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

The alimentary conditioned stimulus after exti was repeatedly applied in conjunction with the sound of tronome which had previously been rendered entirely neu After this procedure had. been continued for some time a action of the metronome with a positive alimentary reflex was occasionally tested, Experiment of 5th December, 1911, os ten applications of the Q Salivary Time A applied Secretion in ing 30 secs. drops during ( ) 30 seconds 12.54 p.m. > Ra powder at a distance 7 eat powder at a distance

Experiment of 1st December, 1911, after 19 applications of the metronome with the extinguished conditioned reflex. Salivary Time Stimulus applied Secretion in during 30 secs. drops during 30 seconds 11.30 a.m. Meat powder at a distance 7 ELAT Meat powder at a distance +metronome’ ` 1 LEDAS; Meat powder at a distance 3 12.7 p.m. Meat powder at a distance . 8 Experiment of 18th December, 1911, after 26 applications of the metronome with the extinguished conditioned reflex.

Time. Stimulus applied Secretion in during 30 sec. drops during 30 seconds. 10.35 a.m. Meat powder at a distance 9 10.47°~,, Meat powder at a distance +metronome 1 FiOS Meat powder at a distance 12 formerly entirely neutral has acquired inhibj properties as a result of repeated applications with the Kaa conditioned reflex. This new inhibitory stimulus of th nd order proves itself in every respect similar in properties se inhibitory stimuli which have been considered already; earn with experimental extinction, conditioned nib and delay. For example, a new inhibitory stimulus of the secod order which is developed with the help of one definite condj&jbbd reflex can exert an inhibitory effect upon other conditione es as well. This is shown in the follow- ing experiment of March, 1911, in which a different dog is employed.

The sound etronome was made always to coincide with an Fae ene ý natural” conditioned alimentary reflex; subse- quently tf) ect of the metronome was tested upon an artificial It can be seen also in the above experiment that the effect of the conditioned inhibitory stimulus of the second order is not limited to the time of its actual administration, but reveals itself also in a definite inhibitory after-effect which is subject to summation. Furthermore, conditioned inhibitory stimuli of the second order are, equally with the primary ones, subject to dis-inhibition by agencies belonging to the group of external inhibitors. We thus come to the following conclusion: when perfectly neutral stimuli fall upon the hemispheres at a time when there prevails a state of inhibition they acquire an inhibitory function of their own, so that when they act subsequently upon any region of the brain which is in a state of excitation they produce inhibition.

A point to which further reference will be made should be men- tioned here, namely, that every extraneous stimulus whic the hemispheres and remains without any further c the animal, if repeated causes the spontaneous de cortical inhibition. Therefore, in the type o described, it is always necessary to ascertain extent to which the acquisition of inhibitory properties le neutral stimuli depends on their simultaneous borg th inhibitory stimuli, and to what extent these inhibitor operties have developed independently.

The analysing and synthesizing activity of the cerebral hemispheres: (a) The initial generalization of conditioned stimuli. (b) Differential inhibition. Strmuxti which evoke conditioned reflexes are perpetually acting as signals of those agencies in the environment which are in themselves. immediately favourable or immediately destructive for the organism. Such signals are drawn sometimes from only one elementary pro- perty of the environing agencies, sometimes from a whole complex of these properties. This is possible only because the nervous system possesses on the one hand a definite analysing mechanism, by means of which it selects out of the whole complexity of the environment those units which are of significance, and, on the other hand, a syn-. thesizing mechanism by means of which individual units can be integrated into an excitatory complex. Thus in studying the nervous activity of the cerebral cortex it is necessary to deal with two further and distinct phenomena, one involving a neuro-analysis and the other involving a neuro-synthesis. The analysing and synthesizing functions of the nervous system constantly superi se themselves upon and interact with one another.

Every type of nervous system presents e or less complex analysing apparatus which readily admits ubdivision into what we may term the nervous analysers. Forex ple, the visual analyser selects the vibrations of light, t or ustic analyser selects the vibrations of sound, and so on. ‘ka ore, each analyser differen- tiates its own selective medium nese environment into a very large number of elementary physiogical stimuli. With regard to the structure of the analysers, @g yh includes, on the one hand, the peri- pheral receptor with 2p fðpfierent nerves, and, on the other hand,

the nerve cells whi at the central termination of the nerve fibres. The peri receptors can be regarded as ‘“‘ transformers,” which, in the Lee any single analyser, are capable of accepting only one defGite form of energy as an adequate stimulus for the initiation nerve impulse. It is obvious that both the peripheral analysing function of the nervous system. Inferior analysing qualities are of course manifested by lower parts of the nervous system, and even by the crudely differentiated nervous substance in those animals which lack a nervous system proper. An organism deprived of its cerebral hemispheres still responds in a great variety of ways to stimuli applied to its receptor surfaces; according to the site of application, the intensity and the quality of the stimuli. However, the highest and most subtle analysing activity of which an animal is capable can be obtained only with the help of the cerebral cortex. It is evident also that only with the progressive development of the analysing activity of the nervous system is the organism enabled to multiply the complexity of its contacts with the external world and to achieve a more and more varied and exact adaptation to external conditions. In contemporary research the study of the analysing function forms a very important section of the so-called physiology of the sense organs. This section has reached a very high state of development in the hands of some of the greatest physiologists, especially Helmholtz, and presents an abundant wealth of data concerning the activities of the peripheral structures of the different analysers and of their cerebral terminations. A good deal is known also about the limits of the analysing functions in man.

But while the study of the physiology of the special sense organs suggests explanations of many complicated cases of the analysing function, and enunciates many fundamental laws to mh this activity conforms, the greater part of the material w as been gathered is of a subjective character, being based ur psychical apperceptions which are the most elementary su ive indications of the objective correlations between a xa environment, This fact constitutes the greatest defect of ction of physiology, since it excludes the study of the an scr function in animals outside man, and therewith all Bie AG} Nae of animal experi- mentation. The method of condjsjonéd reflexes, however, gives over the study of the whole “a most important function of nervous analysis into the ha O%) e purely experimental physiol- ogist. With the help of coined reflexes the scope and limits of the analysing function fferent animals can be exactly deter- mined, and the laws a Co a this function made clear. Although the study of the D logy of analysers has been as yet but little developed, re upon the new lines is making rapid progress and may b ee to add largely to our knowledge of the

mechanism by which the exact correspondence between the organism and its environment is maintained. The first step was to find a method by which the activities of the analysers could be objectively studied in animals by means of visible outward reactions. As was mentioned before, even insignificant changes in the external environment call forth if not a special inborn or acquired reflex activity, then a reaction of orientation (the “ investigatory reflex’). It is obvious that the investigatory reflex can be used to determine the degree to which the nervous system of a given animal is capable of discriminating between various stimuli. If, for example, among the different environing agencies there is present a definite musical tone, any, even slight, alteration of its pitch will suffice to evoke an investigatory reflex in the form of a definite orientation of the ears and maybe of the whole body of the animal in relation to the tone. The same is true even of slight changes in various other elementary or compound stimuli. The investigatory reflex, of course, takes place only provided that the _ structure of the analysing apparatus is sufficiently delicate to register the change in the environment. This reflex can be used for the purpose of our investigation by itself, or, much better, through its inhibitory or dis-inhibitory effects upon conditioned reflexes, since these are the most delicate nervous reactions of which the animal is capable. Howeyer in spite of the high degree of sensitivity mani- fested by the investigatory reflex this reaction j any respects unsuitable as a basis for the study of the alg activity of the nervous system. One of its chief defects at in the case of certain weak stimuli the reaction is on ansient and cannot be repeated, and it is therefore useless for urpose of exact experi- mentation.

The detailed investigati conditioned reflex reaction, on the contrary, provides an K tly suitable method for an exact experimental research in e analysing function. A definite external agent is made, for e = to acquire by our usual technique the properties of a sai conditioned stimulus. By repeated reinforcement this stimulus is strengthened in its new properties, while i mulus nearest to it in intensity, position or quality is alwa trasted by being left without reinforcement, with the result “kat it becomes readily and exactly differentiated from the eat Gpiaied positive conditioned stimulus.

means onditioned reflexes is always preceded by what we call a “period of generalization ” (which may possibly be regarded as some form of synthesizing activity). For instance, if a tone of 1000 d.v. is established as a conditioned stimulus, many other tones spontaneously acquire similar properties, such properties diminishing proportionally to the intervals of these tones from, the one of 1000 d.v. Similarly, if a tactile stimulation of a definite circumscribed area of skin is made into a conditioned stimulus, tactile stimulation of other skin areas will also elicit some conditioned reaction, the effect diminishing with increasing distance of these areas from the one for which the conditioned reflex was originally established. The same is observed with stimulation of other receptor organs. This spontaneous development of accessory reflexes, or, as we have termed it, generalization of stimuli, can be interpreted from a biological point of view by reference to the fact that natural stimuli are in most cases not rigidly constant but range around a particular strength and quality of stimulus in a common group. For example, the hostile sound of any beast of prey serves as a conditioned stimulus to a defence reflex in the animals which it hunts. The defence reflex is brought about independently of variations in pitch, strength and timbre of the sound produced by the animal according to its distance, the tension of its vocal cords

and similar factors. AN Besides this we have encountered in conditioned refi rhs other form of generalization, the vital importance of whi Ox so im- mediately apparent. So far we have been dealing a temporary form of generalization within a single analyseng e case of simul- taneous and delayed reflexes. In the case io Mtioneä long-trace reflexes, with a pause of 1-3 minutes, 1 eralization becomes permanent and of a wider scope. Tfgce“feflexes, like all delayed reflexes, present two phases—an in’ inactive phase based on internal inhibition, and a secon ctive phase based on nervous excitation. All that has been lioni the effect of extra stimuli upon these two phases in t, ahs f delayed reflexes is true also for the two phases in trace r s. The trace reflexes, however, have another characteristi eir own, namely, that they exhibit a permanent and univerSe¥ generalization, involving all the analysers. For example, tO stablish a long-trace conditioned reflex to a tactile ee ae it is found that stimuli which belong to

other anal nd which have never been connected with the given: reflex ba act as conditioned stimuli to the same trace reflex. We shall deal with this phenomenon at some length, since the investigation presents some special points of interest. The following experiments bearing upon this question are taken from a research by Dr. Grossman : A tactile stimulation of the skin is used as the conditioned stimulus for a long-trace reflex to acid, the interval between the end of the conditioned stimulus and the beginning of the unconditioned being one minute. The experiments show the effect of a thermal stimulus at 0°C. and of a given musical tone, both applied for the very first time.

Salivary Secretion in drops during Time Stimulus arte successive minutes Remarks during one minute | from the beginning of the conditioned stimulus O aaa oa 15 troduction of Pp” ae Tone Not reinforced. eames Tactile paras by in- a. 10..,, - Tactile troduction of a; BV he Tactile acid. ' It is thus seen that stim Q which had previously never been -connected with the refle & acid have now acquired the property to excite this reflex. ermore, the stimuli, although applied for the very first oN the same manner as the stimulus used to establish the t Ou ex, their effect being manifested not at the time of their co but chiefly or exclusively after they have been EA ed. This similarity made us inclined to regard them as being © a generalization of the original trace reflex. Of course, the aya e from a few isolated experiments of this type was not

in view of the intrinsic interest of this phenomenon it was subjected to rigid investigation. On the experimental evidence available concerning conditioned reflexes only two further possible explanations of this phenomenon suggested themselves. In the first place a long-trace reflex is always formed slowly and with difficulty, and it was observed in our earlier experiments that before the formation of the trace reflex other conditioned reflexes were very easily established to any chance stimuli which happened to coincide with the actual administration of the unconditioned stimulus and of which the experimenter himself was often the cause. In the case we are speaking of at present the danger of interference by extraneous stimuli was therefore con- siderable, and these experiments had to be repeated in our new laboratory so as to make sure that any possible accidental influence of the experimenter upon the animal was excluded. Under these conditions the generalized character of the long-trace reflex was still found to persist.

The second explanation which suggested itself was as follows: When conditioned reflexes are being established in dogs for the first time, it is found that the whole experimental environment, beginning with the introduction of the animal into the experimental room, acquires at first conditioned properties. This initial reflex could be called, therefore, a conditioned reflex to the environment. t later on, when the special reflex to a single definite and const Ks ulus has appeared, all the other elements of the enviro D gradually lose their special conditioned significance, most rey on account of .a gradual development of internal inhibi However, this inhibition is at first very easily dis-inhibited ae extra stimulus. The following is a striking example of su se which was very common, when, as formerly, the experi D in the room with the dog. The reflex to environne} » had in the given experi- ments just come to an end, the glafpls being now in a resting state except when the special Sit ee stimulus was applied. As soon, however, as I mys d the room, in order for the first time to watch the experi , & copious secretion of saliva was produced by the dog, ¢high’persisted as long as I remained in the room. I myself presentéd in this case the extra stimulus dis- inhibiting the refle: © environment which had only just recently undergone exti . Now it occurred to us that the phenomenon of the mn generalization of the long-trace reflexes might

really be nothing but a dis-inhibition of the reflex to environment. "Iowever, after a thorough examination, this explanation had to be discarded. In the first place, a considerable generalization of long- trace reflexes could easily be observed even in dogs in which the reflex to environment had been deeply inhibited so long ago that it was now almost impossible to dis-inhibit it. In the second place, the supposition of dis-inhibition when followed up necessitated a further assumption which was easily disproved. It has been seen already that dis-inhibition of the inactive phase in delay was obtained immediately on application of the extra stimulus, t.e. without any such latent period as is observed for trace reflexes. If the generalized character of trace reflexes was in reality nothing but dis-inhibition, we should expect all the different stimuli also to act immediately, ' ut as we have seen they act only after their termination and after about the same latent period as the initially established trace reflex. If it is still assumed that the effect is due to dis-inhibition of the reflex to environment, then in the case of trace reflexes all the different stimuli must act for some reason as very powerful extra stimuli which do not dis-inhibit the reflex to environment, but temporarily abolish all conditioned activity by producing a very powerful inhibition (as is also the case with very powerful extra stimuli in delay), and the ensuing dis-inhibition must be brought about by their traces which represent weaker stimuli. This assumption, cae , is contradicted by the following facts. It is known that poh plication of the same powerful extra stimulus is followed b adual diminution of its inhibitory effect, which gives place, ee seen with delay, to dis-inhibition.

But in the case of the eralized stimuli in long- trace reflexes the latent period does minish in spite of repeated applications. This shows that t activity is due to a genuine generalization of the trace refl not to dis-inhibition. Finally, there is this striking fact, eo the case of trace reflexes following the application of various i, which of course are never reinforced, the effect of the special itioned stimulus to which the trace reflex was experimentall ose also becomes temporarily diminished, and the secretion ey fall to zero, a fact which cannot be reconciled with any sup that we are dealing with a dis-inhibition of the reflex to RG ent. Indeed, there is no doubt that this weakening

of the effe@ataf the special conditioned stimulus represents a simple instan KA extinction, as the result of non-reinforcement, of a reflex Thus it isseen that in the course of the establishment of simultaneous and delayed reflexes a temporary generalization develops in the form of a number of accessory conditioned reflexes to associated stimuli. Generalization of the reflexes can be effected also through the whole environment acting on the organism by the sum total of its individual units and leading to the formation of what we may call a synthetic environmental reflex. In other cases, namely, in long- trace reflexes, it is effected in virtue of the intrinsic properties of the nervous system itself, which give a more or less generalized character to the individual external stimuli in their capacity as conditioned stimuli. In many instances, some of which we have referred to above, it is obvious that this fact of generalization of stimuli has a definite importance in the natural correlation between the animal and its environment, but in other cases the generalization can have only a limited or temporarysignificance. In the latter casesthe approximate, general, and under some conditions useful connection with the en- vironment as a whole is replaced by’a precise and definitely specialized connection with a definite stimulatory unit.

The question can now be discussed as to how the specialization of the conditioned reflex, or, in other words, the discrimination of external agencies, arises. Formerly we were inclined to think that this effect could be obtained by two different methods: the first method consisted in repeating the definite conditioned stiaualus a great number of times always accompanied by reinfor nt, and the second method consisted in contrasting the gi definite conditioned stimulus, which was always accompani y reinforce-

At present, however, we are more inclined @)%gard this second method as more probably the only efficac One, since it was ob- served that no absolute ee ever obtained by the use of the first method, even though tle} imulus was repeated with reinforcement over a thousand ti . On the other hand, it was found that contrast by even a si unreinforced, application of an allied stimulus, or by a re single unreinforced. applications ment, with different neighbouring stimuli which wefe ever reinforced. © é

of different members of a sepi allied stimuli at infrequent intervals of days or weeks, led (tow rapid development of differentiation. The method. of contrast ow always employed in our experiments, as leading to a diffengn ation of external agencies in an incomparably quicker time. O We can ng Yow out the development of differentiation between external stimuli in the conditioned reflexes in greater detail. In the first place an interesting observation which remained for a long time without explanation may be considered: It was noticed that when, after a conditioned reflex to a definite stimulus (e.g. a definite musical tone) had been firmly established, the effect of another closely allied stimulus (a neighbouring musical tone) was tried. for the first time, the conditioned reflex which resulted from the new stimulus was frequently much weaker than that obtained with the original conditioned stimulus. On repetition of the stimulus of the neighbouring tone, always, of course, without reinforcement, the secretory effect increased until it became equal to that given by the originally established stimulus, but subsequently on further repetition began to diminish, falling finally to a permanent zero. Thus it appeared that at first the two closely allied stimuli were discriminated straight away, but that later this discrimination for some reason disappeared, only gradually to re-establish itself and finally to become absolute. To provide an explanation of this phenomenon we can revert to an interpretation which was advanced previously for similar events occurring in the process of development of conditioned inhibition. It will be remembered that when, in the formation of conditioned inhibition, a conditioned stimulus was accompanied for the first time by the new stimulus which later acquired the properties of a conditioned inhibitor, the combination produc d either a very small positive effect or else remained totally ¥ Metre Later, although the inhibitory combination was noe Sel by the unconditioned stimulus, it produced again a(Xeflex of full strength, which, however, after further repetitions a ally fell to a permanent zero.

The explanation given in the gageOf conditioned inhibition, and fully borne out by experimenta o dence, was that the additional stimulus elicited on its first appkeatieh an investigatory reflex which immediately produced an ey al inhibition of the conditioned reflex ; on repetition the opis of the investigatory reflex rapidly diminished and the posi ffect of the conditioned stimulus was temporarily restore & later gradually suppressed by the de- velopment of inter a ibition. Similarly, in the case of differen- tiation it is poetic regard stimuli neighbouring on the definite

positive conditioned stimulus as bearing two aspects, one of similarity to, and O her of difference from, the positive conditioned stimulu Qn account of the element which is in common, these NS ing stimuli can act similarly to the patiye conditioned one; it is the presence of the second -factor, of difference, which determines a temporary investigatory reflex, bringing about external inhibition of the excitatory effect, but later serving as foundation for the development of a permanent and final differentiation of allied stimuli.

Salivary Secretion recorded by Time . Stimulus applied during 30 secs. divisions Remarks of scale Experiment of 20th Februar Ea. 3.7 p.m. Object rotating anti-clockwise 26) Not reinforced. | Object rotating clockwise O i Reinforced. The strength of the refle ich is undergoing differential inhibition now diminish ressively with small fluctuations until it reaches a perma : The correctness of this/interpretation is borne out by the striking similarity in detat Gp the development of differentiation and of conditioned inhi . The same variations occur in both cases. The initial dighiiution in the strength of the reflex during the

first few applications of the new stimulus is sometimes succeeded by a transitory increase in strength as compared with the normal, and after this the reflex diminishes steadily below its normal value until it finally attains a permanent zero; in most cases, however, the initial diminution is succeeded by a phase of increase to the normal level, after which the reflex again falls steadily to zero with the development of the final differentiation ; it rarely happens that a development of differentiation is established without such fluctua- tions, or that a gradual diminution of the reflex follows directly upon the sudden initial drop. While, in describing the formation of conditioned inhibition, the fluctuations received a considerable share of attention, no records of experiments were given. A presentation of the analogous experiments on the establishment of differential inhibition will make the matter clear.

In the first series of experiments (see p. 119), which were conducted by Dr. Gubergritz, an object rotating in a clockwise direction served as the positive conditioned stimulus, while the same object rotating in the opposite direction served as the stimulus undergoing differentiation. Amount of Saliva recorded Stimulus by Time applied divisions Remarks during of scale 12.28 p.m. | Tone Reinforced. LOSE Tone Reinforced. ES O Semitone Not reinforced.

The x to the semitone continues to fluctuate, gradually j in strength until at the thirteenth repetition it has The above experiments were conducted on another dog, a musical tone serving as a conditioned alimentary stimulus and its semitone as the stimulus undergoing differentiation. The dog employed in the next series of experiments is the same as was used in the first series. A luminous circle was used for a conditioned alimentary stimulus, and a luminous square of equal surface and equal brightness for the stimulus undergoing differentia- tion. :

Then with small fluctuations the reac nishe progres- sively, until after the eleventh repetit(on) e square becomes Some other interesting points beSides those connected with the interference of the investigato eflex have also come to light in recent experiments.. In th place it has been shown that the development of a differenti of two very closely allied stimuli may be attempted directly nO) the other hand, the same differentiation may be effected in stageg, leading up through the differentiation of more remote stimula There is a considerable difference between the