL ~a sec

Besides the e l e c t r o d e s for picking up the E P s in the auditory c o r t e x bipolar e l e c t r o d e s were implanted in the medial geniculate bodies of two cats. Stimulation was p r o vided by r e c t a n g u l a r c u r r e n t pulses at a voltage of 10-15 V with a threshold of 4-5 V and a duration of 0.2-0.5 m s e e .

The e x p e r i m e n t a l r e s u l t s indicated that an afferent pulse train produced in r e s p o n s e to a single stimulation of the geniculate body causes a change in the r e s p o n s i v e n e s s of the auditory c o r t e x which differs f r o m those produced by the action of a click. In this case there is a l m o s t no r e f r a c t o r y period; its duration is equal to the r e f r a c t o r y period of the t h a l a m u s - c o r t i c a l fibers. When the interval between stimuli is f r o m 5-1"0 m s e c , the total r e sponse to the two stimuli is g r e a t e r than for each s e p a r a t e l y (Fig. 6). When the intervals a r e between 30 and 100 m s e c , the E P amplitude for the second stimulation can be one and a half to two t i m e s g r e a t e r than for the f i r s t , i.e., the r e s p o n s i v e n e s s is not reduced but develops a m a r k e d facilitating effect (Fig. 6). These data agree with the r e s u l t s obtained by other investigators for stimulation of the geniculate body [10] and the thalamus [8]. Fig. 6. Responses evoked in a c a t ' s audito r y c o r t e x for paired stimuli of the medial genieulate body. The numbers on the left a r e the intervals between stimuli, in m s e c .

DISCUSSION

OF

RESULTS

Our experiments showed that a short acoustic click produces a prolonged change of the functional state of the auditory analyzer in an intact cat which is manifested by a decrease of the excitability and a reduction of the ElY amplitude in the auditory cortex for a repeated click. This process can be divided into three periods. The first,lasting 30-50 msec,is characterized by the complete loss of the auditory cortex's ability to react with an evoked potential to a testing click. After this comes a period during which there is a rapid and complete recovery of excitability as determined from the stimulation threshold, and an incomplete (up to about 70%) recovery of auditory cortex responsiveness as detern~tined by the EP amplitude for a testing click. At the end of this period a secondary reduction of the auditory cortex responsiveness occurs. The third period is characterized by a long duration (about 500 reset) and a slow recovery of the EP amplitude for a testing click while there is a complete recovery of excitability as determined by the stimulation threshold. These same three periods in the dynamics of recovery are observed when studying the ElYs of the somatosensory cortex for paired stimuli [14, 18]. The m e c h a n i s m s behind the changes in r e s p o n s i v e n e s s of the c e r e b r a l c o r t e x in these three periods are obviously different. Since the ElY amplitude of the c e r e b r a l c o r t e x depends not only on the p r o c e s s e s

47

in the cortex but also on the ways in which an afferent pulse train is brought to it, the reasons for a reduction of the E P from a testing click can be of both cortical and extracortical origin. The fact that an afferent pulse train arriving at the auditory cortex by a direct stimulation of the geniculate body does not produce a refractory state in it forces us to conclude that the absence of an EP for the testing click during an absolute refractory period is explained, not by the refractoriness of the cortex itself, but by the delay of the testing afferent pulse train in its path to the auditory cortex. Recently it has been shown that when an afferent pulse train passes through the thalamus it induces a prolonged inhibition in it [6, 7]. This inhibition,lasting up to I00 msee,may be the reason for the complete delay or the weakening of a testing afferent pulse train during the absolute refractory period and the initial phase of the relative refractory period. Such a proposal was made in a study of refractoriness in the somatosensory cortex [8]. An inhibition of approximately the same duration developed in the medial geniculate bodies when an afferent pulse train evoked by a click passed through them [9]. But whether this inhibition is the only reason for the ineffectiveness of a testing click during an absolute refractory period is difficult to say. The data of our experiments concerning the close relationship between the absolute refractory period and the electronegative component of the E P from the auditory cortex point to the participation of cortical mechanisms in its origin. Such a relationship has been observed for the visual cortex [12]. It is known that excitation of the reticular formation in the brain stem by any afferent stimulation leads to a reduction in the EP amplitude of the auditory cortex. Therefore it is possible that the reduction of the EP amplitude for a testing click in the intervals from 200-600 msee results from a shoFt-term excitation of the reticular formation by the conditioning click. This assumption is supported by the fact that Nembutal anesthesia shortens this part of the refractory period and makes it less distinct. This same effect is found for a state of alertness,which also points to a connection between the refractory period and the function of the reticular formation of the brain stem. Thus, the reason for the absence and the reduction of EP in the auditory cortex for a testing click with certain intervals between clicks is not the passive lowering of excitability like the ordinary type of refractoriness,but an active cooperation of excitation and inhibition processes in the cerebral cortex, the geniculate bodies, and the reticular formation of the brain stem. Consequently, the widespread designation of this phenomenon by the terms "absolute" and "relative ~ refractory periods is purely arbitrary and is based only on the superficial resemblance. Our experiments have shown that when answering the question of the influence of anesthesia on the recovery of cerebral cortex responsiveness after a conditioning stimulation, it is necessary to take into account both the type and depth of the anesthesia as well as the characteristics of the EP produced by this stimulation. With shallow Nembutal anesthesia a conditioning click can produce both a weakening and a strengthening of the auditory cortex responsiveness. A controlling factor here is the shape of the EP for the conditioning click. CONCLUSIONS The minimum interval between two acoustic clicks for which the second click produces an E P in the auditory cortex of a eat is between 30 and 50 msec. The relative refractory period consists of two parts. The first (50 to i00 msee) is characterized by a rapid recovery, and the second (about 500 msec) by a slow recovery. The recovery of excitability as determined by the stimulation threshold occurs after 80-150 msec. The duration of the absolute refractory period depends on the duration of the initial EP components for the conditioning click. No E P is produced by a testing click during the development of the electropositire excursion and the ascending phase of the initial electronegative excursion of the EP for the conditioning click. Unlike the afferent pulse train produced by a click, the discharge of pulses arriving at the auditory cortex in the case of direct stimulation of the medial gen[culate body does not leave after it a long refractory period. When the interval between stimuli is from 5-10 msec, a summing of the effects from the two stimuli takes place, but when the intervals are from 3 0-i00 msec, a marked facilitating effect is observed. Nembutal shortens the relative refractory period to 200 or 300 msec. Under shallow Nembutal anesthesia a conditioning click may produce either a weakening or a strengthening of the auditory eortex's respons iveness.

48

C h l o r a l o s e c a u s e s a lengthening of the absolute r e f r a c t o r y period which ]s r e l a t e d to its ability to i n c r e a s e the e l e c t r o n e g a t i v e component of the El% The m e c h a n i s m s that d e t e r m i n e the r e f r a c t o r i n e s s of the c e r e b r a l c o r t e x following a conditioning click include p r o c e s s e s that take place both in the c o r t e x itself and in the m e d i a l geniculate bodies and the r e t i c u l a r f o r m a t i o n of the b r a i n stem.

LITERATURE I. 2. 3. 4.

5. 6. 7. 8. 9. i0. Ii. 12. 13. 14. 15. 16. 17. 18.

CITED

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