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

This experiment shows that the application of the thermal component, which by itself was ineffective, led when repeated three times without reinforcement to a complete secondary extinction of the stronger tactile component and to a practically complete extinction of the reflex to the compound stimulus. Hitherto, when referring to the degree of extinction, we have only spoken of the extinction as being partial or as being complete, but we shall now have to extend our conception. Not only must we speak of partial or of complete extinction of a conditioned reflex, but we must also realize that extinction can proceed edna the point of reducing a reflex to zero. We cannot ther judge the degree of extinction only by the magnitude of eflex or its absence, since there can still be a silent extincti yond the zero. This statement rests upon the fact that a ek: ued repetition of an extinguished stimulus beyond the ce the positive reflex deepens the extinction still further. Şu extension of our con- ception serves fully to elucidate the Sermon just described, and it explains why the seemingly inact¥e thermal component when subjected to experimental exting&iDJled to such a profound secondary extinction of the stronger t component. The importance of considering the degree of €X$Netion in all experiments thus becomes evident. The method. ermining the degree of extinction when it goes beyond zero A) explained in connection with the question

which will next be Gigcussed. We shall co r what happens to the conditioned reflexes after they have Sa to experimental extinction and inquire whether they ever regain their original strength. Left to themselves extinguished conditioned reflexes spontaneously recover their full strength after a longer or shorter interval of time, but this of course does not apply to conditioned reflexes which are only just in process of formation. Such reflexes, being weak and irregular, may require for their recovery after extinction a fresh reinforcement by the — underlying unconditioned reflex. However, all those conditioned reflexes which have been fully established invariably and spon- taneously return sooner or later to their full strength. This provides one way of determining the depth of extinction ; it is measured, other conditions being equal, by the time taken for spontaneous restoration of the extinguished reflex to its original strength. Such time interval may vary for the different reflexes from a few minutes to a number of hours. I shall give a few experiments in illustration. The first is an experiment by Dr. Babkin :

Presentation of meat powder a short distance away at intervals of three minutes ; the reflex is not reinforced. any spontaneous recovery of the reflex to the metronome which was partially extinguished just twenty-three minutes ago. Continuation of Demonstration.—The dog is again subjected to the stimulus of the metronome for 30 seconds: the latent period of the conditioned reflex now comes out at 5 seconds and the salivary secretion is 6 drops. At the last reading the latent period was 13 seconds and the salivary secretion 3 drops. Con- siderable recovery has therefore taken place spontaneously during the lecture.

The great differences in rapidity of restoration of extinguished reflexes depend, on a number of factors. The most important factor is the depth of the preceding extinction. The individual character of an animal and its type of general nervous organization also play an important part. Much depends also on the intensity of. the conditioned reflex which was subjected to experimental extinction ; and finally, upon how often the experimental extinction has been repeated. In every case, however, it is possible to accelerate the

_ restoration of an extinguished conditioned reflex. For this purpose it is only necessary to apply the unconditioned stimulus on which the conditioned reflex was built up, either singly, or together with the extinguished conditioned stimulus. This method produces a more or less rapid restoration, according as the conditioned reflex has been extinguished to a greater or less degree. If the extinction has not been carried very far, a single application of the u itioned. stimulus is often sufficient to restore the reflex to full if the extinction has been made profound, repeate are necessary. This means of accelerating the tinguished reflex affords another method of extinction. The further question wheth

acceleration in the ex is greater when the when it is applied in the form of reinforcement cannot be digcu8Sed at present, as it is still under investigation (see Lecture agi All this description of f t extinction may have proved rather wearisome to the re owing to the absence of any under- lying uniformity. Nev, ss they served an important purpose in that through a care consideration of them we were enabled gradually to eon afon a solution of the fundamental question as to the intimate e of experimental extinction. By ruling out one interpre after another we arrived at the conclusion that

I- extinction must be regarded as a special form of inhibition. That | it cannot be regarded as an irreparable destruction of the conditioned | reflex, due to disruption of the respective nervous connections, is I | evidenced by the fact that the extinguished reflexes spontaneously I| | regenerate in course of time. Another possible explanation also I suggests itself : may it not be that the experimental extinction is l- brought about simply by fatigue in some part of the neuro-secretory apparatus involved in the reflex ? This is ruled out by the following evidence. The secretory elements in the gland do not become It | fatigued when the conditioned reflexes are being reinforced, although

they continue indefinitely during an experiment to produce a full Mit | salivary secretion. Moreover, the restoration of an extinguished | ‘reflex is greatly accelerated by a fresh application of the uncondi- | | tioned stimulus, a still further secretory activity of the gland being | | readily obtained although the reflex has been deeply extinguished. | Neither can there be fatigue in the nervous centres of the secretory | reflex. It is sufficient to recall the experiment with the conditioned reflex to the compound stimulus which had two cutaneous com- | ponents—tactile and thermal. The thermal stimulus, which was | the weaker, could not by itself produce even the slightest positive Ili effect: yet none the less its repeated application brought about a Il secondary extinction of the stronger tactile stimulus and even of the I compound stimulus itself. The extinction of the ineffective thermal l | stimulus was at no time accompanied by any abt ositive activity i of the nervous elements, and it is difficult to CORRE that a part of

Hl | the central nervous system underwent <a Rae linea previous activity. Again, it would seem that if w. re to admit the pos- sibility of any fatigue in the satan ed , we could expect only WV the reflex to the thermal stimulus t gO fatigued, but we find | | also an extinction of the reflex tol ctile stimulus which was not | brought into activity at ae uring the repeated stimulation of the thermal receptors. y a process of elimination we are | forced to the conclusion ein experimental extinction is based on IM inhibition, and if we Roo the facts which have been described, in i the light of this n nearly all of them become perfectly il} intelligible. | The i EDs rhythmic fluctuations in the reflexes sometimes observed SEs the process of experimental extinction can now ined as a manifestation of the struggle which is taking

| easily b i place en the nervous processes of excitation and inhibition before one or other of them gains the mastery. Similarly it becomes quite easy to understand the part played by the individuality of the animal. We have all observed for ourselves how the inhibitory processes in the nervous system of human beings are seldom of the same intensity in any two people, and numerous examples in the further course of these lectures will make it clear that a precisely similar variation obtains in the nervous system of animals. |

It is clear that the more vigorous a conditioned. reflex, or in other words the greater the intensity of the excitatory process, the more intense must be the inhibitory process in order to overcome it, and therefore the greater the number of unreinforced repetitions necessary to bring about complete extinction. Again, it was seen that a re- petition of the non-reinforced conditioned stimulus was necessary to produce a ‘sufficient summation of the inhibitory after-effect for complete experimental extinction, and it is reasonable to suppose that the shorter the intervals between successive repetitions of the stimulus the more quickly will the required intensity of the inhibitory process be obtained. This also was found to be the case. As a result of repetitions of experimental extinction on the same animal the zero level of a fresh extinction of the reflex is. reached more rapidly. This shows that inhibition like excitation is facilitated by repetition. The fact itself is well known from observation of ourselves and others, but abundant experimental evidence for anima ill be afforded during the further course of our study of Bh itioned reflexes. ,

The influence exerted by experimental extingijén on reflexes other than the one undergoing extinction, incl unconditioned reflexes as well as homogeneous and he neous conditioned process from its point of initiation thro the entire nervous struc- ture of the hemispheres. This process wil be fully discussed in one of the later lectures. . We have now to consider in still another important feature which has already been not assing, but was left unexplained, namely the frequent devi i@s observed in the curve of experimental extinction. These BE e represent sudden rapid. strengthenings in the intensity of .tareflex which is undergoing extinction, and they depend on t roduction of any accidental stimuli into the experimental exVwonment. Some extraneous sound or shadow

reflexes, must be regarded as the result of ng of the inhibitory finding its way into the room produces at once a rapid strengthening of the reflex, and of course a similar effect is produced by different extra stimuli which we ourselves apply on purpose in order to study this phenomenon experimentally. — I shall describe first of all an observation which for a long time we were at a loss to interpret. A natural conditioned reflex to meat powder, which, as we know from contrel experiments, after extinction récovers its initial value spontaneously in something between a half and one hour, is again extinguished to zero. This time, however, instead of waiting for the spontaneous recovery of the reflex a weak solution of acid is immediately introduced into the dog’s mouth, and after the termination of the secretion produced by the acid (about five minutes) meat powder is again presented at a short distance. This time although nothing like half an hour has elapsed the con- ditioned alimentary reflex is found to be almost completely restored, At first sight the accelerated recovery of the extinguished reflex seems paradoxical, since we know already that positive conditioned reflexes are always quite definitely specific—a definite stimulus rigidly evoking a definite reaction—but in this case a stimulus to an extinguished conditioned alimentary reflex has had its full strength restored through the single application of a stimulus to a heterogeneous unconditioned reflex, namely the defence reflex to acid. And there can be no doubt that although the secretory component of the two reflexes is effected through the same glands yet are distinctly heterogeneous in nature, since the defence r o acid differs sharply from the alimentary reflex to fo @pth as regards the composition of the saliva secreted and KAN the character of the motor response. Without attem or the present to give any explanation we can designat bservation from a purely matter of fact point of view as sting of a sudden removal by an extraneous reflex of the inhi®itory process set up by experimental extinction.

The whole group of cakey of which the above is an illustration have one common fe In all of them the removal of the inhibition is only rary, persisting no longer than the extra stimulus respo i® or the removal of inhibition and its after- effect. It is integasting to mention in this connection a disagreement which ar eB the members of the staff in our laboratory before ct of the restorative effect of acid upon an extinguished alimentary conditioned reflex had been indubitably established. Some of the workers admitted this restorative effect without question, while others disputed it. However, the experimental side of the question turned out to be right in both cases. The cause of the discrepancy was clearly brought out in Dr. Zavadsky’s researches. It appeared that previous observers had overlooked the fact that their conditions of experimentation were not fully identical. Those workers who accepted the restorative effect had tested the extin- guished conditioned alimentary reflex immediately or only a few moments after the salivary secretion in response to the acid had ceased, while the others had tested the extinguished reflex after allowing a considerable interval of time to elapse. Realizing the difference in the experimental procedure of the two sets of: workers Dr. Zavadsky was able in his experiments to obtain all the different stages that had been reported by other workers.. Two of his experi- ments, performed on the same day, are given below.

Salivary Secretion in drops during one minute. Stimulus applied Time during aa ay one minute From From submaxillary Parotid gland gland 2.28 p.m. ‘Meat powder pre- 16 12 2 40> ., | sented at a dis- 9 A 262 >} tance out of 7 oD ar eR, reach of. the 5 : 3.18 5; animal. 0 Qo S20) Acid introduced into the dog’s xO mouth: The flow Q of saliva con- O sequent on this K ceased at 3 hrs. acid and the subsi t testing of the extinguished reflex was in the above exper 7 minutes 10 seconds.

Time during one minute From From submaxillary Parotid gland gland f Meat powder con- 3.34 p.m. (ape | Meat powder pre- 3.46 ,, sented at a ais | 10 8 | tance. 347. {Most powder con- sumed. Abs, | Meat powder pre- 7 6 4.25 trs - sented at a dis- 4 3 4:35. ,; | tance. l 0 ADIN, Acid introduced into the dog’s mouth. The re- sulting salivary secretion ceased at 4 hrs. 54 min. 20 sec. p.m. Meat powder pre- 4.55 p.m. | sented at a ts | ins 5 tance. The time interval between the end of the tion produced by acid and the subsequent testing of the olsu reflex was in this experiment 40 seconds.

Seyen minutes after the salivar Weretion to acid had ceased the restoration of the condition entary reflex was minimal, only one gland showing any a When, however, the reflex was tested only 40 seconds afte the aas secretion to acid had ceased, a considerable re eye of the alimentary conditioned reflex was found, toa oth glands. By this and si eriments the temporary nature of the restoration of ui Sri reflex in response to other extra stimuli was cay emonstrated. The restorative effect was in no way confine e administration of acid but was produced also

by any oth tra stimulus. A further example from Dr. Zavadsky’s ae illustrates this general case. The experiment was con- Time during one minute From From submaxillary Parotid gland gland 1.53 p.m. | Meat powder pre- | 11 7 1.58 ;, - sented at a dis- 4 2 ERRI I tance. 0 0 2 Bie; Same + tactile stimulation of skin. 3 l 2I. Same + knocks un- ; der the table. 2 1 218 ©; Meat powder at a distance. 0 0 220° 5; Prof.Pavlov enters the room con- taining the dog, talks, and stays for two minutes. 2.23 >, Meat powder at a distance. 5 2 228 5. ,, Same. 0 0

Nortr.—Previously to this experiment it had been repeatedly shown that neither the tactile nor the auditory stimulus, nor the entry of Prof. Pavlov into the experimental rao, produced any secretory effect at all. This experiment leaves no doubt that the extingui alimentary conditioned reflex is restored both by the actua presence of the extra stimulus (tactile and auditory), and by ~Olter-etiect (after- effect of stimulus of my entering the room In all the experiments which have been described the restoration of the extinguished reflexes Msts only for a few minutes, depending on the duration of the extr&timulus and its after-effect. In the case, however, of certain ial extra stimuli already men- tioned in connection with ext inhibition, stimuli which are of a protracted nature, the AN tive effect is felt throughout the whole course of experi ®© extinction, which is therefore never smoothly progressive agd)jcan never be brought down to and kept at the zero level of reflex. í

We have GN scuss another important observation bearing on the same poi uring the whole period of our work we observed on many occasions the simultaneous existence of several different reflexes, leading of course to an interaction between them which resulted either in predominance of one or another reflex or in their mutual neutralization. Thus, if we make a tactile stimulation of the skin the stimulus to a conditioned reflex, it frequently happens that we are bothered with an interference from the unconditioned ` reflex response to the cutaneous stimulus itself, in the form of the scratch reflex or some sort of quivering reflex. This may, in rare cases, be so troublesome that the conditioned reflex never reaches a stable value. Exactly the same thing happens sometimes with musical tones of exceedingly high pitch, it being in some dogs impossible to overcome the difficulty of the resulting sharp motor response. All such powerful unconditioned stimuli exercise an external inhibitory influence which perpetually interferes with all positive conditioned reflexes. But it is obvious that these persistent _ extra reflexes should exert a still more powerful disturbing influence upon the normal course of the inhibitory processes’ underlying extinction, since inhibition is in every respect more labile than excitation. I shall be giving a number of examples substantiating this statement in a further lecture, when the whole matter will be subjected to a rigorous experimental analysis. All the considerations put forward in this lecture permit us to regard the temporary restoration of the reflex which is in process of extinction, or which is already extinguished, as based upon the remo f an inhibitory process. We therefore describe this phenome s a dis-inhibition, a term we shall always use in the future e wish to denote a temporary removal of inhibition.

The next question is, whether OYistinction can be drawn between the case of the restoratj an extinguished conditioned reflex resulting from fresh agpp¥ecdtions of the appropriate un- conditioned stimulus, and t Ae which has just been termed dis-inhibition. Our exper ts show that undoubtedly such a distinction does exist. Cie first case, when restoration is effected by the special PON stimulus: underlying the reflex which has undergone ext Ton, such restoration is permanent. In the second case, r, when the restoration is effected under the influence of an\dlien stimulus, such restoration is only temporary.

| As to the fol reason for this difference it is not possible to say very he experimental evidence available up to the present. owever, no doubt that in the first case, just as has already been shown in the second, we are dealing with a removal of inhibition. Any hypothesis of an irreparable destruction of the conditioned reflex in the process of experimental extinction cannot possibly stand for a moment, since in every case of extinction the reflex — invariably becomes spontaneously restored in a longer or shorter time.

The question of the difference in the mode of restoration in these two cases probably goes much deeper, involving the intimate nature of the nervous process underlying dis-inhibition. Regarding the nervous mechanism of dis-inhibition we cannot hope at present to _ approach anything like a fundamental conception, since as yet we know little about the real nature either of the inhibitory process, or of the excitatory process, or of their mutual relations.

I should like, however, specially to direct your attention to one very important feature which repeatedly enforces notice. We have seen that the very same extra stimuli, which, when they evoke strong extraneous reflexes, produce external inhibition of the positive conditioned reflexes, produce, when their effect is weak from the start or weakened by repetition, dis-inhibition of the conditioned reflexes which were made to undergo extinction. Many examples of this will appear in the next lecture. We are now afforded some justification for regarding dis-inhibition, as we did a short while ago, as being the “ inhibition of an inhibition.” By this we do not pretend, however, to explain the underlying TARR of dis- inhibition. Q

The main conclusion of our discussion of xperimental evidence described in this lecture can be su up briefly as follows. A stimulus to a positive conditioned can under certain definite conditions readily be transfor o a stimulus for a negative or inhibitory conditioned r OS transformation is fairly rapid, smooth and ogres ce becomes obvious therefore that in our further study of the function of the cerebral hemispheres we shall necessarily be dealing Canty with positive but also with negative or inhibitory conditi eflexes.

Internal inhibition (continued) : (b) Conditioned inhibition. THE fourth lecture was devoted entirely to the study of the first type of internal inhibition, which was termed experimental extinction. In extinction the positive conditioned stimulus is temporarily transformed into a negative or inhibitory one by the simple method of repeating it several times in succession without rein- forcement. In the present lecture we shall consider the second type of internal inhibition, which has also been investigated in some detail.

The method of experimentation is as follows. A positive con- ditioned stimulus is firmly established in a dog by means of the usual repetitions with reinforcement. A new stimulus is now oc- casionally added, and whenever the combination is applied, which may be at intervals sometimes extending to hours or days, it is never accompanied by the unconditioned stimulus. this way the combination is gradually rendered ineffective, so e conditioned stimulus when applied in combination with th itional stimulus loses its positive effect, although when annie al and with con- stant reinforcement it retains its full pow

We have been accustomed in our i phenomenon by the name of congffi cannot be regarded as especially a priate since the development - of experimental extinction also Abject to equally rigid conditions. The use of this term, in fact, ~an only be justified by historical considerations. Since w, re concerned in this case with the participation of an NE l stimulus, the whole phenomenon was confused at first Wan ternal inhibition. It was only later when its character of } , as distinct from external, inhibition became firmly establishédAhat the prefix “ conditioned ” was added. As will be sho her on, this form of inhibition might more appro- priately h¢VyYbeen termed “ differential inhibition.”

especial interest. While illustrating the varied complexity of the phenomena involved, it demonstrates at the same time the value of the experimental method as providing a satisfactory means for the analysis of this very complexity into simple general principles. : For this reason we shall discuss conditioned inhibition in considerable detail. The first point of importance in the establishment of a conditioned inhibition is its dependence on time relations between the applica- tions of the two stimuli in the inhibitory combination. Conditioned inhibition is developed with comparative ease in all those cases where the duration of the positive stimulus overlaps that of the additional stimulus. In our experiments it is usual to start the additional stimulus a few seconds (generally from 3 to 5) before the positive stimulus, but provided there is an overlap it is immaterial whether the commencement of the additional stimulus precedes, coincides with, or even follows by a few seconds, that of the positive stimulus. If, on the other hand, the additional stimulus is removed as soon as the positive stimulus is applied, so that the two stimuli never coincide, the development of the conditioned inhibition may be a matter of considerable difficulty, and accompanied by restlessness and various defence reactions of the animal. If, finally, a pause of several seconds is introduced between the termination of the additional stimulus and the beginning of the positive stimulus no inhibition develops at all. On the contrary, in the maj coy cases, when this pause reaches a duration of about ten soqos the addi- tional stimulus itself acquires the properties of a e stimulus. This has been discussed already as the general d of formation of positive conditioned reflexes of the second It is only with exceptionally powerful additional stimu for example as a powerful motor-car hooter, that the a an be increased to so

much as twenty seconds and a c tioned inhibition still be developed. An example from the È of Dr. Frolov will serve to illustrate the latter case : A motor-car hooter wa ghd to act for 10 seconds, when, ec after a pause of a further onds, the alimentary conditioned stimulus of a metro Owas applied. The first application of the hooter did not Cho least diminish the magnitude of the succeeding reflex+ tion. When, however, the same combina- tion had been ted several times in succession, and always without we the reflex began gradually to diminish.