Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex

effect, it does not necessarily mean that the inhibition is also with- drawn from the regions surrounding this area : under certain definite conditions the process of excitation in some areas of the cortex may prevail over the co-existing and somewhat weakened inhibition of the surrounding areas. It is, indeed, our experience that conditioned reflexes in which the phase of initial generalization has not been succeeded by sufficient specialization, as well as reflexes of small intensity of excitation, become very easily disturbed, both by external and by internal inhibition. Those strong conditioned reflexes in which the specialization has been carried to a high degree, on the other hand, are not so easily influenced by external or internal inhibition. The question of stability and of resistance of strong and well-developed positive and negative conditioned reflexes will be considered in the thirteenth lecture.

In order to elucidate these exceptional phenomena, which appeared to contradict the more general rule for the irradiation and concentration of inhibition, the following experiments were specially conducted by Dr. Pavlova. Conditioned reflexes were established to several stimuli belonging to different analysers, particular attention being directed to main- taining an equal number of repetitions of all the various stimuli, always, of course, with their reinforcement. After the maximum intensity of every one of the reflexes had been reached, a conditioned inhibition was developed for one of the reflexes b sual method of non-reinforcement of the corresponding sti hen applied. in combination with the conditioned inhibit he strengthening of the conditioned inhibition was carrie he point when the tion of the correspond- es. Of course, during the inhibition all the positive

ing stimulus became limited to two period of development of the co reflexes continued to be ing this stage of the experiment all tHe other stimuli, which up till then had not been combined wi he conditioned inhibitor, were now for the first time teste ompany with the inhibitor. It was found that these refle o, with only one exception, became free of inhibitory after-af%ct two minutes after initiation of the con- ditioned inhibit The exception in question was a weak conditioned * Gitex to the appearance of a somewhat dim light,

which wad Ip d to be still inhibited for some time after the that one and the same intensity of inhibition, when it had ceased to produce any diminution of the stronger reflexes, nevertheless exercised a definite action upon the weaker ones. This dependence of the inhibitory after-effect upon the intensity of the secondarily inhibited reflexes is only one of several factors complicating the otherwise smooth progress of irradiation and con- centration of inhibition. In the next lecture we shall describe another factor of much greater importance, producing a more profound disturbance in the progress of irradiation and concentration.

We shall turn now to a discussion of the experimental evidence with regard to the after-effect of the excitatory process ; this has not, however, yet been studied to so great an extent as the after- effect of the inhibitory process. The first experiments upon the after-effeet of excitation were conducted by Dr. M. Petrova on similar lines to the early experi- ments on the irradiation and concentration of inhibition. Five separate apparatuses for tactile stimulation of the skin were arranged at equal distances apart along the hind limb of a dog, the first of the series being placed over the paw and the fifth over the pelvis. In one animal the tactile stimulation of the paw (No. 1) served to evoke

evoke a conditioned alimentary reflex. Owing to initi efaliza- tion of the reflex, the stimulation of the remaining p (Nos. 2, 3, 4 and 5) also evoked at first positive alimentary r es or defence reflexes to acid. These places, however, my became dif- ferentiated by the method of contrast, givi ally a zero effect, so that only the stimulation of the paw r ned positive. At the beginning of each experiment the tactilégtimulation of the paw (the positive stimulus) was applied during {)séconds, and the amount of

salivary secretion was recorded d the first half and during the second half of the period of ulation. After 30 seconds of stimulation the reflex was rêi d. Then, after some interval of time, the paw was again lated, but now the stimulation was continued during only the girst 15 seconds, and was followed immedi- ately by stimulation of or another of the inhibitory places during the second 15 sé s. Results were obtained as follows. On changing the stiykyJus from the positive place (No. 1) to the nearest DUAIS S No. 2) the salivary secretion during the second

period of 15 seconds was diminished only slightly, or even not at all, as compared with the secretion observed during the first period of 15 seconds (action of apparatus No. 1). But when the remotest i apparatus (No. 5) acted during the second period of 15 seconds, the salivary secretion was considerably diminished. Stimulation of the | places Nos. 3 and 4 under identical conditions gave rise to a secretion | intermediate between that evoked, at places Nos. 2 and 5, the ) secretory effect at place No. 3 exceeding that at place No. 4.

In further experiments not only was the salivary secretion during the 15 seconds of the actual stimulation of the inhibitory places recorded, but also the entire secretory after-effect. In these experiments the difference in the effect of the nearer and the more remote stimuli was exhibited almost. without exception and with great distinctness. For example, stimulation of the nearest place (No. 2) evoked a far more prolonged and greater secretion than did stimulation of the remotest place (No. 5). The following experiments illustrate this case. The dog used in these experi- ments had its tactile conditioned reflexes established with the use of acid.

Salivary Secretion in drops during successive periods of 15 seconds, Salivary Secretion in : drops during successive periods of 15 seconds, starting with the preliminary application of the positive 2 T8 No. 1 N 2,10 Reinforced The results of iG Reina are very definite. In the cases when the ene Kos ulus, No. 1, was succeeded by the nearest inhibitory one, , the amount of salivary secretion recorded different Nisa 7, 7, 12 drops respectively, while the secretion

during the remaining period (action of No. 2 plus the secretory after-effect) was 15, 11 and 16 drops respectively. In the cases when the positive stimulus, No. 1, was succeeded by the remotest stimulus, No. 5, the secretion during the first 15 seconds amounted to 6, 8 and 8 drops respectively, while the secretion during the action of the remotest stimulus, No. 5, plus the secretion during the period of the excitatory after-effect amounted to only 3, 6 and 6 drops respectively. The total secretion subsequently to the first 15 seconds was, therefore, in the first case (with stimulus No. 2) definitely greater than during the 15 seconds’ action of the positive stimulus No. 1, and in the second case (with No. 5) definitely smaller. On changing from the positive stimulus, No. 1, to the nearest inhibi- tory stimulus, No. 2, the secretory after-effect persisted during 45-60 seconds, while on changing to the remotest stimulus, No. 5, the secretory after-effect persisted during only 15 seconds. These results assume a still greater significance when it is remembered that the inhibitory properties of the nearest stimulus, No. 2, ought to be greater than those of the remotest stimulus, No. 5, since, as we have learned already, the more delicate the differentiation the more intense is the underlying inhibitory process. .That this actually was the case was definitely established by special experi- ments, in which the inhibitory after-effects from Nos. 2 and 5 were tested upon the positive reflex from No. 1 and upon some conditioned stimuli from other analysers. The following is mple of the inhibitory after-effect upon the positive reflex C place No. 1,

as tested 44 minutes after the use of the Kee ry stimuli Nos. 2 and 5 respectively. The experiments Wary i dog as before. | Conditia) Salivary : stirĝylu Secretion in Time a d drops during 30 seconds The inhibitory after-effect following stimulation of place No. 2 was still very powerful after an interval of 44 minutes, whereas the inhibitory after-effect following stimulation of place No. 5 was negligible. In view of the results of these further experiments, there seems to be no reason to doubt that the excitation evoked by the positive stimulus in the previous experiments exerted a greater effect on the place nearest to it than on the place most remote.. The whole phenomenon must, therefore, be regarded as a result of an irradiation of excitation.

Some experiments upon the irradiation of excitation have recently been repeated by Dr. Podkopaev. The places for tactile stimulation were taken on the skin of one side of the body, the first apparatus being arranged over the fore paw and the remaining apparatuses being placed at equal-distances apart along the trunk and hind limb of the dog, the last being arranged. over the hind paw. Stimulation of the area over the fore paw was given the properties of a positive alimentary conditioned stimulus, while stimulations of the remaining areas were given the properties of negative stimuli. The salivary secretion was recorded during successive periods of five seconds. Usually the positive conditioned stimulus was con- tinued during 30 seconds and was then immediately reinforced, but for the purpose of the tests the stimulus was occasionall act during only 15 seconds, and was followed by a seconds before the reinforcement. In this manner Atas possible to follow the course of the secretion during 30 s€cends when the stimulus was acting throughout, and also durį Çò seconds when the stimulus was abbreviated to the firs O With the longer stimulation the ratio of the sec during the first 15 seconds to that during the second 15 S8¢conds came out as 1: 2-4;

` in the case of the abbreviated stimulajion it came out as 1 : 1-25. In the next stage of the experinfMyts the positive stimulus after acting during the first 15 secon) was followed immediately by an inhibitory stimulus which A Oye | the succeeding period of 15 seconds, and this eee as given by stimulation of a nearer or a remoter place. io of the amount of secretion during the first 15 seconds to AS sta the second 15 seconds came out in the former cas ae 1-35, and in the latter case as 1 : 0-53. It is obvious {ydrefore, that in this case, as in the previous experimen: RY excitatory after-effect of the positive stimulus

was strong in places nearest to it and negligible in the remoter places. Neither this last nor the preceding series of experiments permits us to determine with which phase of the movement of excitation we have to deal, whether the phase of irradiation or the phase of con- centration. Experiments are at present being conducted in which the inhibitory stimuli are applied, not immediately on cessation of the positive stimulus, but after a short pause of varying length. Unfortunately, the direct experimental data concerning irradiation and concentration of excitation at our disposal are limited to the facts described above. There are, however, a number of isolated observations which have some bearing upon this subject, and these will now be described.

One of our animals—a sheep-dog—had a very exaggerated form of what might be called “the warding reflex.” The experimenter [Dr. Besbokaya], who always worked with the dog in a separate room, used to prepare it for the experiment without the slightest difficulty, placing it in the stand and adjusting the various apparatuses for recording and for stimulation. Under Dr. Besbo- kaya’s control the experiments on conditioned reflexes proceeded quite smoothly, but the moment a stranger appeared in the experi- menting room the dog always developed a powerful aggressive reaction, especially if the stranger (in that case myself) took the place of the experimenter. Applying the establishe ppitioned ali- mentary stimulus under these circumstances, igor obtained a secretory reflex considerably above normal, og consuming its food with obviously exaggerated msc di This aggressive reaction evoked by my presence became minished in intensity when I intentionally kept quite sti e only part of the reflex still remaining was the gaze of t Os , which was kept fixed on me. The conditioned “iments gina now produced a much smaller salivary secretion than normal, and sometimes even no secretion at all. As soon, ever, as I relaxed my attitude, and especially if I moved ab e aggressive reaction returned in full - vigour, and the saling and motor reactions in response to the conditioned alim t@y stimulus became considerubly increased in their intensity. \tJis interesting to note that although this dog grew calm in oF presence of a stranger who kept quite still, the aggressive ion returned after each reinforcement after the end of the e

even though the stranger remained motionless. There appears to be only one possible explanation of this phenomenon. The powerful excitatory effect of the intruder and his movements must have irradiated over the hemispheres, and increased amongst others the excitability of that area which is related to alimentary functions. When, however, this excitation diminished on account of weakening and concentrating of the external stimulus the reverse took place, and the excitability of remoter areas became diminished as com- pared with the normal (effect of external inhibition). In a similar manner the eating of food temporarily heightened the excitability of the centre for the aggressive reaction.

A similar phenomenon was observed in experiments conducted [by Dr. Prorokov] with another exceptional dog. In this dog tactile stimulation of the skin evoked a peculiar and very definite motor reaction, akin to a reaction to tickling, and possibly of a sexual character. It elicited also a conditioned alimentary reflex which was abnormally powerful, exceeding any of the other conditioned alimentary reflexes to non-tactile stimuli. As a rule, tactile stimula- tion of the skin evoked first of all its own special motor reaction, and was then succeeded after 10-15 seconds by an exceedingly powerful alimentary motor reflex. When the special motor reaction was over- come by the use of a technique which will be discussed in the last lecture, the conditioned alimentary reflex to the tactileystimulus became reduced to a normal level, and became less RY ful, for example, than homogeneous reflexes to auditory uli. This phenomenon must unquestionably be regarded as her example

of irradiation of excitation. We are inclined to give a similar nein also to. the following fact. In the case of interactg conditioned reflexes based, upon the use of different sient stances the inhibition of some of them by others does not lop immediately, but only after an interval of some minute is quite conceivable that at first an irradiation of extatign Č Initiated from the cortical point corresponding to some defing mentary substance. The excita-

bility of neighbouring poi ereby becomes temporarily increased, and it is only afterv, hen the excitation gets weakened and concentrated, that thaiAhibition reveals itself [experiments of Dr. Savitch]. ° A similar ent can be advanced in explanation of some unexpect tion of heterogeneous conditioned reflexes to thermal stimuli. A conditioned reflex to acid was established by the application of a thermal stimulus of 0° C. to the skin of the animal, and a conditioned alimentary reflex by means of a thermal stimulus of 47°C. The development of these reflexes required a considerable time, and when finally established they were found to show the following extremely interesting peculiarity. If the conditioned reflex to cold (based on acid) was repeated with reinforcement several times in succession, the alimentary conditioned reflex to heat when evoked directly after- wards became transformed into a defence reflex to acid similar to that obtained with the stimulus of cold. Reversely, cold applied after several reinforcements of the conditioned alimentary reflex to heat brought about an alimentary reaction instead of the customary reaction to acid. The fact of this transformation was quite distinct and beyond doubt, since the nature of the reflex could easily be established by, for example, observing the motor reactions of the animal, which differ considerably for the two reflexes. In the case of the alimentary reflex the dog turned towards the experimenter, dividing its gaze between him and the source of food and smacking its lips ; in the case of the reflex to acid the dog turned away from the experimenter, snorting and whining, shaking its head and making ejective movements with its tongue. Moreover, the reflexes could easily be distinguished by the composition of the saliva from the submaxillary salivary gland, which in the case o at, alimentary reflex was thick and mucous, and in the case o SS flex to acid, fluid or watery. These points of difference stablished in the given case with especial care. The persist nd prolonged.

trans- formation of one reflex into the other he more strange and unexpected, since such a confusion LXcrogeneous reflexes never occurs in other analysers; for hays in the acoustic analyser, even if the two stimuli to hte on reflexes differ by less than a tone no transformation occùrð, although it might be. thought that the difference betwee o neighbouring tones was of much smaller vital RAEC n the difference between heat and of the analysers Yorjheat and cold. As is well known the peripheral ends of ge ysers are interspersed with one another over the

Shishlg eS he cortical ends of the two analysers overlap and it is tempting, therefore, to conjecture that the cortical ends are inter- spersed similarly to the peripheral ends. In that case irradiation from the one analyser to the other would be facilitated and such a confusion between the reflexes as is illustrated in the above experiments can be accounted for. I shall now return to a further consideration of a phenomenon which was referred to in the preceding lecture in connection with the analysing activity of the cortex, but which, from the point of view of its intimate mechanism, has to be dealt with, in this section. This concerns the initial generalization of any recently established con- ditioned stimulus, representing an apparently spontaneous positive activity of stimuli which had never previously been applied, but which constitute an allied group with the stimulus which had ac- quired conditioned properties on account of reinforcement. As an example we may take a series of experiments on the cutaneous analyser conducted by Dr. Anrep. Six apparatuses for tactile stimulation of the skin were arranged at definite places along one side of the body. The action of apparatus No. 0 at a place on the thigh was given positive alimentary conditioned properties in a short trace reflex. The remaining places were distributed as follows:

No. 1 on the hind paw, No. 2 on the pelvis, No. 3 on the middle of the trunk, No. 4 on the shoulder, No. 5 on the fore leg, No. 6 on the front paw. Stimulation of any of the latter place Nod a spontaneous conditioned secretory effect, although no hem had been combined with food; the intensity of thei retory effect decreased very gradually with the distance fro e place on the thigh, No. 0. The table gives the average es obtained with one of the three dogs employed for ej experiments. The amount of saliva is expressed in dre@pse4ch equal to 0-01 c.c. The secretion was recorded for the mA cecorids during which the stimulation lasted. ? |

The stability of g QMation varies considerably in different dogs, and it is not A OS a to keep it on a definite level during a series of experiment If the stimuli were applied to new places, and reinforced time, the reflexes would become completely . generalized, + & e other hand, if these stimuli remained without reinforcement an opportunity would be provided for the develop- ment of a differential inhibition. In view of this, and in order to secure more or less exact and comparable results which could be verified by repetition, the stimuli were alternately applied with and without reinforcement, or else the applications were repeated at long

It remains now to inquire into the intimate mechanism of generalization, and the following interpretation seems best to agree with our present knowledge. It may be assumed that each element of the receptor apparatus gains representation in the cortex of the hemispheres through its own proper central neurone, and the peri- pheral grouping of the receptor organs may be regarded as projecting itself in a definite grouping of nervous elements in the cortex. A nervous impulse reaching the cortex from a definite point of the peripheral receptor does not give rise to an excitation which is limited within the corresponding cortical element, but the excitation irradiates from its point of origin over the cortex, diminishing in intensity the further it spreads from the centre of excitation, exactly as we have seen in the case of inhibition. Just as the initial point of cortical excitation becomes connected with the centre of the uncon- ditioned reflex, so also do the secondary points of cortical excitation, and this leads to the formation of many accessory reflexes. These reflexes decrease progressively in strength with increasing distance of the secondarily excited areas from the point imary cortical excitation, since the magnitude of the conditi flex is rigidly determined by the intensity of excitation. ThiSaterpretation agrees with Dr. Koupalov’s observation that an e imental generalization of a localized tactile reflex develops ally and progressively, and that complete equalization of ct of stimulating any place of the skin is only reached noe iderable time. This, of course,

is only natural, since conditione@eflexes from points further removed from the primary excitation appear considerably later, the accessory tactile reflexes being m GQ yweaker to start with at the remoter places. The explanatio Chixced as regards the initial generalization of simultaneous.p@yitioned reflexes within a single analyser can be effectively applied to the type of universal generalization which is observed jndgng-trace reflexes (Lecture VII). In the case of the long pau ween the conditioned and the unconditioned stimulus, the ¢ ion initiated by the conditioned stimulus has time to

spread over the entire cortex, and in this manner all the individual cortical points can now link up with the centre for the unconditioned reflex as soon as this becomes excited. In the case of a short pause between the two stimuli the centre for the unconditioned reflex determines a concentration of the conditioned excitation towards itself, thus limiting any extensive irradiation over the general mass of the hemispheres. Mutual induction of excitation and inhibition: (a) Positive induction. (b) Negative induction.

THE first aspect of the intimate nature of the cortical activity which came into our field of investigation concerned the irradiation and concentration of excitation and inhibition. Only considerably later was a second aspect of this activity discovered and also subjected to a rigorous analysis. This second aspect concerned the reinforcing effect exerted by the one process upon the other, both in respect to the cortical points directly excited or inhibited and those into which the excitation or inhibition has irradiated.

This effect will be referred to as inductton—a term introduced by E. Hering and C. S. Sherrington. Induction is mutual, or reciprocal, excitation leading to increased inhibition and inhibition leading to increased excitation. The former is referred to as “the phase of negative induction,” and the latter as “ the phase of positive induc- tion,” or, briefly, “ negative ” and “‘ positive ” indugti The phenomenon of induction was obsérved. as so frequently happens in scientific researches for a long time obscured in our minds by Ke) ea of a smoothness of propagation of nervous processes, and. irregularities which are now known to be the result of in , were always attributed to various casual disturbing influe ca Hanger of which is so great in our investigations. The first inkeation of induction was found in the experiments of Dr. Kogan, hich, as was shown in the ninth lecture (page 161), the fa of a place on the skin following

immediately upon the ç te extinction of another definite place much further remo ealed almost invariably a state of increased excitability. N Pinas investigation of this phenomenon was, however, attengptgd, until recently some experiments of Dr. Four- sikov ae] attention upon the matter and led to its energetiç investiga The following is one of the earlier experiments of Dr. Eo v. A conditioned alimentary reflex was established to tactile stimula- tion of the front paw. A similar tactile stimulation of the hind paw was differentiated, the inhibition being complete, so that not a single drop of saliva appeared in response to stimulation of the inhibitory place.