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

This diminution persisted even when the pause was lengthened to 20 seconds. Experiment of 28th December, 1924. Second application of the motor-car hooter. à Duration of Salivary Time Stimulus Stimulation Secretion Remarks in seconds in drops - 1.41 p.m. Metronome 30 9 Reinforced. 1.48 _,, >, = (Hooter 10 0 235% | Pause 20 i ARES aa | Metronome 30 6 a Experiment of 21st January, 1925. Thirteenth application of hooter. à Duration of Salivary Time Stimulus Stimulation Secretion Remarks | in seconds: in drops 1.58 p.m. Metronome 30 8-5 Reinforced. 2.9 >. £ (Hooter 10 0 33 rL O £3 E [Pause a 0 AEN ee UNtsigencnne 30 1 :

We thus find that the time interval between the CY imul required to produce either conditioned inhibition, or al tively a positive conditioned reflex of the second. order, var cording to the in- tensity of the additional stimulus. This iùgency of the ultimate significance of the additional stimul n the time relations is interesting as evidence of an en between the antagonistic processes of excitation and inhi ie) The following interpretati f the complete difference in the final character. assumed a additional stimulus under so slight

a difference in the experi al conditions seems best to agree with the experimental resyl hen the additional stimulus or its fresh trace left in the he eres coincides with the action of the positive stimulus, there esult some sort of special physiological fusion of the effect ote stimuli into one compound excitation partly differing OR partly resembling the positive one. It will be shown in seventh lecture, that in response to closely related N Cri as neighbouring tones or tactile stimulation of adjacent

places of the skin, the same events take place as in the development of conditioned inhibition. If one stimulus with the help of the usual procedure is given positive conditioned properties, the neighbouring stimuli belonging to the group also give at first a positive conditioned reaction, whereas later, on their systematic repetition without reinforcement, they lose their excitatory properties and acquire inhibitory properties instead. This result corresponds exactly with the successful development of conditioned inhibition in the experi- ment mentioned at the beginning of this lecture. When, however, the additional stimulus is separated from the application of the positive conditioned stimulus by a longer interval, the union in the ~ hemispheres into a single compound excitation does not materialize ; instead, the stimuli act upon the hemispheres as two distinctly separate events, and the additional stimulus acquires new properties as a positive conditioned stimulus of the second order. The older conditioned stimulus acts in this case in exactly the same capacity as an unconditioned stimulus in the usual method of establishing conditioned reflexes of the first order. A strong stimulus has a prolonged after-effect ; it is, therefore, still capable of being fused by the hemispheres with the conditioned stimulus into a special new compound even after a long interval of time. It is easy to understand from this point of view that in the establishment of conditioned inhibition the interval which can be made between the end of the additional stimulus and the beginning of the positive oe Se stimulus must depend directly upon the strength of tha itional stimulus. Whether our interpretation be the cor yone or not, the phenomena are encouraging to the experimentĘ&N? that in every case he can discern a remarkable regularity mK se very complex activities of the cortex.

mention must be made of certain very aS phenomena obtained with some apparently normal animals andi animals in which surgical had produced a condition of the nervous. system. In these cases even perfectly TUN s“&pplications of the additional and of the positive conditio imuli led not to conditioned inhibition but to the developmeQt þf a definite positive conditioned reflex of the second order, Afhis generally persisted for a very long time, but later it ofteaXgxppened that the secondary conditioned reflex and also eee inhibition were both present simultaneously.

interference with the hemisphe obviously increased excitabili Such a case is represented in the following experiment of Dr. Kasherininova : 3 Tactile stimulation of the skin was used in this dog as the con- ditioned stimulus for the defence reflex to acid, and a metronome served for the additional stimulus in an inhibitory combination which, of course, was never reinforced. The twenty-fifth application of the inhibitory combination evoked a secretion of only 3 drops of saliva during one minute, whereas the conditioned stimulus applied singly evoked a secretion of 29 drops during one minute. After thirty-four repetitions of the inhibitory combination the stimulus of the metronome applied singly elicited a salivary secretion of 8 drops, although prior to its participation in the inhibitory com- bination it had no excitatory effect whatsoever. It is clear, therefore, that the metronome coupled with the tactile stimulus exerted a strong inhibitory influence, but it acquired at the same time some of the excitatory properties of the tactile stimulus, so that when used alone it behaved as a conditioned stimulus of the second order. Though cases such as that just described have been observed but rarely, it is quite possible that a transitory acquisition of weak excitatory properties of the second order is of no infrequent occur- rence in the development of conditioned inhibition.

The course of development of conditioned inhibition is not always the same. In some cases the first addition of a new stimulus to the positive conditioned stimulus immediately cones diminution, or even in a complete disappearance, of the ‘Sud ioned reflex. With successive repetitions of the combin the reflex comes back nearly to its original level and then in falls slowly to zero. In other cases the first few combinatio ult in an augmentation of the reflex as compared with the Qn isolated action of the conditioned stimulus, and only sub eq) eħtly does the reflex gradually diminish to zero. In yet a thi group it is found that an initial diminution in the strength of Me reflex is followed by a phase of augmentation above normgPand then again the reflex slowly falls to a permanent zero. roe different phases which are observed during the establi t~of the conditioned inhibition depend entirely upon the i sity of the extraneous reflex which is evoked e additional stimulus. The initial diminution

of the reflex hich occurs in one group during the first few applica- tions of the\gembination is due undoubtedly to external inhibition. When dditional stimulus applied singly evokes in the dog a strong investigatory reflex it is found that its addition to the positive conditioned stimulus exerts an inhibitory influence from the very start. When on the other hand the stimulus evokes only a mild investigatory reflex, the preliminary phase of diminution of the reflex is absent and the first applications of the combination produce an increase in the reflex. This increase is undoubtedly due to dis-inhibition, since—as we may state now in anticipation of a future discussion—the positive effect of the majority of conditioned stimuli in our experimental reflexes is almost invariably preceded by a phase of internal inhibition. The inhibitory process is removed by the investigatory reflex, provided that the latter is not sufficiently strong also to influence the excitatory component of the conditioned reflex. The third group of cases, in which the initial diminution in strength of the reflex is followed by an increase above normal, depends on the gradual weakening through repetition of the extraneous reflex produced by the additional stimulus. The external inhibition brought about by the investigatory reaction is at first strong enough to inhibit the conditioned reflex, but, on weakening, produces only dis-inhibition and so increases the strength of the reflex response. Several examples of this will be given at the end of the present lecture.

It is obvious that any agent in nature may be used as a stimulus for the development of a conditioned inhibition, supposing of course that the organism is provided with the requisite or or the perception of such an agent. The records of actual ex ents will afford numerous illustrations of the different t of stimuli employed. As was mentioned before, not only the acta Dimulu but also its trace in the hemispheres can be use the development of conditioned inhibition. The trace must ver, in all cases be as recent as possible. The establishm@qt of conditioned inhibition when the interval between the stim is more prolonged can be brought about only by the use AP pxceptionally strong additional stimuli. Once, however, the ory combination has been firmly established the pause bet he end of the additional stimulus and the beginning of sitive conditioned stimulus may- be extended even to so (hugh as one minute without the inhibitory effect of the combination being impaired.

Some experi have been made in which the time inter- val was itse ployed as a stimulus for internal inhibition. The following experiment by Dr. Krjyshkovski illustrates such case : A definite tone served as a positive conditioned stimulus to acid, while a tactile stimulation of the skin served for the additional stimulus in the inhibitory combination. The inhibitory combination in this dog had been habitually applied for some other special purpose at the 19th-20th minute after the last introduction of acid. This led to the result that at the 19th-20th minute only a minimal salivary secretion was produced by the action of the positive con- ditioned stimulus when it was applied singly.

successive applications Salivary Secretion of the Reinforced in drops during Conditioned Stimulus one minute in minutes In these experiments the interval of 19 to 20 minutes had itself acquired sufficient inhibitory properties to abolish or greatly reduce the effect of the conditioned stimulus. The rate of development of conditioned inhibition as well as its completeness (absolute or relative inhibition) sie Yorend upon a number of conditions. Of first importance in nnection is the individuality of the animal, the excitable Mhibitable character of its nervous organization. In some wie establishment of a conditioned inhibition takes a long tim never becomes absolute ; in other dogs an inhibition cee yee and firmly established.

after very few repetitions of t itory combination. A further important oes the intensity of the additional itory combination. For example, ishtovt, a metronome was used for a us to acid, while a thermal stimulation d as the additional stimulus. It was found stimulus at a temperature of 4 to 5° C. the first stimulus employed in the i in an experiment by positive conditione of the skin was ent that with the indication of nditioned inhibition could be observed only after the 30th a Glpation of the inhibitory combination, and the inhibition was not complete even after 145 applications. With the use, BANGS of a thermal stimulus at a temperature of 1°C., in an ex-

periment performed on the same animal after an interval of four months, only twelve repetitions of the inhibitory combination were required to establish complete inhibition. The rate of formation of conditioned inhibition depends, again, on the character and the relative intensity of the additional stimulus in comparison with the conditioned stimulus. In some dogs it was found impossible to establish complete inhibition by the addition of a thermal stimulus at 45° C., when the sound of a metronome served as the positive conditioned stimulus. When, however, a visual stimulus was employed as the positive conditioned stimulus in a similar combination, a conditioned inhibition could be readily produced [experiment by Dr. Foursikov].

Finally, it should be mentioned that, although other factors may remain constant, the first establishment of a conditioned inhibition in a dog takes more time than any succeeding one. I have not up to the present given any conclusive evidence that the phenomenon of conditioned inhibition is really in the nature of an inhibition at all, and is not merely a passive disappearance of the positive conditioned reflex owing to the compound stimulus re- maining habitually unreinforced. The proof that the phenomenon actually does represent a real inhibition will be gradually brought out as we proceed with our study of the experimental evidence.

It will be of interest, first of all, to study the nature and ultimate function of the additional stimulus on which conditione ibition depends.. This can, of course, be determined only by 45g out the action of the additional stimulus in different mo ions of the experiment. Tested singly after the conditione Wition has been fully established it produces no positive effec 1, The action of the additional stimulus can be tested, cf by applying it in which it has never previously been iated. In such a case the inhibitory properties of the addjtidw4l stimulus become clearly revealed, the result being an i iate diminution in the positive reflex response. This is trye nly in the case of homogeneous reflexes, but also in the Ny of heterogeneous reflexes, and the inhibitory effect may even to the unconditioned reflexes themselves. These f are clearly exhibited in the following experiment by Dr. Sens

Three sine conditioned reflexes have been established in the dog we iş experiment, the three stimuli being the flash of a lamp, a rotating object, and the tone C sharp of a pneumatic tuning fork. Two independent conditioned inhibitions of the reflex to rotation have also been firmly established, one by the use of tactile stimulation of the skin and the other by the use of a metronome. In the first experiment the flash of the lamp is for the first time accompanied by a tactile stimulation of the skin :

Time Stimulus applied Saliva in during 1 minute drops during 1 minute 1.38 p.m. Rotating Object. 16 1507 Flash of lamp. 17 2.14, Flash of lamp +tactile stimulus. 2 PADAT Routine reinforcement of rotating object by feeding -- 243 Rotating object + tactile stimulus (the usual in- hibitory combination) 0 In the second experiment the tone C sharp is for the first time accompanied by the metronome. It will be seen NN e additional stimulus when applied for the first time in th &)Combination produced a diminution almost to zero in the cortditdoned reflex response.: It follows, therefore, that when an addG@pnal stimulus is used with an alien homogeneous con- ditioned er its inhibitory property becomes thereby immediately revealed. N This same property is seen also when the additional

stimulus is combined for the first time with a heterogeneous conditioned reflex, a case which is illustrated in the following experiment : A dog has a conditioned alimentary reflex which has been established by the use of the metronome, while the addition of a whistle provides a powerful inhibitory combination. Besides this . a conditioned reflex to acid has been established in response to tactile stimulation of the skin. The metronome and the tactile stimuli belong therefore to heterogeneous conditioned reflexes, and the positive effect of one of them (i.e. the metronome) is completely inhibited by the sound of the whistle. The whistle is now for the first time combined with the heterogeneous tactile conditioned stimulus [experiment by Dr. Babkin].

In all the foregoing experiments it had, of course, to be shown before any particular additional stimulus was used in c ination with an alien positive conditioned stimulus that it woul exercise any effect of external inhibition. With this precaugh/it seems to me that the experiments justify our conclusion ae where a con- ditioned inhibition has been firmly established thes ditional stimulus itself acquires inhibitory properties which manifested outside the parent combination. The additiona rays is therefore termed in our investigations the conditioned plier.

It is made clear by further expertwfents that a real inhibitory effect is also produced by the jį itory combination itself, the inhibitory process persisting as er-effect which may be detected some considerable time afte stimulus of the inhibitory combina- tion itself has been re ved) The inhibitory after-effect exerts its influence not only BR particular reflex to the conditioned stimulus employed the combination, but also upon all other conditioned refle whether homogeneous or heterogeneous. In this connection We may consider the following experiments :

In the first experiment a rotating object serves as an alimentary conditioned stimulus, and a tone of 30,000 vibrations produced by a Galton’s whistle as its conditioned inhibitor [experiment by Dr. Nikolaev]. Salivary ; Time Stimulus applied Secretion in . during 1 minute drops during 1 minute 3.5 p.m. Rotating object 7 320 (3, Rotating object 6 339.. Rotating object + tone 0 308 eire Rotating object 1 After withdrawal of the stimulation by the inhibitory combina- tion the effect of the positive conditioned stimulus is weakened for several minutes, and only regains its normal strength by degrees.

In the next experiment a rotating object serves as a conditioned stimulus for the defence reflex to acid, and a musical tone serves as a conditioned. alimentary stimulus. A tactile stimulus is used as a conditioned inhibitor for the alimentary reflex. [Experiment by Dr. Ponisovsky.] 30 seconds 12.23 p.m. Rotating DNN 5 bs PA Rotating ebj 12 EAGER Tone + tądNe stimulus 0 12.48 Rotatin ject l This experiment d Djirates that the heterogeneous conditioned reflex also become inished as a result of the inhibitory after- effect of the cg Q ion. When the bitor is applied by itself alone, and not together with the -e@statory stimulus in combination with which it was originall veloped, it can also be demonstrated to produce an

The degree to which positive conditioned reflexes are influenced by the conditioned inhibitor varies inversely as their relative physio- logical strength. An experiment may be taken from a paper by Dr. Leporsky in illustration of this point : Three independent conditioned alimentary reflexes have been firmly established to a rotating object, to the flash of several electric lamps, and to a musical tone. A tactile stimulation of the skin has been established as a conditioned inhibitor for each of the three reflexes, so that in combination with any one of them separately it reduces the reflex to zero. All three positive stimuli applied together produce a much greater salivary secretion than any one of them applied singly, showing summation of conditioned reflexes. The experiment shows the effect of the application of the conditioned inhibitor in conjunction with the simultaneous action of all three positive stimuli.

Amount of Time Stimulus during , Saliva in 1 minute drops during 1 minute 1.40 p.m. Tone 21 LORT Rotating object + tone + flash 32 2.10- y; Rotating object 2: a ae Simultaneous application of all | three positive conditioned | ~\ stimuli + tactile stimulus A Lala Rotating object + tactile stimu- AN lus à & The conditioned inhibitor, therefore, altho reduced to zero every one of the positive conditioned refle n singly, could only partially inhibit-the reflex secretion x by all three acting simultaneously (2.27 p.m.). Q

Two further details concerning ibitory after-effect remain to be considered. The first is henomenon of summation of but several times in succes so much the more will the strength and duration‘ of the i y after-effect be increased. Two ex- periments of Dr. Chebotayéva, carried out on successive days, serve to illustrate the cas Gm point : A ting ole N as a conditioned alimentary stimulus, and a rotating AS S or conditioned inhibitor. Amount of Time Stimulus during Saliva in 30 seconds drops during 30 seconds First experiment : 3.32 p.m. Metronome 5 3.40 ,, Metronome 6 3.50 ,, Metronome +rotating object 0 Sides os Metronome 3 44 ,, Metronome 5

12.59 p.m Metronome 7 L628). Metronome é 8 Lbs: Metronome + rotating object 1 i ee Metronome +rotating object 0 L264, Metronome 2 | Bs Pe Metronome 6 In the first experiment it is seen that the conditioned reflex, which, to start with, was of considerable strength, when tested by a fresh application of the positive conditioned stimulus 14 minutes after the cessation of a single application of the inhibitory combina- tion (3.52 p.m.), is diminished by half (7.e. from 6 drops to 3). In the second experiment, taking place on the following day, the reflex, when tested 54 minutes after the second of tw essive applica- tions of the inhibitory combination (1.25 pps diminished by three-quarters (t.e. from 8 drops to 2).

The second important detail conce effect is that its duration becomes sho the inhibitory after- the experiments proceed the earlier experiments for something over an hour, but ae se of succeeding experiments gradually’ becomes reduced nly a few minutes or seconds. The following two experiments òf Dr. Nikolaev which were performed, with an interval of ov months’ continuous experimentation, provide an illustrati my case in question : A rotating obj NS vides the stimulus to a conditioned ali- mentary reflex øA a given tone serves as its conditioned inhibitor. In the first expewment the inhibitory after-effect was obvious for over 20 m Gps ; six months later, after continual practice, it was absent s n as 34 minutes after the single application of the

Salivary Time Stimulus during Secretion in 30 seconds . drops during 30 seconds The problem of experimental destruction of the inhibitory properties of the conditioned inhibitor applied singly or in its in- hibitory combination is very complex and has not yet been fully worked out. I shall mention, therefore, only some of the better established facts. It is obvious that a complete abolition of the inhibitory properties of the combination should most readily be brought about by reversing the technique employed in its formation —1t.e. by systematically reinforcing the inhibitory combination by the appropriate unconditioned reflex. An experimag Dr.

Krjishkovski will illustrate this process : Q A given tone of a pneumatic tuning fork provi conditioned - stimulus to acid, while a tactile stimulation of in serves as the aliváry Time Stimulus during etion in Routine l minute ` ps during |, acid. ee 5 Inhibitory epN ition >; | Every application i] 93 E ” QO ” 1 of the inhibitory pera ” ” 3 | combination was 1] 49 3 P © ” 5 followed up by DAE © » 10 injectionof acid TE BD; A ” 14 | into the mouth.

It is interesting to note that if alternately with every application of the reinforced inhibitory combination we repeat the reinforced positive conditioned stimulus, a very considerable retardation in the progress of destruction of the inhibition is produced. This matter will be subjected to rigorous experimental analysis further on (Lecture XI). Quite distinct from the above process of gradual weakening of the inhibitory combination is the case in which the disturbance appears suddenly and as suddenly disappears. Extra stimuli belonging to the group of mild external inhibitors, as we may term them, influencing the animal during the action of the inhibitory combination instantaneously restore to something of its normal value the positive conditioned reflex which underwent conditioned inhibition. It is evident that external inhibition has brought about the removal of conditioned inhibition, and that we deal again with the phenomenon of dis-inhibition. These relations may be illustrated by the following experiments of Dr. Nikolaev :