Slow potential changes from cat cortex and classical aversive conditioning

399

Electroencephalography and Clinical Neurophysiology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

SLOW

POTENTIAL CLASSICAL

CHANGES AVERSIVE

FROM

CAT CORTEX

CONDITIONING

AND

1,2

JOHN R. CHIOR1NI, P H . D . 3

DhTsion of Electroencephalography and Neurophysiology. Psychopathic Hospital, and Department of Psychiatry, Unil:ersity of lowa, College of Medicine, Iowa City, Iowa 52240 (U.S.A.) (Accepted for publication: July 17, 1968)

INTRODUCTION

Variations in the amplitude of surface slow potential changes (SPCs) have been reported to occur in both man and infrahuman species when subjected to certain experimental procedures. Gumnit (1961) reported shifts in amplitudes of SPCs which decreased with repeated presentation of auditory stimuli and recovered when changes in the stimulus characteristics occurred. Rusinov(1960) and Mnukhina (1961) demonstrated increases in SPC amplitude during progressive trials of classical conditioning. Mnukhina (1961), Walter et al. (1965) and McAdam (1966) reported decreases during later conditioning trials. Similar decreases were observed by Rowland and Goldstone (1963) and Low et al. (1966) during progressive extinction trials in differential conditioning and by Rusinov (1960) in classical conditioning. SPCs have been related to periods of behavioral arousal, both spontaneous (Caspers 1961; Wurtz 1965) and experimentally induced (Rowland and Goldstone 1963; Wurtz 1966), as well as to electrical stimulation of the midbrain reticular formation (MRF) (Arduini 1958). Rusinov (1960) and Mnukhina (1961) have interpreted the SPC which occurs during the 1 This report formed the main portion of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, in the Department of Psychology, University of Iowa, August, 1966. 2 This research was supported in part by the Public Health Service Pre-Doctoral Fellowship Grant MH22, 388-03 from the National Institute of Mental Health. 3 Present address: Personnel Research Division (HRPA), Lackland Air Force Base, Texas 78236.

development of a conditioned response as a sign of the formation of temporary connections between CS and UCS. However, it is possible that the SPCs related to the development of conditioned responses (CRs) may only be indices of generalized arousal, meditated by MRF. Since MRF is a midline structure, activity within it should be reflected in fairly generalized and bilaterally symmetrical changes in cortical activity. If SPCs which increase with the development of CRs and decrease with habituation are indices of energizing or arousing factors, they should similarly be generalized and bilateral. Such symmetry has been observed in SPCs following stimulation of MRF as well as in some conditioning situations. Rusinov (1960) and Mnukhina (1961), however, both reported that, following the initial diffuse cortical shifts, localized SPCs in sensori-motor areas and primary sensory areas occurred concomitantly with the development of CRs. Whereas diffuse SPCs may be related to activity in the MRF, these localized shifts suggest that another neural system is involved in the elicitation of SPCs associated with learning. The present investigation was designed to study the course of variations in SPC amplitude during classical aversive conditioning (leg flexion). To measure the degree of cortical localization of the SPC, bilateral electrode placements were chosen. Since localized changes in SPC amplitude may be related to the formation of temporary connections during conditioning, data were obtained from two groups of identically treated subjects, one group showing a high level of conditioning ("learning group") Electroeneeph. elin. Neurophysiol., 1969, 26:399-406

400

J.R. CHIORINI

and one group showing only a limited amount of conditioning ("non-learning group"). A peripheral measure of "orienting" was included by measuring eye movements, and placing the source of the CS asymmetrically. METHODS

Subjects were 19 experimentally naive adult cats. Epidural Ag-AgC1 electrodes (Rowland 1961) were stereotaxically placed (Jasper and Ajmone Marsan 1960) over right and left anterior (F29.5, L3) and posterior (F26.5, L7) sigmoid gyri. All operations were carried out aseptically using sodium pentobarbital anesthesia. The reference electrode for SPC recording was an Ag-AgC1 disk clipped to the pinna of the right ear or to the skin of the back of the neck. During the training trials, each cat was placed in a canvas and aluminum sling and suspended from the ceiling of a conditioning chamber located in a sound-proofed room. The conditioned stimulus (CS) was a 1600 msec, 1000 p/sec, 80 dB, complex tone generated by a Grass S-4 stimulator and presented to the right of the animal's head. During the last 100 msec of the CS an unavoidable 150 V DC shock (UCS) was delivered to the left hind paw via screw-clamp electrodes. The UCS was generated by a Grass S-4 and SIU-4B stimulus isolation unit. The CS and UCS terminated simultaneously. Although McAdam et al. (1965) demonstrated that 500 msec was the optimal inter-stimulus interval for establishing classical aversive CRs in the cat, a relatively long interval (1500 msec) was chosen in the present experiment. In preliminary research it was found that with this interval a stable and high level of conditioned responding could be obtained in about 50 ~o of the animals tested. Thus, two groups of subjects (learning and non-learning) could be obtained while holding parameters of the experimental situation constant. This long interval also provided a more adequate period of time for observing SPCs in the CS-UCS interval. Horizontal eye movements were measured by Ag-AgC1 disk electrodes placed at the outer canthi. These electrodes were held in place by an "Ace" elastic bandage which assisted in immobilizing the head of the animal. Leg flexions were

measured by passing a loop of line around the left hind leg of the animal and securing it to a wheel which turned a 10 kf2 potentiometer that formed one arm of a Wheatstone bridge. Displays of eye movements, leg flexions and SPCs were made with a Grass Model 7 polygraph using Grass Model 7PIA DC chopper pre-amplifiers. A sample trial is shown in Fig. 1. A calibration signal introduced in ser;es with the electrodes, and written out on th,z EEG record 250 msec before the presentation of the CS, was

IP$1.SOMATOSENSO2YIOfOuVC©I I~f __. ._

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,

CONTRA.SO:.'AI~SENS~_ Y. ~ IPSI. MOTOR

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EYE MOVEMENT

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•

II

^ "

"

._. "I'LL

- -MOVE. - - - -LEFT ~..~__ CR / -

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jr CS ,

J UCS

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Fig. I Sample of polygraph display of electroencephalographic responses from ipsilateral and contralateral somatosensory and motor cortex (an upward deflection indicates negativity of the cortex relative to the reference) of eye movement and of anticipatory leg flexion. Temporal relationship of the CS and UCS is illustrated at the bottom of the figure.

included with each trial. Measurements for the SPC were of the maximum deflection in mm relative to a predetermined baseline during the inter-stimulus interval. Using this method, fortythree SPCs were measured independently by the author and by another trained observer; the correlation between them was +0.94 (F = 295.0 d f = 1/41, P < 0.001). Prior to starting any experimental procedures each animal was allowed a one week period for post-operative recovery. On each experimental day, approximately 15 min before being placed in the apparatus, each animal was given 30-35 mg of bulbocapnine hydrochloride i.m. This made it Electroenceph. clin. Neurophysiol., 1969, 26:399--406

DC CHANGESAND CONDITIONING

-6oq

~:: ~re tractable but did not seem to impair con-

d.~tioning (McAdam et al. 1965) 1. Each animal was given 30 trials per day. Fifty habituation trials (30 on day 1, 20 on day 2) were ~,:ven with CS alone. Paired C S - U C S trials were ti~en presented (10 on day 2, 30 per day thercafl-~') until a criterion of 90 % CRs in a block of 30 tri ! ; for a single day was reached, or until , '4 ::;d been given. To develop a stabilized ~ R in am ".Is reaching the 90 % criterion, additional CS -LCS pairings were given in a number equal to h t the trials required to reach criterion. Followin~ his period of "overlearning", extinction trials with CS alone were given until conditioned responding had decreased to 10 % orless in a block of 30 trials. Those cats failing to reach the 90 % criterion of conditioning in 300 trials were given an arbitrary total of 100 extinction trials. At the completion of the experiment, each animal was sacrificed under sodium pentobarbital anesthesia and the brain perfused with normal saline and 10 % formalin. Data based upon electrodes which penetrated the cortex were not included in the statistical analysis. The conditioning and extinction data for the learning group were analyzed in 10ths of total trials. The habituation data for both groups, and the extinction data for the non-learning group, were analyzed in blocks of 10 trials. The CS-UCS trials for the non-learning group were analyzed in blocks of 30 trials (i.e., in 10ths of the arbitrary totals). Unless otherwise stated all results discussed as "significant" were so at or below the P = 0.05 level.

401

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I Leorning Group: SornotooensoryCodex ~ C0ntroloteral p .a. • ,:.--.-o Ipsilateral ,s , . 4 ~ -40 ~ - ~ Log Rexions fl"

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HABITUATION IOths OF 5ths OF BLKS OF CONDITIONING OVER- lOthsOF EXT)NCTION iO TRIALS LEARNING

~

Fig. 2 Acquisition data for conditioned leg flexions in the learning group, together with the amplitude data of the SPCs from contralateral and ipsilateral somato-sensory cortex. ditioned leg flexions in the learning group, together with the amplitude data of the SPCs from somato-sensory cortex, contralateral and ipsilateral to the left hind leg (the leg to which the UCS was delivered and the movements of which were scored as CRs). SPCs from both contralateral and ipsilateral areas showed significant decreases in amplitude over progressive habituation trials as measured by Ftests for trend (Hayes 1963). During the C S - U C S interval SPCs from contralateral somato-sensory cortex were larger during conditioning (F = 5.13, d f = 1/9, P < 0.05) and overlearning (F = 7.89, d f = 1/9, P < 0.025) than were those from the ipsilateral area. Both areas showed decreases in the amplitude of the

RESULTS

TABLE I

The mean number of trials to reach the 90 % criterion of conditioned leg flexion in the learning group (N = 11) was 118 (range: 40-220) trials. The mean number of trials to reach the 10% criterion of extinction was 108 (range: 40-370) trials. Eight cats failed to exceed 36 % CRs in any block of 30 trials during 300 C S - U C S pairings; these formed the non-learning group. Fig. 2 presents the acquisition data for con-

SPC amplitude differences between the last habituation trial and the 5th conditioning trial (t tests for related measures)

1 G. Marsh, working in this laboratory, has recently obtained data suggestingthat bulbocapnine hydrochloride, in the same doses used here, makes conditioning more rapid, perhaps by diminishing extraneous movementproduced stimuli (personal communication).

Learning group

Area

Non-learning group

t

df

t

df

Somato-sensory ipsilateral contralateral

1.84" 1.37

9 9

0.31 1.02

6 6

Motor ipsilateral contralateral

1.93* 2.46*

9 9

1.05 2.27*

7 7

* P< 0.05. Electroenceph. clin. Neurophysiol., 1969, 26: 399-406

402

J.R. CHIORINI -60-

"60-

-I

-50 t

Non-Leorning Group:Somotosensory Cortex

-40-

Controloterol e--.~ Ipsiloterol

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* - - " Leg Flexions

-3o

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-20

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Non-Learning Group:Motor Cortex

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•'--" Contralatero[ c-~ Ipsilateral • ""* Leg Flexions n:8

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+lO

HABITUATION CONDITIONING EXTINCTION BLKSof IO BLKSof ~0 TRIALS 8LKS of IOTRIALS TRIALS

HABITUATION CONDITIONING BLKSof I0 BLKSof 30 TRIALS TRIALS

Fig. 3 Acquisition data for conditioned leg flexions in the n o n learning group together with amplitude data of the SPCs from contralateral and ipsilateral somato-sensory cortex.

Fig. 5 Acquisition data for conditioned leg flexions in the n o n learning group together with amplitude data of the SPCs from contralateral and ipsilatera[ motor cortex.

Learning Group : Motor Cortex

-60-

Ipsiloteral Leg Flexions ~ - *

-50~-40-

g

Learninq Group Non-Leorning Group (ps o e o ~.^ m use ~ [psifoterol~ m I n n=7 Oonrole erol~b° o nso y ~ ContralateraFSo o use sort ~ * Ipsiloterol ~'Mo or H Ipsiloterol~ -Mo o n=8 -60q ~ CnnlrolateraP ~ 3 Controluteraf/

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EXTINCTION BLKSof I0 TRIALS

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HABITUATION BLKS OF JO TRIALS

IOths OF CONDITIONING

5ths OF OVER- IOths OF EXTINCTION LEARNING

Fig. 4 Acquisition data for conditioned leg flexions in the learning group together with amplitude data of the SPCs from centralateral and ipsilateral motor cortex.

SPCs during overlearning, measured by F tests for trend, although CRs remained at asymptote during this phase of training. There was an increase in mean negativity of the SPCs in the first block of extinction trials in both contralateral and ipsilateral areas (Table l) compared with the last block of overlearning trials. During extinction trials both areas showed significant decreases in SPC amplitude over progressive blocks (F tests for trend). Fig. 3 presents the acquisition data for the conditioned responses for the non-learninggroup, together with the SPC data from somato-sensory cortex. Analyses of variance showed no statistically significant differences between mean am-

+40

i

V- ~

'

~

LAST Ist 5 CS-UCS HABITUATION PAIRINGS TRIAL

I

LAST 1st 5 CS-UCS HABITUATION PAIRINGS TRIAL

Fig. 6 Amplitude data of SPCs for the first five CS U C S pairings and the last habituation trail for both the learning and non-learning groups.

plitude for the SPCs from the two areas during any of the phases of training, nor was there any significant change in SPC amplitude over progressive habituation or conditioning trials. The nonlearning group did show significant increases in mean amplitude of SPCs in the first block of extinction trials (Table I) and significant decreases in amplitude over progressive extinction trials (F tests for trend), in both areas. Fig. 4 presents SPC amplitude data from contralateral and ipsilateral motor cortex in the learning group together with the conditioned leg flexion data. Both areas showed significant decreases in SPC amplitude during habituation (F Electroenceph. olin. Neurophysiol., 1969, 26 : 399-406

403

DC CHANGES AND CONDITIONING

tests for trend). During conditioning, SPCs from contralateral motor cortex showed a greater mean amplitude than those from the ipsilateral area (F = 5.44, d r = I / 9 , P < 0.05). A separate analysis of a trials effect for contralateral motor cortex during this phase was non-significant. SPCs for the contralateral area did show a decrease in amplitude during overlearning (F test for trend). Both areas showed significantly larger negative SPCs on the first block of extinction trials than on the last block of overlearning trials (Table I), and both areas showed a decrease in SPC amplitude over extinction trials (F tests for trend). In addition to this trials effect, during extinction SPCs from contralateral motor cortex were significantly greater in mean amplitude than those from the ipsilateral area (F = 5.57, df = 1/9, P < 0.05). Fig. 5 presents the SPC data from the motor cortex of the non-learning group together with the CR acquisition data. There were no significant area differences in SPC amplitude during any of the phases of training. The only significant change in amplitude over progressive trials occurred during extinction, both areas showing significant decreases in SPC amplitude (F tests for trend). The only transitory change in SPC amplitude accompanying a change in training procedures occurred as an increase in SPC amplitude during the first block of C S - U C S pairings compared with the last block of habituation trials in the contralateral motor area (Table D. Analyses were also made for the first five CS-UCs pairings and the last habituation trial TABLE II SPC amplitudes for trials on which CRs occurred vs. trials on which no CRs were made. Data taken from learning group, conditioning blocks 3-6 inclusive (t tests for related means) Area

*P < 0.05.

t

df

Somato-sensory ipsilateral contralateral

3.27* 4.40*

7 7

Motor ipsilateral contralateral

2.47* 3.70"

7 7

T A B L E III

SPC amplitude differences between the first block of extinction trials and the last block of CS-UCS pairings (t tests for related means) Learning group

Area

Non-Ie~rning group

t

df

t

df

Somato-sensory ipsilateral contralateral

2.56* 2.29*

9 9

4.72* 2.35*

6 6

Motor ipsilateral contralateral

2.72* 3.22"

9 9

1.37 1.43

7 7

.

.

.

.

.

* P < 0.05.

for both groups (Fig. 6 and Table II). In contrast to the systematic increase in amplitude of the SPCs in all areas which was shown by the learning group, the non-learning group showed considerable trial to trial variability during this period. Comparison was made between the last habituation trial and the fifth C S - U C S pairing for each area in both groups. In the learning group, all areas except contralateral so~nato-sensory cortex showed significant increases in this period but the non-learning group failed to show any such differences. Comparisons were also made for the learning group between SPC amplitudes on trials in which CRs occurred and those in which no CR was present. All CS-UCS trials in blocks 3-6 were used for this comparison. All response vs. non-response differences were statistically significant (Table lII), negative SPCs on trials in which CRs appeared having greater mean amplitude than on trials in which no CR appeared. The mean percentage of eye movements to the right or left, and mean amplitude of SPCs from each cortical area by tenths of trials, did not show systematic covariation in either group of subjects, as tested by Spearman rank order correlations. DISCUSSION

The present investigation has demonstrated variations in the amplitude of cortical slow potentials which are in substantial agreement with those described in other conditioning studies. The decrease in SPC amplitude during habiElectroenceph. c/in. Nettrophysiol., 1969. 2 6 : 3 9 9 - 4 0 6

4C4

J. R. CHIOR1N[

tuation and the increases during the first five CS-UCS pairings seen in the learning group, as well as the decreases over progressive extinction trials seen in both groups, parallel changes which would be predicted to occur in a general level of arousal. The decreases in SPC amplitude during overlearning find parallels in observations of similar decreases in the percent or intensity of cortical low voltage fast activity during overlearning trials (Beck et al. 1958; Thompson and Obrist 1964). These decreases may reflect a process of habituation to the CS-UCS pairing. They may also reflect the occurrence of a spatial shift in maximal excitation, so that a highly restricted focus develops in the cortex but is no longer sampled by the specific electrode placements (Mnukhina 1961). During CS-UCS pairings localized enhancement of SPCs occurred in cortical areas presumed to be directly involved in the elaboration of the conditioned reponse. These differences between contralateral homologous cortical loci, in contrast to the bilaterally symmetrical shifts shown during habituation and extinction, may reflect processes related to the actual mechanism of the CR, since differences were also seen in SPCs during trials in which a CR appeared, in contrast to trials in which CRs failed to appear. Since two types of SPCs have been observed, bilaterally symmetrical and unilateral, it seems probable that two separate neural systems be involved in their production. Activity in the MRF is commonly associated with increased arousal or activation. Stimulation of M R F produces desynchronization of the cortical EEG ("arousal" in the physiological sense) and also development of concomitant diffuse bilateral cortical negative SPCs (Arduini 1958). This suggests that those symmetrical changes in SPC amplitude seen during habituation and extinction reflect activity in the midbrain reticular formation and represent changes in a general level of arousal. With respect to the unilateral SPCs, related to the actual development of CRs, Arduini (1958) has shown that ipsilateral negative shifts in "sensori-motor" cortex may be elicited by electrical stimulation of nucleus ventralis lateralis. Jasper et al. (1955) reported recruiting responses (in other studies usually associated with cortical negativity) in ?osterior sigmoid gyrus following

repetitive stimulation of nucleus ventralis anterior, although the effects of electrical stimulation of this structure on cortical negativity has not yet been specifically studied. The involvement ofn. ventralis anterior in mediation of conditioned responding has been previously suggested by studies of John and Killam (1959) and McAdam (1962), and lesions in this area have been reported to alter or eliminate thalamically induced cortical recruiting responses (Weinberger et al. 1965). It thus appears possible that the local SPCs observed during conditioning and overlearning represent activity originating in the ventral thalamus and that this region may be involved in the development and maintenance of the conditioned response. SUMMARY

1. The present study was designed to investigate variations in the amplitude of slow electrical potential changes (SPCs) recorded from motor and somato-sensory cortex of cats undergoing classical leg flexion conditioning. Eleven cats (the "learning group") reached the behavioral criterion of 9 0 ~ CRs. Eight cats who failed to reach conditioning criterion in 300 trials formed a "non-learning" comparison group. 2. In the learning group negative SPCs from all cortical areas sampled showed decreasing amplitudes during habituation and extinction. All areas, with the exception of the motor cortex ipsilateral to the left hind leg of the animal (the leg receiving the UCS), showed similar decreases in amplitude during overlearning. Slow potential changes from the non-learning group showed consistent decreases only during extinction trials. In the learning group, negative SPCs recorded from contralateral motor and somato-sensory areas during conditioning showed greater mean amplitude than those recorded from the respective ipsilateral areas. In the non-learning group no significant differences appeared between contralateral and ipsilateral areas during CS-UCS pairings. 3. No relation between SPCs and eye movement could be established. 4. It was concluded that the SPC reflects two processes representing the activity of two or more subcortical systems. The equal bilateral changes in SPC amplitude seen in both the learning and Electroenceph. olin. Neurophysiol., 1969, 26:399~,06

DC CHANGES AND CONDITIONING

non-learning groups were attributed to a diffuse activation via a system such as the midbrain reticular formation. The enhancement of negative SPCs restricted to the contralateral motor and somato-sensory cortices in the learning group, during acquisition, was attributed to a "localized" activation system. This could involve the nucleus ventralis lateralis and nucleus ventralis anterior of the thalamus. The diffuse system may be involved in the maintenance of general levels of "arousal", or "activation", a view commonly held. The localized system may be necessary for the elaboration and development of conditioned responding, but does not appear to be necessary for the maintenance of such behavioral activity. RESUME VARIATIONS LENTES DE POTENTIEL AU NIVEAU DU CORTEX DU CHAT ET COND1TIONNEMENT ADVERSIF CLASSIQUE

1. Cette 6rude a pour but de rechercher les modifications de I'amplitude des variations lentes de potentiel 61ectrique (SPCs) enregistr6es au niveau du cortex moteur et somato-sensoriel de chats sous conditionnement classique de flexion de la jambe. Onze chats ("groupe bien entrain6") ont atteint le crithre comportemental de 90 ~ de r4ponses conditionn6es. Huit chats qui n'ont pas atteint ce crithre de conditionnement aprhs 300 essais ont constitu6 un groupe de comparaison ("non entrain6"). 2. Dans le groupe entrain4 les SPCs nOgatifs au niveau de toutes les aires coricales 6chantillonn6es montrent des amplitudes d4croissantes pendant l'habituation et l'extinction. Toutes les aires,/i l'exception du cortex moteur ipsilat6ral de la patte post6rieure gauche de l'animal (jambe recevant les UCS) montrent des diminutions similaires d'amplitude pendant le sur-apprentissage. Les variations lentes de potentiel dans le groupe non-entrain0 ne montrent de diminution nette que pendant les essais d'extinction. Darts le groupe entrain6, de SPCs n6gatifs enregistr6s pendant le conditionnement au niveau des aires motrices et somato-sensorielles contro-lat6rales, montrent une amplitude moyenne plus grande que ceux enregistr6s au niveau des aires ipsilat6rales correspondantes. Dans le groupe non-entrain6 aucune diff6rence significative n'apparait

405

entre les aires contro-lat6rales et ipsilat6rales pendant les couplages CS-UCS. 3. Aucune relation n'a pu ~tre 6tablie entre les SPCs et les mouvements oculaires. 4. Les auteurs concluent que le SPC reflbte deux processus repr6sentant l'activit6 de deux syst6mes sous-corticaux ou plus. Les variations d'amplitude du SPC 6gales des deux c6t6s, observ6es dans les deux groupes entrain6 et non-entraTn6, sont attribu6es/t une activation diffuse au travers d'un systbme tel que la formation r6ticulaire de la ligne m6diane. Le fait que l'augmentation des SPCs n6gatifs est limit6e aux cortex moteur et somato-sensoriel contro-lat6raux dans le groupe entraTn6, pendant l'acquisition, est attribu6 5, un systbme d'activation "localis6". Celui-ci pourrait inclure le noyau ventral lat6ral et le noyau ventral ant6rieur du thalamus. Le syst6me diffus peut 6tre impliqu6 dans le maintien des niveaux g6n6raux d"'arousal" ou d'"activation", point de vue g6n6ralement accept6. Le syst6me localis6 peut 8tre n6cessaire 5. l'61aboration et au d6veloppement des rdponses conditionn6es, mais ne semble pas 8tre n6cessaire au maintien d'une telle activit6 comportementale. The author wishes to express his appreciation to Professor J. R. Knott, who directed this research, and to Dr. D. W. McAdam, for their assistance and suggestions.

REFERENCES

ARDUINI,A. Enduring potential changes evoked in cerebral cortex by stimulation of brain stern reticular formation and thalamus. In H. H. JASPERet al. (Eds.), Reticular formation of the brain. Little, Brown and Co., Boston, 1958: 333-351. BECK, E. C., DOTY, R. W. and KO0t, K. A. Electrocortical

reactions associated with conditioned flexion reflexes. Electroenceph. clin. Neurophysiol., 1958, 10: 279-289. CASPERS,H. Changes of cortical D. C. potentials in the sleep-wakefulness cycle. In G. E. W. WOLSTENHOLME and M. O'CoYNOR(Eds.), The nature of sleep. Little, Brown and Co., Boston, 1961 : 237-253. GUMN~T,R. J. The distribution of direct current responses evoked by sounds in the auditory cortex of the cat. Electroenceph. clin. Neurophysiol., 1961, 13: 889-895. HAYES,W. L. Statistics for psychologists. Holt, Rinehart and Winston, New York, 1963, 719 p. JASPER,H. H., and AJMONE MARSAN,C. ,4 stereotaxic atlas of the diencephalon of the cat. Nat. Res. Council of Canada, Ottawa, 1960. JASPER,H. H., NAQUET,R. and K~NG, E. E. Thalamocortical recruiting responses in sensory receiving areas in the cat. Electroenceph. elin. Neurophysiol., 1955, 7:99-114.

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406

J . R . CHIORINI

JOHN, E. R. and KILLAM, K. F. Electrophysiological correlates of avoidance conditioning in the cat. J. Pharmacol. exp. Ther. 1959, 125: 252-274. Low, M. D., BRODA, R. P., FROST, J. D., KELLAWAY, P. and GOL, A. Surface-negative slow potential shift associated with conditioning in man. Neurology (Minneap.), 1966, 16: 771-782. MCADAM, D. W. Electroencephalographic changes and classical aversive conditioning in the cat. Exp. Neurol., 1962, 6: 357-371. MCADAM, D. W. Slow potential changes recorded from human brain during learning of a temporal interval. Psychon. Sei., 1966, 6: 435-436. MCADAM, D. W., KNOTT, J. R. and CHIORINI, J. R. Classical conditioning in the cat as a function of the CS-US interval. Psychon. Sci., 1965, 3: 89-90. MNUKHINA, R. S. Electroencephalographic analyses of the mechanism of temporary connexion closure. Parlor J. higher nerv. Activ., 1961, 11: 359-366. ROWLAND, V. Simple non-polarizable electrode for chronic implantation. Electroenceph. elin. Neurophysiol., 1961, 13: 290-291. ROWLAND, V. and GOLDSTONE, M. Appetitively condi-

tioned and drive-related bioelectric baseline shift in cat cortex. Electroenceph. clin. Neurophysiol., 1963, 15: 474-485. Rus~Nov, V. S. General and localized alternations in the electroencephalogram during the formation of conditioned reflexes in man. Electroeneeph. clin. Neurophysiol., 1960, Suppl. 13: 309-319. THOMPSON, L. W. and OBRIST, W. D. EEG correlates of verbal learning and overlearning. Eleetroenceph. clin. Neurophysiol., 1964, •6:332-342. WALTER, W., COOPER, R., MACCAI.LUM, R. C. and COHEN, J. The origin and significance of the contingent negative variation or "expectancy wave". Electroenceph. clin. Neurophysiol., 1965, 18: 720. WEINRERGER, N. M., VELASCO, M. and LINDSLEY, D. B. Effects of lesions upon thalamicatly induced electrocortical desynchronization and recruiting. Electroenceph. clin. Neurophysiol., 1965, 18: 369-377. WURTZ, R. H. Steady potential shifts during arousal and deep sleep in the cat. Electroenceph. olin. Neurophysiol., 1965, 18: 649-662. WURTZ, R. H. Steady potential correlates of intracranial reinforcement. Electroenceph. c#n. Neurophysiol., 1966, 20 : 59-67.

Reference: CHtORINI, J. R. Slow potential changes from cat cortex and classical aversive conditioning. Electroenceph. clin. Neurophysiol., 1969, 26: 399-406.

ANNOUNCEMENT INTERNATIONAL

SYMPOSIUM

ON THE

PATHOGENESIS

OF EPILEPSY

Varna, October 7 8, 1969

The Epilepsy Research Group at the Department for Medical Sciences, Bulgarian Academy of Sciences, on the basis of its traditional collaboration with the Institute of Neurology-Academy of Rumanian Soc. Republic, the Institute of Physiology-Czechoslovakian Academy of Sciences, the Institute of Physiology-Georgian Academy of Sciences and the Institute of Clinical and Experimental Neurology Georgian Department of Health, will organize an International Symposium on the Pathogenesis of Epilepsy. This Symposium is to be held at the Black Sea resort "Zlatny Pyasatsy" ("The Golden Sands"), near Varna, on the 7th and 8th October, 1969. Different aspects of the physiopathology, pathomorphology and biochemistry of clinical and experimental epilepsy will be treated in separate sessions. Invited lecturers will also discuss some basic problems of Brain Research. Reports and communications can be presented in

English, French, German or Russian. Applications for participation and titles of subjects should be sent as soon as possible. Summaries will be expected no later than March 31, 1969. The full text of reports and communications should be sent before June 30, 1969. Arrangements will be made for the publication of the Proceedings of the Symposium. We shall appreciate any kind of cooperation in the organization of the Symposium and especially the largest possible diffusion of information about this initiative among interested colleagues. Further information can be obtained from the Organizing Committee at the following address: Epilepsy Research Group, Sofia 31, Bulgaria. Organizing Cornmittee: President - Academ. G. Usunoff, Secretary - Dr. E. Atsev, Treasurer - Dr. D. Tchavdarov.

Electroenceph. clin. Neurophysiol., 1969, 26:406