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

up with the positive phenomena of nery sxtitation. But before dealing with these I consider it adviss to give a brief description of inhibition of centres as observed. the field of unconditioned reflexes. The present state of ha Deical knowledge enables us to recognize under normal coy{yfrons two types of central inhibition. These may be H ere and indirect, or internal and external respectively. On the hand we are familiar with the direct inhibitory action aoa afferent nerves, or of certain physical and chemical pr ies of the blood, upon definite nervous centres

controlling ref N tion, circulation, locomotion and so on. On the other hand the central nervous system supplies numerous cases of indirect inhibition. In the latter cases inhibition of the activity of a given centre is brought about as the result of an activity of some other centre, which activity has arisen in its turn as a result of excitation from some afferent nerve or from some change in the composition of the blood. Instances of other kinds of inhibition are provided by those complex unconditioned reflexes generally known as “instincts.” Thus many insects, especially when in the larval stage, become immobilized and drop down at the slightest touch. This is obviously a case of direct inhibition of the entire nervous apparatus of locomotion. Another example may be taken from the newly hatched chick, which manifests straightway a pecking reflex to the visual stimulus of small objects or patches of light and shade. If, however, a strongly irritant and injurious substance gets taken up, the pecking reflex becomes inhibited immediately and is replaced by a defence reaction leading to a rejection of the irritating substance. This is a result of an interaction of two active centres, and is an example of external inhibition.

Conditioned reflexes are also subject to these two types of central inhibition. Since the indirect or external inhibition of conditioned reflexes does not differ in the least from the corresponding inhibition of unconditioned reflexes I shall take this first. The following is a very simple case, and one of common occurrence in our earlier experiments. The dog and the experimenter wgy\d be isolated. in the experimental room, all the conditions rÒÑQAin g for a while constant. Suddenly some disturbing fact uld arise—a sound would penetrate into the room ; some qu change in illumination would occur, the sun going behind a Q or a draught would get in underneath the door, and ma Ons some odour with it. If any one of these extra stimuli eħed to be introduced just at the time of application of the coMditioned stimulus, it would inevitably bring about a more or less prdggunced weakening or even a complete disappearance of the re Myesponse, depending on the strength of the extra stimulus. terpretation of this simple case does not present much Na The appearance of any new stimulus immediately e e investigatory reflex and the animal fixes

all its approp jat pe ena organs upon the source of disturbance, pricking ned ears, fastening its gaze upon the disturbing agency, and sni D air. The investigatory reflex is excited and the RN reflex is in consequence inhibited. These extra stimuli have also another important influence. Every stimulus, however rapidly it may disappear, is effective not only while it lasts, but also for some time after its cessation while its after-effect lasts. On account of this, if the conditioned stimulus is applied shortly after the extra stimulus, the reflex reaction will be partly inhibited. Different extra stimuli, whether accidental or deliberate, produce after-effects of different length. Some may still have a powerful after-effect after 2 or 3 minutes, others after 10 or 12 minutes, and still others after several days. These longer after-effects occur especially with alimentary, and gustatory stimuli in general, and in many experiments they have to be taken seriously into account. The effect of exciting an extra reflex will, of course, vary according as the conditioned reflex has only freshly been formed or has already been firmly established. It is obvious that an old-established reflex is not likely to be so easily inhibited as a recent one. In our old laboratory the neglect to provide against external stimuli often led to a curious complication when I visited some of my co-workers. Having by himself established a new con- ditioned reflex, working in the room with the dog, the experimenter would invite me for a demonstration, and then everything would go wrong and he would be unable to show anything at all. It was I who presented the extra stimulus: the investigatory reflex was immediately brought into play: the dog gazed at me, and smelled at me, and of course this was sufficient to inhibit ever cently established reflex. Another example is very simi f one experimenter had worked with a dog and establishe e firm and stable conditioned reflexes, conducting numerous periment with them, when he handed the animal over to ano experimenter to work with, all the reflexes disappeared considerable time.

The same happened when the dog wa: nged over from one research room to another. In the case of extra stimuli, wh awaken specialized reflex responses, the resulting inhibition g&€xtremely profound. Examples of such extra stimuli are, the æ or sound of game for sporting dogs, of cats for some dogs; ing under the floor for others, and | so on. Not only such red stimuli, but also any very strong | or unusual stimulus exINbjs a prolonged and pronounced inhibitory

. after-effect. With -pGgect to strong or unusual stimuli dogs can be divided. into two ps. Some respond in a manner which may be termed positi e. aggressively—barking furiously and, barin ~\ The Carl F. Shepard Memorial Library ne) ILLINOIS COLLEGE OF OPTOMETRY Ò ; 3241 S. Michigan Avenue their teeth. Others exhibit a defence reaction of a passive nature— they try to get free and run away or they stand like lumps of stone HY | absolutely motionless; and sometimes they will shiver violently | and crouch down in the stand ; or they may urinate, a most unusual occurrence for dogs while in the stand. In these dogs inhibition ii predominates. This case can also be classified as a type of indirect or external inhibition of conditioned reflexes, for the inhibition I | originates primarily in other parts of the brain than that where the | reflex response is initiated. Ii All the cases of external inhibition which have just been discussed Ii || are of a temporary nature; and on this account the stimuli producing ii these effects are termed temporary inhibitory stimuli. If they act HHI upon the animal repeatedly and are not reinforced with any uncon- | ditioned or conditioned stimulus, sooner or later they become lost I | upon the organism and lose all their inhibitory properties. | An experiment providing an admirable illustration of this Hal 3 condition occurred during the first two lectures, without the audience Ht being aware of it. The dog used for the experiment with the metro- | nome (p. 22) had been kept in the lecture theatre during the whole || of one lecture previous to the demonstration, while one of my I co-workers repeatedly tried the experiment. As a matter of fact

| the experiment was at first a failure, the reflex being inhibited, and HI the animal only freed itself gradually from the inhibitory influence i of the unfamiliar surroundings. N | || Some years ago, when I was delivering oe conditioned reflexes, I had to proceed in a sj stration of experiments. The dogs wer from the very beginning of the course ctures, and my co-workers constantly repeated the S preliminary to the actual lecture demonstration, and omy fth this precaution could the | experiments be successfully cepted In this series of lectures, | || | unfortunately, the present pxaVOidable and unforeseen circumstances

to occasional demons, keeping chiefly to a description of experiments and ~rches conducted in our laboratories. In aC) e temporary external inhibition, a second type Hi of external inhNeion can be distinguished. This may be termed es of lectures on r manner for demon- d in the lecture theatre | permanent’ 6grnal inhibition, since it does not weaken through | repetition) t constantly maintains its strength. An example may | þe un the reaction to acid. If the dog is given food, even

some considerable time before the conditioned stimulus to the acid is applied, this reflex cannot be obtained. What happens is that the alimentary centres inhibit the reaction to acid for a considerable time after they themselves are set into activity, and this happens always, no matter how often the experiment may be repeated. It is therefore a permanent external inhibition which does not undergo any sort of spontaneous extinction. Such cases are not at all rare. If a conditioned reflex is established to introduction of acid, any careless administration—too much acid, or too concentrated, or applied too often—produces a severe irritation of the buccal mucous membrane and produces a permanent inhibition of conditioned reflexes which lasts until the pathological condition is removed. Again, the dog may have some laceration of the skin which may be constantly irritated by the supporting loops of the stand. The defence reflex will then become dominant while the conditioned reflexes, and especially the mild defence reflex to acid, will be in- hibited. Many more examples of this nature could be quoted. I will give one or two more. In one case an experiment may be running quite smoothly, and suddenly all conditioned reflexes begin to fail, and finally disappear altogether. The dog is taken out, allowed to urinate, and then all the reflexes return to normal. Evidently stimulation of the centre for micturition had inhibited the conditioned reflexes. Another example may be chosen in the season wken the females are in heat. If the males have been housed near Anales before the experiment, it is found. that all their onding refiexes are inhibited in greater or less degree. It is obvipG@Nt% this case that the inhibition derives from the sexual centres eN ac hemispheres.

In face of such numerous potential sources of jaNDition we see that our term of “ conditioned ” reflexes is verf¥@ypeopriate. Yet these conditions can readily be controlled, ang the-disquieting factors can be eliminated. All of them produce ex l inhibition, and I should like to give a final summing up of eir Character : No sooner does any extra nervous excitation Oy’ 1 the central nervous system than it immediately makes it e felt in diminishing or abolish- ing conditioned reflexes, but orarily only, as long as the causative stimulus or its after-effefF i

Internal inhibition of conditioned reflexes : (a) Extinction. Towarps the end of the last lecture I discussed external inhibition of conditioned reflexes, as exhibited in numerous cases of temporary clashing between conditioned reflexes and other extra excitatory processes in the brain, and we saw how this clashing led to weakening, more or less profound, and sometimes even to the disappearance of the conditioned reflexes. ; In the second type of inhibition, which may be termed internal inhibition, the positive conditioned stimulus itself becomes, under definite conditions, negative or inhibitory ; it.now evokes in the cells of the cortex a process of inhibition instead of the usual excita- tion. Conditions favouring the development of conditioned reflexes of the negative or inhibitory type are of frequent occurrence, and these reflexes are met with not less frequently than reflexes of the positive or excitatory type.

The most striking difference between external and internal inhibition is that, whereas under the conditi scribed in the preceding lecture external inhibition is prod. n the very first (Nrbition on the other hand always develops progressively, he) ten very slowly, and in many cases with difficulty. I shall start by describing thatit internal inhibition which was first encountered in our AS s, and shall trace the growth Demonstration.—The of g is an example of an experiment illustrating the first grou nternal inhibitions.

We are taking fo, periment the same dog that was used in the second oc the conditioned reflex to the sound of the metronome. I ng the reflex the metronome is sounded for 30 seconds duri hich the secretion of saliva is measured in drops, and at the Ge time the interval between the beginning of the stimulus the beginning of the salivary secretion is recorded. This RG is customarily called the latent period, although as will be seen later some other term might more usefully have been employed. Stimulation by the metronome is not followed in this particular experiment by feeding, i.e. contrary to our usual routine the conditioned reflex is not reinforced. The stimulus of the metro- nome is repeated during periods of 30 seconds at intervals of two minutes. The following results are obtained :

F Secretion of Saliva yn in Drops during 30 seconds 5 EAE ee The continuation of this experiment must be left over until later on in the lecture when it will be possible to add a further important detail. One detail, however, already stands out quite clearly, namely that repeated application of a conditioned stimulus which is not followed up by reinforcement leads to a weakening of the conditioned reflex. If the experiment had been pushed further there would have come a stage when the reflex would entirely dis- appear. This phenomenon of a rapid and more or less smoothly progressive weakening of the reflex to-a conditioned. igen which is repeated a number of times without et hen y appro- priately be termed extinction of conditioned reflex uch a term has the advantage that it does not imply hypothesis as to the exact mechanism by which the p enon is brought

relating to experimental extinction conditioned reflexes, but before I can discuss this it is necessar~¥o make a few remarks about the terminology which will be loyed. Formerly we made a distinction between “‘ natural ? “ artificial ” conditioned reflexes, “natural” reflexes bein e which appeared to be formed spontaneously as a res Orne natural association of, for example, the sight and smell we with the eating of food itself, or of the procedure of introgSging acid or some rejectable substance with the acid or the rgfaeéable substance itself, while “ artificial ” reflexes . were those Whigs ould be formed as a result of artificially associating

with the food or rejectable substance stimuli which in the ordinary course of events have nothing in common with food or the rejectable substance. At the present time, however, we know that there is not the slightest difference in properties between all these reflexes. I mention this fact here because the numerous experiments of the earlier period of our work were carried out with the “ natural ” conditioned reflexes, and it is from these that I shall draw many examples in the present lecture. All the numerous artificial stimuli which we now use every day in our experiments were important to us at the time of those experiments because they provided easily controlled, exact, and regularly reproducible stimuli, and because they could be applied to check the correctness of our concep- tion of the mechanism by which. natural conditioned reflexes are formed. At present.the artificial stimuli predominate in impor- tance because of the vast field of research they have unfolded to us and because they came ultimately to provide the most important material for our investigation.

The progress of experimental extinction is often subject to fluctuation. The fluctuations of an otherwise smooth curve may be brought about both by external and internal factors. To obtain a smooth curve of extinction of a conditioned reflex it is necessary to maintain the unreinforced conditioned stimulus rigidly constant in character and strength; the environing experimental conditions also must remain absolutely constant. Very widgNluctuations in the reflex undergoing experimental extinction aragyt do occur in the case of a natural conditioned stimulus, for ex e the presentation of food, which may be held at one time furtl ay from the animal than at another, or which may be held sy GQnary or slightly moving. With an artificial conditioned stimulu he other hand, it is quite easy to obtain an exact repetition stimulus and so to avoid this cause of disturbance in theéxeurve of extinction. With regard to variations in the experimen g) conditions it is only natural that any marked changes in the@vironment, such, for example, as any introduction of strong ẹ stimuli which would produce external - inhibition, should al cÉ the smoothness of the curve of experi- mental extinction (Nich strong stimuli abruptly diminish all

conditioned refi€xeg, including of course reflexes undergoing extinc- — tion, but thg-yeflexes reappear when the disturbing stimuli are ae greater interest attaches to the effect of extra l intensity. Such stimuli produce a temporary weakening, not of the reflex, but of the progress of the experimental extinction. An example can be seen in the fifth repetition. of the conditioned stimulus in our lecture demonstration (p. 49). . The rise in the reaction from 5 drops to 7 drops definitely coincided with some small disturbance produced by the audience. This effect of extra stimuli of small intensity is of great importance for the physiology of the hemispheres, and we shall return to it later in this lecture.

Even with stimuli of constant strength, and with constant environing conditions, fluctuations in the curve of experimental extinction are sometimes observed. These fluctuations are of rhythmic character and are evidently due to some internal factors. These factors affect directly the nervous processes involved in ex- perimental extinction, and we shall come across examples on frequent occasions in the further course of our discussion. The rate of experimental extinction, measured by the period of time during which a given stimulus must be applied at definite regular intervals without reinforcement before the reflex response becomes zero, depends on numerous conditions. First among these come any individual peculiarities of the nervous organization of the animal. Under the same set of external conditions some animals will have the conditioned reflexes rapidly extinguished, while in others the whole process will be much delayed. In excitable dogs the reflexes ate mostly slow of extinction, but in és animals extinction is rapid. Clearly also the extent to whi reflex has gained a firm footing is an important point : are ich has only recently been established -is likely to be less oP me than an older one and is likely to suffer extin the more quickly. The rapidity of extinction depends also į eat measure upon the intensity of the unconditioned ante ying the conditioned

In this connection the followi eriments by Dr. Babkin are of interest : O An unconditioned reflex Or iven quantity of a 1% extract of quassia introduced into og’s mouth produces on an average of ten experiments Q:. of salivary secretion. A conditioned reflex established. on' basis of this unconditioned one produces 0-3 c.c. during © rG-sninute of stimulation. A definite quantity of 0:1% aqueous O of hydrochloric acid evokes in the same dog an DANN eflex measuring on an average of five experiments

5-2 c.e. The corresponding conditioned reflex gives 0-9 c.c. during one minute of stimulation. The conditioned stimuli are ‘‘ natural ” ones, namely, the presentation. of quassia or of acid, as the case may be, at some distance from the animal. Every other condition of the experiment is maintained rigidly constant. The following table illustrates the experimental extinction of each of the two conditioned reflexes in this animal. Extinction of the Extinction of the Conditioned Reflex to Conditioned Reflex to Hydrochloric Acid. Extract of Quassia. Secretion of Saliva Secretion of Saliva in c.cs. during in c.cs. during one.minute one minute 1-00 c.c. 0-35 c.c.

Yet another important factor in determining the rate of experi- mental extinction is the length of pause between successive repetitions of the stimulus without reinforcement. The shorter the pause the more quickly will extinction of the reflex be obtained, and in most cases a smaller number of repetitions will be requiged. The con- ditions may be illustrated from an experiment byQr-)Babkin : The conditioned stimulus was provided. by mA owder presented to the dog at a distance for one minute ; t mulus was repeated several times in succession, and was of not reinforced. The five series of extinctions given below y rere carried out on the same animal in a single day. Between er Rte extinctions the dog was given a rest, and the reflex was orced by feeding with meat

conditioned stimuli required to produce the next experimental ex- tinction is less, until in the end a zero reaction results in some dogs from only a single application of the unreinforced conditioned stimulus. g A circumstance of especial interest is that experimental extinction of any single conditioned reflex results, not only in a weakening of that particular conditioned reflex which is directly subjected, to the extinction (primary extinction), but also in a weakening of other conditioned reflexes which were not directly subjected to extinction (secondary extinction). This latter phenomenon involves not only those conditioned reflexes which were based upon a common un- conditioned reflex with the primarily extinguished one (homogeneous conditioned reflexes), but also those which were based upon a different unconditioned reflex (heterogeneous conditioned reflexes). Sometimes secondary extinction reaches a profound degree, involving even the unconditioned reflexes. The latter case is illustrated by the following experiment of Dr. Perelzweig :

A given quantity of hydrochloric acid produced on an average in a dog a salivary secretion of 6 c.c. After several extinctions of the corresponding conditioned defence reflex, in which the condi- tioned stimulus was tactile, application of the acid itself pro- duced only 3-8 c.c. Even more striking results were obtained in connection with the secondary extinction of hgafpgeneous con- ditioned reflexes, and our experiments were mai cerned with these. In every case of secondary extinction egree to which the

primary experimental extinction is cary4d)is of first importance. A primary experimental extinction of ined reflex which is carried to its final stages smooth he many small points of difference between secondary pak lons of different conditioned reflexes, but a primary extinctioQ'W ich is carried to only a moderate degree leaves these small d@yrences well pronounced. It may be stated, other things bein ual, that the extent to which homo- geneous conditioned undergo secondary extinction is deter- mined by their rel physiological strengths. The strength of its turn, depends on whether it has long been

established, op, tiẹ number of times it has been refreshed by rein- forcement, $ on whether or not the reinforcement has been disconti and for how long. The extent of secondary extinction ES o on whether, and on how often, the reflex has previously been subjected to experimental extinction, and on whether or not it is reinforced immediately before the primary extinction is begun. The greater the strength of the conditioned reflex as compared with the reflex which is subjected to primary extinction, the less does it undergo secondary extinction ; on the other hand, if the stronger reflex is subjected to primary experimental extinction the weaker conditioned reflex undergoes complete secondary extinction. The following experiments bearing on this subject were performed by Dr. Babkin :

A dog has three conditioned reflexes to acid, one depending on the sound of a buzzer, a second on the sound of a metronome and a third on a tactile stimulation of the skin. Every conditioned stimulus was continued always for 30 seconds. The first experiment shows the relative strengths of the reflexes. Salivary Secretion Time Conditioned Stimulus in drops during 30 seconds In the following experiment a primary extinction reflex to the metronome was produced by repeating the E at intervals

On primary extinction of the conditioned reflex of medium . strength the weaker tactile reflex also was completely extinguished, while the stronger reflex (to buzzer) was still partly active. These results were corroborated by further experiments in which the order of testing the secondarily extinguished reflexes was reversed. The same dependence of the degree of extinction upon the strength of the conditioned reflexes is seen when the conditioned reflex undergoing extinction is one to a compound stimulus composed of several distinct elements which can be applied either simultaneously or else independently. Primary extinction of the reflex to the compound stimulus is always accompanied by secondary extinction of the reflexes to its individual components. Supposing there are two components of equal physiological strength, then the primary extinction of the one leads to simultaneous secondary extinction of the other, while the reflex to the compound stimulus is usually considerably diminished. Where, however, the two components of the compound stimulus are unequal in strength, primary extinction of the stronger reflex leads to a complete extinction of the reflex to the weaker component, while extinction of the weaker (unless carried beyond zero) leads only to a partial weakening of the reflex to the stronger component. The primary extinction of the stronger reflex leads also to a complete extinction of the reflex to the compound stimulus. The question of interrelation betweeg the different individual components in a compound stimul be discussed

further in a future lecture. In the meantime we must endeavour AAN nd a correct inter- pretation of the phenomenon of experi tal extinction, and we shall find it important in this connectj give attention to a case in which the weaker of the two c ents in a conditioned reflex is completely overshadowed by onger, the weaker when tested separately producing no posityreweflex effect. When in such a case the weaker stimulus is apg singly several times in succession, without reinforcement, results, nevertheless, an extinction not only of the i stronger stimulus, but also of the reflex

to the compound us. An experiment by Dr. Perelzweig can be taken to ill this point : A compountgonditioned reflex has been established on a basis of the TRO reflex to acid. The individual components of the compou imulus are a tactile stimulus and a thermal stimulus of Rou the experiment given below the animal is stimulated by the compound stimulus and by its components separately. In every case the stimulus is applied during one minute. 12.0 noon Compound 12.25 p.m. Compound Compound Thermal Thermal Thermal Tactile Compound Compound Tactile