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

Reflexes, like the driving-belts of machines of human design, may be of two kinds—positive and negative, ex S and inhibi- tory. Although the investigation of these refl aS y physiologists has been going on now for a long time, it is a not nearly finished. Fresh reflexes are continually being dis ed. We are ignorant of the properties of those receptor o for which the effective stimulus arises inside the organis the internal reflexes them- selves remain a field unexplored “A’he paths by which nervous impulses are conducted in thg-eemtral nervous system are for the most part little known, or © penne at all. The mechanism of inhibitions confined vwé the central nervous system remains quite obscure : we nething only of those inhibitory réflexes which manifest t ‘“eIves along the inhibitory efferent nerves. Furthermore, t bination and interaction of different reflexes are as yet insuiSeintly understood. Nevertheless physiologists are

succeeding*g&ope and more in unravelling the mechanism of these machine) activities of the organism, and may reasonably be exp elucidate and control it in the end. To those reflexes which have long been the subject of physiological investigation, and which concern chiefly the activities of separate organs and tissues, there should be added another group of inborn . reflexes. These also take place in the nervous system, ‘and they are the inevitable reactions to perfectly definite stimuli. They have to do with reactions of the organism as a whole, and comprise that general behaviour of the animal which has been termed “ instinctive.” Since complete agreement as regards the essential affinity of these reactions to the reflex has not yet been attained, we must discuss this question more fully. We owe to the English philosopher, Herbert Spencer, the suggestion that instinctive reactions are reflexes. Ample evidence was later advanced by zoologists, physiologists, and students of comparative psychology in support of this. I propose here to bring together the various arguments in favour of this view. Between the simplest reflex and the instinct we can find numerous stages of transition, and among these we are puzzled to find any line

.of demarcation. To exemplify this we may take the newly hatched chick. This little creature reacts by pecking to any stimulus that catches the eye, whether it be a real object or only a stain in the surface it is walking upon. In what way shall we say that this differs from the inclining of the head, the closing of the lids, when something flicks past its eyes ? We should call this last a defensive reflex, but the first has been termed a feeding instinct : although in pecking nothing but an inclination of the head aes vement of the beak occurs. Qy

It has also been maintained that instincts are complex than reflexes. There are, however, exceedingly co x reflexes which nobody would term instincts. We may take ing as an example. This is very complex and involves the co- ion of a large number of muscles (both striped and plain) oe over a large area and usually employed in quite different tions of the organism. It involves also a secretory activity Ye part of certain glands which is usually evoked for a quite dj t purpose.

Again, it has been assu the long train of actions involved in certain instinctive AN affords a distinctive point of contrast with the reflex, whic“tsNégarded as always being built on a simple scale. By way of ne we may take the building of a nest, or of dwellings in RK, by animals. A chain of incidents is linked together : mat is gathered and carried to the site chosen ; there it is built ANS strengthened. To look upon this as reflex we must assume that one reflex initiates the next following—or, in other words, we must regard it as a chain-reflex. But this linking up of activities is not peculiar to instincts alone. We are familiar with numerous reflexes which most certainly fuse into chains. Thus, for example, if we stimulate an afferent nerve, e.g. the sciatic nerve, a reflex rise of blood pressure occurs; the high pressure in the left ventricle of the heart, and first part of the aorta, serves as the effective stimulus to a second reflex, this time a depressor reflex which has a moderating influence on the first. Again, we may take one of the chain reflexes recently established by Magnus. A cat, even when deprived of its cerebral hemispheres, will in most cases land on its feet when thrown from a height. - How is this managed ? When the position of the otolithic organ in space is altered a definite reflex is evoked which brings about a contraction of the muscles in the neck, restoring the animal’s head to the normal position. This is the first reflex. With the righting of the head a fresh reflex is evoked, and certain muscles of the trunk and limbs are brought into

` play, restoring the animal to the standing posture. This is the Some, again, object to the identification of instincts with reflexes on this ground : instincts, they say, frequently depend upon the internal state of an organism. For instance, a bird only builds its nest in the mating season. Or, to take a simpler case, when an animal is satiated with eating, then food has no longgÑany attraction and the animal leaves off eating. Again, the e ås true of the sexual impulse. This depends on the age of organism, and on the state of the reproductive glands ; and <Q rsiderbi influence is exerted by hormones (the product D he glands of internal secretion). But this dependence ca Noe claimed as a peculiar property of “instincts.” The in W of any reflex, indeed its very presence, is dependent on fke irritability of the centres, which in turn depends constantly on (D physical and chemical properties of the blood (automatic stim@ytion of centres) and on the interaction of reflexes.

Last of all, it is iñes held that whereas reflexes determine only the activities gle organs and tissues, instincts involve the activity of the drganism as a whole. We now know, however, from the recent investigations of Magnus and de Kleijn, that standing, walking a maintenance of postural balance in general, are all nothing eflexes. Tt follows from all this that instincts and reflexes are alike the inevitable responses of the organism to internal and external stimuli, and therefore we have no need to call them by two different terms. Reflex has the better claim of the two, in that it has been used from the very beginning with a strictly scientific connotation.

The aggregate of reflexes constitutes the foundation of the nervous activities both of men and of animals. It is therefore of great importance to study in detail all the fundamental reflexes of the organism. Up to the present, unfortunately, this is far from being accomplished, especially, as I have mentioned before, in the case of those reflexes which have been known vaguely as “ instincts.” Our knowledge of these latter is very limited and fragmentary. Their classification under such headings as “alimentary,” “defen- sive,” “ sexual,” “ parental” and “ social ” instincts, is thoroughly inadequate. Under each of these heads is assembled often a large number of individual reflexes. Some of these are quite unidentified ; some are confused with others; and many are still only partially appreciated. I can demonstrate from my own experience to what extent the subject remains inchoate and full of gaps. In the course of the researches which I shall presently explain, we were completely at a loss on one occasion to find any cause for the peculiar behaviour of ananimal. It was evidently a very tractable dog, which soon became very friendly with us. We started off with a very simple experiment. The dog was placed in a stand with loose loops round HAG , but so as to be quite comfortable and free to move a pace ogee othing more was done except to present the animal re y with food at intervals of some minutes. It stood elute at first, and ate quite readily, but as time went on it beea cited and struggled to get out of the stand, scratching at the nawing the supports, and so on. This ceaseless muscular n was accompanied by breathlessness and continuous salve, which persisted at every experiment during several weeks, the‘wnimal getting worse and worse until it was no longer fitted fo researches. For a long time we remained puzzled over th al behaviour of this animal.

We tried out experimentall erous possible interpretations, but though we had had perience with a great number of dogs in our laboratories we d not work out a satisfactory solution of this strange belie, until it occurred to us at last that it might be the expres of a special freedom reflex, and that the dog simply ww remain quiet when it was constrained in the stand. This reflex was overcome by setting off another against it—the reflex for food. We began to give the dog the whole of its food in the stand. At first the animal ate but little, and lost considerably in weight, but gradually it got to eat more, until at last the whole ration was consumed. At the same time the animal grew quieter during the course of the experiments : the freedom reflex was being inhibited. It is clear that the freedom reflex is one of the most important reflexes, or, if we use a more general term, reactions, of living beings. This reflex has even yet to find its final recognition. In James’s writings it is not even enumerated among the special human “instincts.” But it is clear that if the animal were not provided with a reflex of protest against boundaries set to its freedom, the smallest obstacle in its path would interfere with the proper fulfilment of its natural functions. Some animals as we all know have this freedom reflex to such a degree that when placed in captivity they refuse all food, sicken and die.

As another example of a reflex which is very much neglected we may refer to what may be called the investigatory reflex. I call it the “ What-is-it ?”’ reflex. It is this reflex which brings about the immediate response in man and animals to the slightest changes in the world around them, so that they immediately orientate their appropriate receptor organ in accordance with the perceptible quality in the agent bringing about the change, making full investi- gation of it. The biological significance of this seh is obvious. If the animal were not provided with such a r its life would hang at every moment by a thread. In ma S reflex has been greatly developed with far-reaching resu eing represented in its highest form by inquisitiveness—t ent of that scientific method through which we may h day to come to a true orientation in knowledge of the Ami us.

Still less has been done ove the elucidation of the class of negative or inhibitory reflex stincts) which are often evoked by any strong stimulus or y weak stimuli, if unusual. Animal hypnotism, so-called, o this category. are inborn in th of definite reflexes, I must again emphasize how important iisto compile a complete list comprising all these reflexes with Ogir adequate classification. For, as will be shown later on, al N remaining nervous functions of the animal organism are Pan these reflexes. Now, although the possession of such

reflexes as those just described constitutes the fundamental condition for the natural survival of the animal, they are not in themselves sufficient to ensure a prolonged, stable and normal existence. This can be shown in dogs in which the cerebral hemispheres have been removed. Leaving out of account the internal reflexes, such a dog still retains the fundamental external reflexes. It is attracted by food ; it is repelled by nocuous stimuli ; it exhibits the investigatory reflex, raising its head and pricking up its ears to sound. In addition it exhibits the freedom reflex, offering a powerful resistance to any restraint. Nevertheless it is wholly incapable of looking after itself, and if left to itself will very soon die. Evidently something important is missing in its present nervous make-up. What nervous activities can it have lost ? It is easily seen that, in this dog, the number of stimuli evoking reflex reaction is considerably diminished ; those remaining are of an elemental, generalized nature, and act at a very short range. Consequently the dynamic equilibrium between the inner forces of the animal system and the external forces in its environment has become elemental as compared with the exquisite adaptability of the normal animal, and the simpler balance is obviously inadequate to life.

Let us return now to the simplest reflex from which our investi- gations started. If food or some rejectable substance finds its way into the mouth, a secretion of saliva is produced. The purpose of this secretion is in the case of food to alter it ea the case of a rejectable substance to dilute and wash it o he mouth. This is an example of a reflex due to the = and chemical properties of a substance when it comes into ¢ t with the mucous membrane of the mouth and tongue. addition to this, a similar reflex secretion is evoked whe bg are placed at a distance from the dog and the r ge organs affected are only those of smell and sight. Even Ee, kal from which the food has been given is sufficient to cola alimentary reflex complete in all its details ; and, further, retion may be provoked even by the sight of the person w ght the vessel, or by the sound of his footsteps. All these dmerable stimuli falling upon the several finely discriminating/@isance receptors lose their power for ever as soon as the hemisph are taken from the animal, and those only which have a ci effect on mouth and tongue still retain their

power. The O advantage to the organism of a capacity to react to the toimu is evident, for it is in virtue of their. aċtion that food finding its way into the mouth immediately encounters plenty of moistening saliva, and rejectable substances, often nocuous to the mucous membrane, find a layer of protective saliva already in the mouth which rapidly dilutes and washes them out. Even greater is their importance when they evoke the motor component of the complex reflex of nutrition, t.e. when they act as stimuli to the reflex of seeking food.

Here is another example—the reflex of self-defence. The strong carnivorous animal preys on weaker animals, and these if they waited to defend themselves until the teeth of the foe were in their flesh would speedily be exterminated. The case takes on a different aspect when the defence reflex is called into play by the sights and sounds of the enemy’s approach. Then the prey has a chance to save itself by hiding or by flight. How can we describe, in general, this difference in the dynamic balance of life between the normal and the decorticated animal ? What is the general mechanism and law of this distinction? It is pretty evident that under natural conditions the normal animal must respond not only to stimuli which themselves bring immediate benefit or harm, but also to other physical or chemical agencies— waves of sound, light, and the like—which in themselves only signal the approach of these stimuli; though it is not the sight and sound of the beast of prey which is in itself harmful to the smaller animal, but its teeth and claws.

Now although the signalling stimuli do play a,axt ån those com- paratively simple reflexes we have given as e es, yet this is not the most important point. The essential feaķùxe of the highest acti- vity of the central nervous system, with h we are concerned and which in the higher animals most prokaply belongs entirely to the hemispheres, consists not in the f that innumerable signalling stimuli do initiate reflex reactiongQn the animal, but in the fact that under different conditions the same stimuli may initiate quite different reflex reactions ; ig) conversely the same reaction may be initiated by different li.

In the above-m ed example of the salivary reflex, the signal at one timg,i particular vessel, at another time another ; under certain co\difions one man, under different conditions another —strictly deyeading upon which vessel had been used in feeding and which had. brought the vessel and given food to the dog. This S O makes the machine-like responsive activities of the organism still more precise, and adds to it qualities of yet higher perfection. So infinitely complex, so continuously in flux, are the conditions in the world around, that that complex animal system which is itself in living flux, and that system only, has a chance to establish dynamic equilibrium with the environment. Thus we see that the fundamental and the most general function of the hemi- spheres is that of reacting to signals presented by innumerable stimuli of interchangeable signification.

Technical methods employed in the objective investigation of the functions of the cerebral hemispheres.—Response to signals as reflex action.—Unconditioned and conditioned reflexes.—Necessary conditions for the development of conditioned reflexes. In the previous lecture I gave an account of the reasons which led us to adopt, for the investigation of the functions of the cerebral hemispheres, the purely objective method used for investigating the physiological activity of the lower parts of the nervous system. In this manner the investigation of the cerebral hemispheres is brought into line with the investigations conducted in other branches of natural science, and their activities are studied as purely physiological facts, without any need to resort to fantastic speculations as to the existence of any possible subjective state in the animal which may be conjectured. on analogy with ourselves. From this point of view the whole nervous activity of the animal must be regarded as based firstly on inborn reflexes. These are regular causal connections between certain definite external stimuli acting the organism and its necessary reflex reactions. Such inbox exes are com- paratively few in number, and the stimuli s g them in action act close up, being as a rule the genre Qc and, chemical properties of the common agencies whic ect the organism. The inborn reflexes by themselves are ina e to ensure the continued existence of the organism, especi e the more highly organized animals, which, when deprive oftheir highest nervous activity, are permanently disabled, and t to themselves, although retaining all their inborn reflexes, sogp,cease to exist. The complex conditions of everyday existence nee a much more detailed and specialized correlation betwee Les fmal and its environment than is afforded

by the inborn re alone. This more precise correlation can be established on ough the medium of the cerebral hemispheres ; and we "Gy that a great number of all sorts of stimuli always of a general character which determine the inborn reflexes, and that this is the only means by which a most delicate adjustment of the organism to the environment can be established. To this function of the hemispheres we gave the name of “ signalization.” Before passing on to describe the results of our investigation it is necessary to give some account of the purely technical side of the methods employed, and to describe the general way in which the signalizing activity of the hemispheres can be studied. It is obvious that the reflex activity of any effector organ can be chosen for the purpose of this investigation, since signalling stimuli can get linked up with any of the inborn reflexes. But, as was mentioned in the first lecture, the starting point for the present investigation was determined in particular by the study of two reflexes—the food or “‘ alimentary ” reflex, and the “defence ” reflex in its mildest form, as observed when a rejectable substance finds its way into the mouth of the animal. As it turned out, these two reflexes proved a fortunate choice in many ways. Indeed, while any strong defence reflex, e.g. against such a stimulus as a powerful electric current, makes the animal extremely restless and excited; and while the sexual reflexes require a special environment—to say nothing, of their periodic character and ‘their dependence upon age—the ali- mentary reflex and the mild defence reflex to rejectable substances are normal everyday occurrences. ~\

It is essential to realize that each of these two owe es—the alimentary reflex and the mild defence reflex to rej ‘NES —consists of two distinct components, a mtg a a secretory. Firstly the animal exhibits a reflex activity di towards getting hold of the food and eating it or, in the cage Q jectable substances, towards getting rid of them out of the (mð uèh ; and secondly, in both cases an immediate secretion of galiva occurs, in the case of food, to start the physical and chem)! processes of digestion and, in the case of rejectable substanc€M,to wash them out of the mouth. We confined our experimen Sos entirely to the secretory component of the reflex : liéd motor reactions were taken into account only where ther e special reasons. The secretory reflex presents many important advantages for our purpose. It allows of an extremely accyrate measurement of the intensity of reflex activity, since e the number of drops in a given time may be counted or aek ain may be caused to displace a coloured fluid in a ek ole laced graduated glass tube. It would be much

more difficult to obtain the same accuracy of measurement for any motor reflex, especially for such complex motor reactions as accom- pany reflexes to food or to rejectable substances. Even by using most delicate instruments we should never be able to reach such precision in measuring the intensity of the motor component of the reflexes as can easily be attained with the secretory component. Again, a very important point in favour of the secretory reflexes is the much smaller tendency to interpret them in an anthropomorphic fashion—7.e. in terms of subjective analogy. Although this seems a trivial consideration from our present standpoint, it was of importance in the earlier stages of our investigation and did undoubtedly influence our choice.

For the purpose of registering the intensity of the salivary reflex all the dogs employed in the experiments are subjected to a prelimi- nary minor operation, which-consists in the transplantation of the opening of the salivary duct from its natural place on the mucous membrane of the mouth to the outside skin. For this purpose the terminal portion of the salivary duct is dissected and freed from the surrounding tissue, and the duct, together with a small portion of. the mucous membrane surrounding its natural opening, is carried through a suitable incision, to the outside of the cheek in the case of the parotid gland, or under the chin in the case of the submaxillary gland. In this new position the duct is fixed by a {aarete which are removed when the wound has healed. Asar the operation the saliva now flows to the outside, on to t eek or chin of the animal, instead of into the mouth, so tha measurement of the secretory activity of the gland is great facilitated. It is only necessary for this purpose to adj mall glass funnel over the opening of the duct on to the 19); for this we find a ‘special cement prepared according to peat of Mendeléeff + most useful. As an alternative, very suitable‘add accurate as a recording apparatus is a hemispherical bulb w also can be hermetically sealed on to the skin. From the bu ject two tubes, one pointing up and the other pointing do A e latter tube is used for drawing off the saliva which col Oring each observation, while the former tube, connects by air smission with a horizontal graduated glass. tube filled with colGared fluid. As the saliva flows into the hemispherical bulb the oged fluid is displaced along the graduated tube, where

the amount of secretion can be read off accurately.: Further, it is not difficult to fix up an automatic electrically-recording device which will split up the displaced fluid into drops. of exactly equal volume and reduce any ‘lag in the movement of the fluid to a minimum.* To come to the general:technique of the experiments, it is impor- tant to remember that our research deals with the highly specialized j Fic. 1.—The apparatus used for recording the salivary secretion in set tioned reflexes, - .A, hemispherical bulb which is fixed.over.the fistula. .aaa, BS ing. tube leading through the partition separating the animal’sroomirom the experimen (connecting the bulb A to the registering apparatus, B. bb, tube connecting the bulb ottle, C.

After. each observation a vacuum; is created in the bottle C by; deggreS¥jon of the rubber balloon D; the saliva accumulating in A is thus sucked away. 5 Go the observation A is automatically disconnected from C and connected with the mS v apparatus. During the aspirations of the saliva from bulb A the latter is autom Ify disconnected from the registering apparatus. activity of the cerebral cortex, a signaling apparatus of tremendous complexity and of, most.. exquisite, sitivity, through which. the

In almost all the experiments Cui in' these lectures the ainouit: of salivary secretion is, for the) sal niformity, given in drops. It. was, however, only in the very earli od of the research—before the separation of the experimenter from th al was made—that the actual number of drops falling from a sma fixed over the fistula was counted, and only a few of these experiment: given. In the great majority of the experiments the salivary secretiqn measured by the displacement of water in a gradu- ated tube or by tk tric; recorder, allowing a much greater accuracy. of measurement. CO dings so obtained have been- converted, in the tables, into drops: Sw in some experiments it will be noticed that the number of drops BR to an accuracy of one-tenth.

animal is influenced by countless stimuli from the outside world. Every one of these stimuli produces a certain effect upon the animal, and all of them taken together may clash and interfere with, or else reinforce, one another. Unless we are careful to take special pre- cautions the success of the whole investigation may be jeopardized, and we should get hopelessly lost as soon as we began to seek for cause and effect among so many and various influences, so intertwined. and entangled as to form a veritable chaos. It was evident that the experimental conditions had to be simplified, and that this simpli- fication must consist in eliminating as far as possible any stimuli outside our control which might fall upon the animal, admitting only such stimuli as could be entirely controlled by the experimenter. It was thought at the beginning of our research that it would be sufficient simply to isolate the experimenter in the research chamber with the dog on its stand, and to refuse admission to anyone else during the course of an experiment. But this precaution was found- to be wholly inadequate, since the experimenter, however still he might try to be, was himself a constant source of a large number of stimuli. His slightest movements—blinking of the eyelids or movement of the eyes, posture, respiration and so on—all acted as stimuli which, falling upon the dog, were sufficient to vitiate the experiments by making exact interpretation of the results extremely difficult. In order to exclude this undue influenge\on the part of the experimenter as far as possible, he had to RY ioned outside the room in which the dog was placed, an n this precaution proved. unsuccessful in laboratories not s ly designed for the — study of these particular reflexes. Th ironment of the animal, even when shut up by itself in a is perpetually changing.

Footfalls of a passer-by, chance corkveyations in neighbouring rooms, slamming of a door or vibratid&{ from a passing van, street-cries, even shadows cast through th&Qindows into the room, any of these casual uncontrolled stimu i @jling upon the receptors of the dog set ral hemispheres and vitiate the experi- ese disturbing factors a special laboratory ute of Experimental Medicine in Petrograd, the funds bein povided by a keen and public-spirited Moscow he primary task was the protection of the dogs

from unc ed extraneous stimuli, and this was effected by sur- roundingXke building with an isolating trench and employing other (four to each floor) were isolated from one another by a cross-shaped corridor ; the top and ground floors, where these rooms were situated, were separated by an intermediate floor. Each research room was carefully partitioned by the use of sound-proof materials into two compartments—one for the animal, the other for the experimenter. For stimulating the animal, and for registering the corresponding reflex response, electrical methods or pneumatic transmission were used. By means of these arrangements it was possible to get some- thing of that stability of environmental conditions so essential to the carrying out of a successful experiment.

Another point should be mentioned—although in this respect the means at our disposal still leave something to be desired. In analysing the exceedingly complex influence of the external environ- ment upon the animal, the experimenter must be able to exercise full control over all the conditions obtaining during the course of any experiment. He should therefore have at his disposal various instruments for affecting the animal by different kinds of stimuli, singly or combined, so as to imitate simple natural conditions. But we were often handicapped by the conditions in- which we had to work and by the shortcomings of the instruments at our disposal, for we always found that the cerebral hemispheres were sensitive to far finer gradations of stimulus than we could furnish.

It is possible that the experimental conditions I hay, may raise somewhere the objection of being abnorma However it is hardly likely, in view of the infinite met with under natural conditions, that we shal) quite unprecedented in the life of the animal. with any phenomenon of vast complexity absolutely necessary to isolate the different single factors i Ai so as to study them independently, or in arbitrary grou Kn which we can keep the individual units under control. Butas a matter of fact the same