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

objection and the same answer equally to the whole of animal physiology. For instance, t thods of vivisection and of the study of isolated organs an ues, which aim at the same isolation of different individua ta Xions, have been constantly employed, and we may safely s of physiology are-d@»yto the successful application of such methods of control. In experiments it is the whole animal which is placed unas ited number of rigidly defined conditions, and

only by this method is it: possible to study the reflexes independently of one another. ! The foregoing remarks give an idea) of:our general aim and of the technical side of our methods: T propose to introduce:you to the first and’ most elementary principles of the:subject. matter of ‘our research by means of a few demonstrations: Demonstration.—The dog used in the following experiment has been operated upon as described previously: ‘It can be seen that so long’ as no special stimulus ‘is applied the salivary: glands remain quite inactive. But when>the-sounds -from a: beating metronome ave allowed to fall) upon the ear, a salivary secretion begins after 9 seconds, and inthe course of 45 seconds: eleven drops have been secreted: ©The activity of the salivary gland: has) thus ‘been’ called into’ play ‘by ‘impulses of sound-a «stimulus: quite valienoto food: This activity of the salivary gland cannot be regarded as anything else than a component of the alimentary reflex. Besides the secretory, the’ motor component’ of the food reflex isialso'very apparent in experiments of this kind. In this:very experiment: the dog: turns in the direction from which it has been customary to present the food and begins to ‘lick its lips vigorously::

This experiment is an example ofa central nervous activity depending: on. the integrity of the: hemispheres.:; A idecerebrate dog would never have responded by salivary secretion o any stimulus of the kind. It is’obvious also that the underly inciple of this activity is signalization.» The sound) of them ome is the signal for food;and the animal reacts to the signe the same way as if it were food; no distinction: canbe ed. between the effects produced. on the animal by thesounds e beating metronome and showing it real food. O

Demonstration ——Kood “iss œ the animal. The: salivary secretion begins’ after v5 ‘sec » and six drops: are ‘collected in the course’ of 15 seconds. e effectis the same as that observed with thesounds of them ome. | Itis again a case of signalization, and is due'to the aci the hemispheres. That the effect NS t and smell -of food.is not due to.an inborn reflex, but to Te, which has been acquired in the course of the aulimal’s own intéfidual existence, was shown by experiments carried out by Di@Povich in the laboratory of the late. Prof, Vartanov, Dr. Zit Kopi took several young puppies away from their: mother "N hem for a considerable time only on milk.» When the `

puppies were a few months old he established fistulae of their salivary ducts, and was thus able to measure accurately the secretory activity of the glands. He now showed these puppies some solid food— bread or meat—but no secretion of saliva was evoked. It is evident; therefore, that the sight of food does not in itself act as a direct stimulus to salivary secretion. Only after the puppies have been allowed to eat bread and meat on several occasions does the sight or smell of these foodstuffs evoke the secretion.

The following experiment serves to illustrate the activity of the salivary gland as an inborn reflex in contrast to signalization : Demonstration.—Food is suddenly introduced into the dog’s mouth ; secretion begins in 1 to 2 seconds. The secretion is brought _about by the physical and chemical properties of the food itself acting upon receptors in the mucous membrane of the mouth and tongue. It is purely reflex. This comparatively simple experiment explains how a decerebrate dog can die of starvation in the midst of plenty, for it will only start eating if food chances to come into contact with its mouth or tongue. Moreover, the elementary nature of the inborn reflexes, with. their limitations and inadequacy, are clearly brought out in these experi- ments, and we are now able to appreciate the fundamental importance of those stimuli which have the character of signals.

Our néxt step will be to consider the question of the nature of signalization and of its mechanism from.a purely physiol l point of view. It has been mentioned already that a reflex i vitable reaction of the organism to an, external stimulus ght about along a definite path in the nervous system. No that in: signalization all the properties of a r are present. In the first. place an: external- stimulus isi reqs Q -This was given in our: first: experiment--by the: sounds of S These sounds falling on the, auditory receptor. of; th (S caused the propagation of:an:impulse ‘along the, auditory Ao In the brain the impulse was transmitted to. the secretory, es of the salivary glands, and passed’ thence. to. the. glands g them to active secretion. It is, true, that'in the experi ith the metronome an interval of several, seconds elapse Keon the beginning of the stimulus and the beginning of the t secretion, whereas the time interval for the inborn reflex Gepretion was only 1 to 2 seconds. The longer latent period wa wever, due to some special conditions of the

generally speaking the reaction to signals under natural conditions is as speedy as are the inborn reflexes. We shall be considering the latent period of signalization in fuller detail in a further lecture. In our general survey we characterized a reflex as a necessary reaction following upon a strictly definite stimulus under strictly defined conditions. Such a definition holds perfectly true also for signalization ; the only difference is that the type of the effective reaction to signals depends upon a greater number of conditions. But this does not make signalization differ fundamentally from the better known reflexes in any respect, since in the latter, variations in character or force, inhibition and absence of reflexes, can also be traced to some definite change in the conditions of the experiment.

Thorough investigation of the subject shows that accident plays no part whatever in the signalizing activity of the hemispheres, and all experiments proceed strictly according to plan. In the special laboratory I have described, the animal can frequently be kept under rigid experimental observation for 1 to 2 hours without a single drop of saliva being secreted independently of stimuli applied by the observer, although in the ordinary type of physiological — laboratory experiments are very often distorted by the interference of extraneous and uncontrolled stimuli.

All these conditions leave no grounds for regarding the phenomena which we have termed “ signalization ” as being a Nyction else than reflex. There is, however, another aspect. of t stion which at a first glance seems to point to an essential ae nce between the better known reflexes and signalization. H hrough its chemical and physical properties, evokes the saliv eflex in every dog right from birth, whereas this new type ed as reflex—‘ the signal reflex ’’—is built up gradually i pantie of the animal’s own individual existence. But igs e considered as a fundamental point of difference, and can (0) old as a valid argument against

employing the term “reflex ” for this new group of phenomena ? It is certainly a sufficie ment for making a definite distinction — between the two t CS) eflex and for considering the signal reflex — in a group diigo the inborn reflex. But this does not invalidate in AS) ay our right logically to term both “ reflex,” since the von distinction does not concern. the character of the response A e part of the organism, but only the mode of formation of the KA x mechanism. We may take the telephonic installation INS ustration. Communication can be effected in two ways.

My residence may be connected directly with the laboratory by a private line, and I may call up the laboratory whenever it pleases me to do so; or on the other hand, a connection may have to be made through the central exchange. But. the result in both cases ‘is the same. The only point of distinction between the methods is that the private line provides a permanent and readily available cable, while the other line necessitates a preliminary central connec- tion being established. In the one case the communicating wire is always complete, in the other case a small addition must be made to the wire at the central exchange. We have a similar state of affairs in reflex action. The path of the inborn reflex is already completed at birth ; but the path of the signalizing reflex has still to be com- „pleted in the higher nervous centres. We are thus brought to consider the mode of formation of new reflex. mechanisms. A new reflex is formed inevitably under a given set of physiological conditions, and with the greatest ease, so that there is no need to take the subjective states of the dog into consideration. With a complete understanding of all the factors involved, the new signalizing reflexes are under the absolute control of the experimenter; they proceed according to as rigid laws as do any other physiological processes, and must be regarded as being in every sense a part of the physiological activity of living beings. I have termed this new group of reflexes conditioned reflexes to distinguish them from the inborn or unconditioned reflexes. The term “conditioned” is becoming more more generally employed, and I think its use is fully jus fn that, compared with the inborn reflexes, these new refl actually do depend on very many conditions, both in their f ion and in the maintenance of their physiological activity.

ourse the terms “conditioned ” and “ unconditioned ” c replaced by others of arguably equal merit. Thus, for & , we might retain the term “inborn reflexes,” and call the type “ acquired reflexes ” ; or call the former “ species ‘reflexes ” Sivice they are characteristic of the species, and the latter ‘i i from animal to animal in as different times and under ual reflexes ” since they vary , and even in the same animal at ent conditions. Or again we might reflexes ” and the latter “‘ connection

reflexes.” There should b theoretical objection to the hypothesis of the formation of ne ysiological paths and new -connections within nervous system is to establish most complicated and delicate cor- respondences between the organism and its environment we may not unnaturally expect to find there, on the analogy of the methods used by the technician.in everyday experience, a highly developed con- nector system superimposed on a conductor system. The physiologist certainly should not object to this conception seeing that he has been used to employing the German conception of “ Bahnung,’’ which means a laying down of fresh physiological paths in the centres. Conditioned reflexes are phenomena of common and widespread occurrence: their establishment is an integral function in everyday life. We recognize them in ourselves and in other people or animals under such names as ‘“‘ education,” “ habits,” and “ training ;” and all of these are really nothing more than the results of an establish- ment of new nervous connections during the post-natal existence of the organism. They are, in actual fact, links connecting definite extraneous stimuli with their definite responsive reactions. I believe that the recognition and the study of the conditioned reflex will throw open the door to a true physiological investigation probably of all the highest nervous activities of the cerebral hemispheres, and the purpose of the present lectures is to give some account of what we have already accomplished in this direction.

We come now to consider the precise conditions under which new conditioned reflexes or new connections of nervous paths are established. The fundamental requisite is that anyeAternal stimulus which is to become the signal in a conditioned e$ eX must overlap in point of time with the action of an unco ned stimulus. In the experiment which I chose as my ex e the unconditioned stimulus was food. Now if the intake*eAfhod by the animal takes place simultaneously with the acy f a neutral stimulus which has been hitherto in no way relat food, the neutral stimulus readily acquires the property Aeroting the same reaction in the animal as would food itself. is was the case with the, dog.em- ployed in our experiment the metronome. Qn several occasions this animal had been sts ted by the sound of the metronome and: immediately present th food—.e., a stimulus which. was neutral of itself had QO perimposed. upon, the action of the inborn

alimentary refléx.) We observed that, after several repetitions of the combined. stiGwlation, the sounds from.the metronome,had acquired the prop of stimulating salivary secretion and of evoking the ANSE ions. characteristic, of the alimentary, reflex. The. first demonstration was nothing but an example of such a conditioned stimulus in action. Precisely the same-occurs with the mild defence reflex: to rejectable substances. Introduction into the dog’s mouth of a little of an acid solution brings about a quite definite responsive reaction. ‘The animal sets about getting rid of the acid, shaking its head» violently, opening its mouth and making movements with its tongue. At the same time it produces a copious salivary secretion. The same reaction will infallibly be obtained from any stimulus which has previously been applied a sufficient number of times while acid was being introduced into the dog’s mouth. Hence a first and most essential requisite for the formation of a new conditioned reflex lies in a coincidence in time of the action of any previously neutral stimulus with some definite unconditioned stimulus. Further, it’ is not-enough that there should be overlapping between the two stimuli y: it is also and equally necessary that the conditioned stimulus should begin to operate before the unconditioned stimulus comes into action. .

If this order is:reversed, the unconditioned stimulus being applied - first. and the neutral stimulus second, the conditioned reflex cannot. be established at all. Dr. Krestovnikov performed these experiments with many different modifications and controls, but the effect was always the same. The following are some of his results : In-one case 427 applications were made in succession of the odour of ‘vanillin together with the introduction of acid int e dog’s mouth, but the acid was always made to precede t llin by some 5 to'10 seconds::. Vanillin failed to: acquire t conditioned stimulus.: However; in the succeedj which the: order: of stimuli was reversed, th ur, this time of amyl acetate, became an effective. conditj Stimulus after only 20. combinations.. With another dog A uzzing of an electric

bell set: going 5 to 10 seconds after adfinistration of food failed to establish a conditioned alimentary refl&seven after 374 combinations, whereas the regular rotation of Sp ject infront of the eyes of the animal, the rotation beginnir Oore the administration of food, . acquired the properties: of SGhizioned stimulus after only 5 com- binations. = The: electri Ley: r set: going: before the administration of food: established aX\gghditioned, alimentary. ‘reflex: after only a single combinatioy.

Dr. Krestovig S experiments were carried out on five dogs, and the result(®@8 always negative when the neutral stimulus was applied, whether 10 seconds, 5 seconds or only a single second after the beginning of the unconditioned stimulus. During all these experiments not only the secretory reflex but also the motor reaction of the animal was carefully observed, and these observations always corroborated one another. We thus see that the first set of conditions required for the formation of a new conditioned reflex encompasses the time relation between the presentation of the unconditioned stimulus and the presentation of that agent which has to acquire the properties of a conditioned stimulus.

As regards the condition of the hemispheres themselves, an alert state of the nervous system is absolutely essential for the formation of a new conditioned reflex. If the dog is mostly drowsy during the experiments, the establishment of a conditioned reflex becomes a long and tedious process, and in extreme cases is impossible to accomplish. The hemispheres must, however, be free from any other nervous activity, and therefore in building up a new conditioned reflex it is important to avoid foreign stimuli which, falling upon the animal, would cause other reactions of their own. If this is not attended to, the establishment of a conditioned reflex is very difficult, if not impossible. Thus, for example, if the dog has been so fastened up that anything causes severe irritation, it does not matter how many times the combination of stimuli is repeated, we shall not be able to obtain a conditioned reflex. A somewhat similar case was described in the first lecture—that of the dog which exhibited the freedom reflex in an exaggerated degree. It c QI be stated as a rule that the establishment-of the first co oned reflex in an animal is usually more difficult than the oe of succeeding ones. It is obvious that this must be OF n we consider that even in the most favourable circumstance, ewe experimental conditions themselves will be sure to provol@ Mero different reflexes—+.e. will give rise to one or other d&turBing activity of the hemispheres. But this statement must be @ lified by remarking that in cases where the cause of these @pontrolled reflexes is not found out, so that we are not able to Arid of them, the hemispheres themselves will help us. For iNX éhvironment of the animal during the ex- tain any powerful disturbing elements, then

periment does no practically agg he extraneous reflexes will with time gradually and spont usly weaken in strength. . The ths Taa determining the facility with which new- con- dition exes can be established is the health of the animal. A good state of health will ensure the normal functioning of the cerebral _ hemispheres, and we shall not have to bother with the effects of any internal pathological stimuli. The fourth, and last, group of conditions has to dq with the properties of the stimulus which is to become conditioned, and also with the properties of the unconditioned stimulus which is selected. Conditioned reflexes are quite readily formed to stimuli to which the animal is more or less indifferent at the outset, though strictly speaking no stimulus within the animal’s range of perception exists to which it would be absolutely indifferent. In a normal animal the slightest alteration in the environment—even the very slightest sound or faintest odour, or the smallest change in intensity of illumination—immediately evokes the reflex which I referred to in the first lecture as the investigatory reflex—‘‘ What is it ?”’— manifested by a very definite motor reaction. However, if these neutral stimuli keep recurring, they spontaneously and rapidly weaken in their effect upon the hemispheres, thus bringing about bit by bit the removal of this obstacle to the establishment of a conditioned reflex. But if the extraneous stimuli are strong or unusual, the formation of a conditioned reflex will be difficult, and in extreme cases impossible.

It must also be remembered that in most cases we are not ac- quainted with the history of the dog before it came into the laberatory, and that we do not know what sort of conditioned reflex Ny been established to stimuli which appear to be of the simpl er aracter. But in spite of this we have, in a large number ee found it possible to take a strong stimulus which evo trong uncon- ditioned response of its own, and still pe it into a conditioned stimulus for another reflex take for example a nocuous stimulus, such as a strong © current or wounding or cauterization of the skin. These are iously stimuli to vigorous unconditioned defence reflexes. Therganism responds by a violent motor reaction directed tovara ova of the nocuous stimulus or to its own removal from i we may, nevertheless, make use even of these stimuli for stablishment of a new conditioned reflex. Thus in one par€cuf%f experiment a strong nocuous stimulus —an electric ee great strength—was converted into an alimentary conditi stimulus, so that its application to the skin

did not evoke t ightest defence reaction. Instead, the animal exhibited a arked alimentary conditioned reflex, turning its head to where it usually received the food and smacking its lips, at the same time producing a profuse secretion of saliva. The following is a record taken from a research by Dr. Eroféeva’: Distance of Part of centile ds a Time |secondary coil Skin Pca e A Motor Reaction in cms. Stimulated 40 eng his 4.23 p.m. 4 usual place 6 In all cases the motor reaction displayed 4.45 ,, 4 oe 5 was that character- istic of an alimen- TEEN 2 new place 7 tary reflex; there was no slightest trace a3 by Ga 0 Eran es 9 of any motor defence 5.45 ,, 0 ES 6 reflex.

After each stimulation the dòg was allowed to eat food for a few seconds. Similar results were obtained from dogs in which cauterization or pricking of the skin deep enough to draw blood was made to acquire the properties of an alimentary conditioned stimulus. These experiments have been apt to upset very sensitive people; but we have been able to demonstrate, though without, any pretension of penetrating into the subjective world of tees, that they were labouring under a false impression. jetted to the very closest scrutiny, not even the tiniest an st subtle objective phenomenon usually exhibited by ani nder the influence of strong injurious stimuli can be observe@S these dogs. No appreci- able changes in the pulse or in eer occur in these animals, whereas such changes are wee t prominent when the nocuous

stimulus has not been convga\_ into an alimentary conditioned stimulus. Such a ee, enomenon is the result of diverting the nervous impulse fro e physiological path to another. This transference is cep Mowever upon a very definite condition —namely, upon LN ative strengths of the two unconditioned reflexes. ies Successful sformation of the unconditioned stimulus for one reflex OO stimulus for another reflex can be brought hen the former reflex is physiologically weaker and

about o aN SS y of less importance than the latter. We are led to this A nocuous stimulus applied to.the dog’s skin was transformed into a conditioned stimulus for the alimentary reflex. This, we consider, was due to the fact that the alimentary reflex is in such cases stronger than the defence reflex. In the same way we all know that when dogs join in a scuffle for food they frequently sustain skin wounds, which however play no dominant part as stimuli to any defence reflex, being entirely subordinated to the reflex for food. Neverthe- less there is a certain limit—there are stronger reflexes than the

- alimentary reflex. One is the reflex of self-preservation, of existence or non-existence, life or death. To give only one example, it was found impossible to transform a defence reaction into an alimentary conditioned reflex when the stimulus to the unconditioned defence reaction was a strong electric current applied to skin overlying bone with no muscular layer intervening. This signifies that the afferent nervous impulses set up by injury to the bone, and signalizing far greater danger than those set up by injury to the skin, cannot acquire even a temporary connection with the part of the brain from which

the alimentary reflex is controlled. Nevertheless, on the whole, the foregoing considerations emphasize the advantage of using the alimentary reflex for most of our experiments, since in the hierarchy of reflexes this holds a very high place. While, as we have seen, very strong and even specialize stimuli can under certain conditions acquire the properties of c tioned stimuli, there is, on the other hand, a minimum strengt w which stimuli cannot be given conditioned properties. a thermal stimulus of 45° C. applied to`the skin can be madeiNO an alimentary conditioned reflex, whereas at 38° to 39° C. (app the skin temperature in the dog) a ther ulus is ineffective [experiments of Dr. Solomonov]. $ , while with the help

of a very strong unconditioned stim it is possible to convert a very unsuitable stimulus—for extfiple, one which naturally evokes a different unconditioned —into a conditioned stimulus, it is exceedingly difficult or ev possible with the help of only a weak unconditioned stimul OO transform even a very favourable neutral stimulus into itioned stimulus. Even where such a conditioned reflex is shcodssfully established, its occurrence results only in a very s Iyeflex response. Some unconditioned stimuli may be permane weak, others may display a weakness which is only tempor arying with the condition of the animal. As an

example AY last we may take food. In the hungry animal food. naturally brings about a powerful unconditioned reflex, and the conditioned reflex develops quickly. But in a dog which has not long been fed the unconditioned stimulus has only a small effect, and alimentary conditioned reflexes either are not formed, at all or are established very slowly. By complying with all the conditions which I have enumerated —which is not a very difficult task—a new conditioned reflex is infallibly obtained. We apply to the receptors of the animal rigidly defined stimuli; these stimuli necessarily result in the formation of a néw connection in the hemispheres with a consequent develop- ment of a typical reflex reaction.