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

a different place on the skin) ; a whistle of moderate strength ; the odour of amyl acetate. IV. Intense odour of camphor; loud whistle; sound of an electric buzzer. Extra stimuli belonging to the first group did not, in this dog, affect either phase of the delayed reflexes. Extra stimuli belonging to the second group exerted an effect only upon the initial phase of the reflex, causing a salivary secretion. Extra stimuli belonging to the third group disturbed both phases of delay: during the first phase a salivary secretion was produced, and during the second phase the secretion which should normally have been present was much dimin- ished. Extra stimuli belonging to the fourth group exercised little or no influence upon the initial phase of the delayed reflex, but completely suppressed the second phase. It may be added that when all due precautions were taken the experiments proceeded as a rule with striking precision.

The following experiments serve to illustrate the grouping of extra stimuli given above : A tactile stimulation of the skin is used as a conditioned stimulus to acid in a delayed reflex. The conditioned stimulus acts continu- ously during three minutes before the administration of acid. The extra stimuli employed are: (1) a thermal cutaneous stimulus of 44°C., (2) a thermal cutaneous stimulus of 0-5°C., (3) an_ddour of amyl acetate, (4) the sound of an electric buzzer. _ <S

iVary Secretion drops per secs. during the olated action of the conditioned stimulus action of the conditioned stimulus, but only during either the first, inactive phase or the second, active phase. This is illustrated in the following experiments carried out on the same dog and under the same cònditions as before. ecretion of Saliva in drops per 30 secs. during the isolated action of the conditioned stimulus CFactile + whistle of medium strength during the second 15 minutes

The extra stimulus of the whistle, which belongs to the third group of external inhibitors, when acting during the first phase of the delayed reflex brought about a secretion of saliva, but when acting during the second phase it diminished the secretion. A certain diminution of the secretion is sometimes observed during the second phase of the reflex in experiments where the extra stimulus is applied during the first phase. This must be due to the after-effect of the extra stimulus.

It now remains to interpret the empirical grouping of the extra stimuli, and to determine the reason of their different action upon the delayed reflexes. All the experimental evidence at our disposal - indicates that the intensity of the effect which they produce depends on their relative physiological strength and the magnitude of the general reactions by which the animal responds to their independent action. The distribution of extra stimuli among the four groups represents a classification according to such physiological strength. In some cases this can be seen by a casual glance at the list itself, where different intensities of the same stimulus appear in different groups.

The difference in the effects of these extra stimuli is also revealed by the motor reaction of the animal (investigatory reflex). With stimuli belonging to the first group there is frequently no motor reaction at all. As we pass on to stimuli belonging to the peaining olénged. Moreover, the inhibitory effect of all these extra Kyu i, which belong, of course, to the group of external nhibitng clearly seen in relation to the second (usually active) p © f the delayed reflexes. The inhibitory effect becomes the ronounced as we ascend from the first to the fourth group u h stimuli belonging to the latter group complete inhibition is&btāfned. It is thus obvious that the classification represents stim@) f progressively increasing physiological effect upon the or: those in the first group exerting the least effect.

In cases where an extra S, allowed to act upon a delayed conditioned reflex mat is found that its inhibitory effect upon the second (usu tive) phase progressively diminishes. This is a further pro v a. and dis-inhibition of delayed reflexes by extra A is closely associated with external inhibition. Two experime Quy be given which were performed on the dog COren S ast experiment : Salivary Secretion in drops per Time Stimulus 30 secs. during the isolated action of the conditioned stimulus

It can be seen from these two experiments how the inhibitory effect of the extra stimulus upon the active phase falls gradually with each repetition, the recovery of the delayed reflex being especially regular in the second experiment. All these experiments, which were carried out one and the same animal, have been repeated with similar re many other dogs. The only variation was in the distrib ity of stimuli among the four groups, which differed slightly fonda animals. This appears only reasonable when it i ed that the intensity of the reactions of different animals in nse to extraneous stimuli varies to a considerable and TAR extreme degree, depending on the individual character of th&gerVous system and on the previous history of the animal.

Thus we come to the c Qa that variations in the effect of different extra stimuli he delayed reflex are determined by differences in the phywiQ@eréal strength of the stimuli. When the strength of the ex imulus is insignificant the delayed. reflex in either of its pis es remains unaffected. When the strength of S is somewhat increased it is only the initial ) phase which becomes affected, being now converted f ESAS: With a greater strength of the extra stimulus (usually active) phase of the delayed reflex also becomes

involved, the secretion being considerably diminished in magnitude, so that it becomes equal to or even smaller than the secretion during the dis-inhibited first phase. Finally, with a maximal intensity of the extra stimulus all conditioned activity disappears, and. the de- layed reflex is represented throughout both phases by a series of zeros. As a result of the different effects upon the two phases of the reflex brought about through external inhibition, two facts relating to the central nervous activities stand out clearly. The first is that the extraneous stimulus acting on the positive phase of the reflex inhibits, and acting on the negative phase dis-inhibits, in either case, therefore, reversing the nervous process prevailing at the time. The second is that the inhibitory process is more labile and more easily affected than the excitatory process, being influenced by stimuli of much weaker physiological strength.

The following was a chance, but instructive, observation with regard to the second point. I meant to illustrate a series of public lectures on conditioned reflexes by demonstration of experiments. The lectures were given at a place remote from our laboratories so that the dogs had to be conveyed and set down in surroundings which were quite unfamiliar. All the five or six experiments with positive conditioned reflexes, in the first lecture, were carried out successfully. On the other hand, in the second lecture, the experi- ments with inhibitory reflexes, again five or six in n er and attempted on the same animals as before, did not sucked „all the reflexes having undergone dis-inhibition. Thus the same extra stimulus of the changed environmental conditi had not the slightest disturbing effect upon the positive Qiitioned reflexes, but exercised a profound influence upon RaQ bitory conditioned. reflexes, even though the effect of the( ex stimulus was now weakened on account of repetition. ese facts will be discussed more fully in a further lecture, in com@%tton with other observations bearing on the interrelations betwéPy excitation and inhibition.

Since the first phase of 2O% ayed reflex, like experimental extinction and conditioned į ttón, involves an inhibitory process, we should expect to find en all three a close similarity. It has already been noticed(in,the cases of extinction and conditioned tion spreads spont: sly to other conditioned reflexes, and that the degree of this go ing is determined by the relative physiological intensity AS reflexes. It has further been observed that if the

inhibition of a net e ositive conditioned reflex, that the inhibi- reflexes involved secondarily are physiologically weaker than the reflex in which the inhibition was developed primarily, the accom- panying secondary inhibition of these other reflexes is complete ; but that if the reflexes involved secondarily are the stronger only a partial inhibition is obtained. In other words, the intensity of the primary inhibition is found to be exactly proportional to the intensity of the excitatory process on which it is based. This holds good also for inhibition of delay. An alteration in either direction of the strength of the conditioned stimulus causes a sharp disturbance in the established relation between the inhibitory and excitatory phases of the delay. The two following experiments on this point were conducted on the same animal as was employed in the preceding experiments :

Salivary Secretion in drops per Time Stimulus 30 secs. during the isolated action of the conditioned stimulus The ex peDrents show a more definite effect in the case of a diminuti << the strength of the conditioned stimulus than in the Just as with extinction and conditioned inhibition, the intensity of the inhibition in delay also depends upon the strength of the unconditioned stimulus. This can easily be shown with conditioned alimentary stimuli, when the delayed reflex is tested in the dog after it had been fed at the usual time and again after a certain period of fast. The following experiment illustrates this point. A whistle of moderate strength is used as a conditioned alimentary stimulus in a conditioned reflex delayed by three minutes.

Experiment of 13th December, 1907. Previously to the experiment the dog was fed at the usual time. Salivary Secretion in drops per Time Stimulus 30 secs. during the isolated action of the conditioned stimulus Experiment of 15th December, 1907. Conducted upon the same dog after two days deprivation of food. Salivary Secretion ~\ in drops per EAN Time Stimulus 30 secs. during t significance of the conditioned ulus is increased through de- privation of food, the inactj inhibitory phase of the delay almost entirely disappear

observed with inhibitiow of delay exactly as with extinction and conditioned inhibitjen. This is clearly evidenced by an experiment such as the foll : ation of the skin serves as a conditioned stimulus eflex, the isolated action of this stimulus being continued during three minutes before the application of the uncon- Salivary Secretion in drops per 30 secs. during the isolated action of conditioned stimulus Reinforced with acid at the end of the 3rd min- ute of isolated action of the conditioned stimulus.

of the 3rd min- ute of isolated action of the conditioned stimulus. The tactile stimulus which normally produced inhibition during the first 14 minutes of its isolated action, when abbreviated so as not to act during the second 14 minutes associated with the excitatory phase of the reflex, led to such a strengthening of the inhibition that on its subsequent application for the full three minutes the positive phase of the reflex was either greatly reduced (11.21 a.m.) or else abolished altogether (12 noon).

It remains now to seek an interpretation of the fact that in the | case of a delayed reflex, the same stimulus has at-first an inhibitory, and later an excitatory, effect. What factor determines these two distinct properties of one and the same stimulus acting under ap- parently identical conditions ? We shall find no difficulty in cor- relating this phenomenon with the experimental evidence already considered in these lectures. In the third and fifth lectures a number of agencies were discussed which can be given either excitatory or inhibitory conditioned properties. In particular the factor of duration of time was shown to act as a real physiological stimulus, and experiments were described in which definite time intervals appeared as effective stimuli. I should like especially to recall to your memory the experiment in which the compound stimulus consisted of an external stimulus related to a definite moment of time (p. 41). In this experiment the external stimulus was with-

out any effect until applied in the neighbourhood of at i moment after the previous administration of acid, b is par- ticular moment was approached the secretory ar made its , appearance and gradually and precisely at a maximum. This is exactly the case with delayed reflexes al he experiments which have just been described in the NS, ROA tho the uncon- ditioned stimulus was added to the aoe stimulus only at the expiration of 3 minutes; in other Sgords, the external stimulus itself plus its: duration for three “on es together constitute the actual compound stimulus whickDyas immediately reinforced, and

it was this particular combina and not the nominal conditioned stimulus which acquired Gi ed properties. The same nominal stimulus at any time preyrwsly to the end of the third minute acted as a component of a ys t stimulatory compound which remained unreinforced by t conditioned reflex and therefore became inhibitory, exac ould any other stimulus if it were not rein- forced. The phenomenon has already been demonstrated in the case SS nditioned inhibition, and illustrations even more

striking will be given in the next lecture, which is to deal with the fourth type of internal inhibition. In the case of delayed. reflexes the significance of the duration of a stimulus can be observed in a very concrete and simple manner. When the external stimulus selected for the formation of a conditioned reflex is applied during a given interval of time, at each successive moment the stimulus forms part of a definite and distinct stimulatory compound. It is well known how soon we get accustomed to stimuli of smell, sound or illumination. This, of course, means that the nerve cells which are being excited pass through a series of successive physiological changes. In accordance with this it is obvious that if a definite unconditioned reflex is repeatedly evoked coincidently with any one particular physiological state of the cerebral cells, it is this definite state and no other that acquires a definite con- ditioned significance. In the next lecture it will be shown to how great an extent the discrimination of different intensities of one and the same stimulus can proceed, and how the stimulus evokes at one particular intensity a positive and at another intensity an inhibitory conditioned reflex. We are thus fortunately provided with a great deal of perfectly good experimental evidence which throws light on the phenomenon of internal inhibition involved in delay.

reckoned with in studying conditioned, reflexes. Ke ow that in order to determine the intensity of excitation. i simultaneous or a short-delayed conditioned reflex in its erous and subtle variations under different conditions, O cessary to apply the appropriate conditioned stimulus sin r a longer or a shorter period of time, combining its actio = of the unconditioned stimulus at the end of this tee = Only in this manner can we get a measurable reflex resp This procedure, even though rarely performed, tends to theglevelopment of a more or less prolonged delay, so that along wit excitatory process there originates also an inhibitory NS investigation thus naturally becomes

complicated, for it į necessary to deal with two simultaneous and antagonistia poe For example, it is not easy by our usual methods to obtate’ the true latent period of conditioned reflexes, since what ften been referred to in our description of experi- ments as ue) “latent period ” represented in actual fact a delay— i.e. V of an inhibitory period, which could appro- priately be termed the “ preliminary inhibitory period,” but which is certainly not the true latent period of the reflex.

In order to determine the true latent period of conditioned reflexes it is necessary to use reflexes which are as nearly as possible simultaneous, the unconditioned stimulus succeeding rapidly on the conditioned after an interval of a fraction of a second. It is only by using such reflexes, and then chiefly their motor components, that convincing experimental evidence can be obtained showing that the length of the true latent period of a conditioned reflex is really of the same order as is the true latent period of reflexes in the lower centres of the brain and spinal cord. Such determinations have only recently been introduced into our investigations, as we did not consider the determination of the true latent period of conditioned reflexes to be of fundamental importance for establishing their nature as reflex. The regularity and infallibility with which these reflexes could be evoked constituted in our opinion the main evidence for their reflex nature, and in this connection differences in the period of latency did not seem of much importance since the latent period of lower reflexes also is known to be subject to fairly wide variations depending on the complexity of the central paths and connections involved in the reflex. It may justly be admitted, however, and with-

out prejudice to our conception of the reflex nature of conditioned reflexes, that the nervous connections are of greater Aes rele in those reflexes which involve the cerebral cortex. The elucidation of many other problems concer ee reflexes, such, for instance, as that of the actual Nse of the ex- citatory process from the moment of its origi LOX is also rendered. difficult by interference from delay. In QP repo individual differences in the character of the nerf ystem may be very helpful. For example, it was shown $gbove that.in some animals the delay is developed with difficulty, that the excitatory process is only slightly, or often not at allgjisturbed. The experimenter, in order to diminish the dae elay, can also take the precaution to abbreviate the isolated the conditioned stimulus to the minimum compatible sig oa a secretion of sufficient magni- tude to allow of aao different variations of the experiment. On the other hand, e are some problems in the elucidation of which the initial} Gass of inhibition may be turned to advantage, as will be sho the next lecture. It can clearly be seen how, when WR into the properties of conditioned reflexes, the

l experimenter has to adapt his methods to the character of the The experimental evidence with which we have been dealing in the last three lectures demonstrates the enormous biological impor- tance of internal inhibition of conditioned reflexes. It is by means of internal inhibition that the signalizing activity of the hemispheres is constantly corrected and perfected. To sum up: If over a given time a signalling, i.e. a conditioned, stimulus is repeatedly presented without the accompaniment of the uncon- ditioned stimulus, then the conditioned stimulus becomes meaning- less to the organism as calling for an unnecessary expenditure of energy, and the stimulus loses, though generally for only a short time, its physiological significance (experimental extinction).

In exactly the same manner, if a conditioned stimulus is repeatedly applied together with another extraneous stimulus and in the com- bination is never followed by, the unconditioned stimulus, the conditioned stimulus loses its positive excitatory conditioned effect, but only in that particular combination and not when applied singly (conditioned inhibition). Finally, if a regular interval of sufficient duration is established between the commencement of a conditioned stimulus and its reinforcement by the unconditioned stimulus, the former becomes ineffective during the first part of its isolated acti during the second part of its action a positive excitatory A. appears, and this increases progressively in intensity as toO ent approaches when the unconditioned stimulus has cu eerily been applied (inhibition of delay).

In the above manner a continuou, =. exact adaptation of the organism to its eee) effected, revealing a most delicate adjustment in the ARE ee es nervous processes of the | higher animals. As has been clearly de phenomena of extinction sent the formation of į ditioned reflexes “oO A. in the last three lectures the itioned inhibition and. delay all repre- y conditioned reflexes. Inhibitory con- ever, also be obtained by a totally different procedure. If itory stimulus is applied, simultaneously and. repeatedly for Wot periods of time together with some neutral stimulus th Cayter also develops an inhibitory function of its own. This ait has been specially examined. by Dr. Volborth, who

presenting them repeatedly to the animal until they ceased to produce any inhibition of the positive conditioned reflexes or any dis-inhibition of the inhibitory reflexes. He then repeatedly allowed. these neutral stimuli to act during short intervals of time either with conditioned reflexes which had just been experimentally extinguished. or with the inhibitory combination in a conditioned inhibition. After repeating this procedure several times he tested the action of these hitherto neutral stimuli upon positive conditioned reflexes, and in this manner successfully demonstrated that they had acquired definite inhibitory properties.

Since in order to become sure of the results these tests had to be repeated several times, Dr. Volborth in one group of experiments accompanied only every alternate test by the unconditioned stimulus. This strict alternation of reinforcement with non-reinforcement was adopted in order to prevent the combination of the positive stimulus’ with the new and hitherto neutral stimulus from acquiring a pre- dominance of positive or negative properties in itself. But most convincing of all was another form of the experiment in which the test was always accompanied by the unconditioned stimulus. In spite of repeated tests of this kind the recently acquired inhibitory properties of the hitherto neutral stimuli were still prominently exhibited. The following are some of Dr. Volborth’s experiments, in which a “ natural ” alimentary stimulus was employed.