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

Conditioned Secretion in of t Time stimulus applied drops during itary during 30 seconds| 30 seconds reaction in seconds It is seen that the secretory effect was increased almost 50% when the positive conditioned stimulus was applied immediately after the termination of the inhibitory stimulus, and, that the latent period of the reflex was definitely shortened. Moreover, the intensity of the motor alimentary reaction of the animal was also considerably increased. Evidently, under the influence of the inhibitory gtimulus

(hind paw), the corresponding cortical area dovelorg ate of inhibition, which is, as we know, retained for some a the termination of the inhibitory stimulus. On the r hand, the cortical area corresponding to the positive stim ‘dian paw), on state of increased excitation. In this j e the interpretation is helped by the spatial relation of t points of stimulation. The problem, however, becomes mo mplicated when both the positive and negative stimuli act Q one and the same cortical

point, as, for example, in. oe differentiation of a stimulus according to its intensity or g to its continuous or interrupted character, or as regards th uency of interruption in one and the same type of stimulu phenomenon of induction can, never- theless, be seen in ee also. The following is another experi- ment taken frome oursikov’s research : A condition mentary reflex is established to a rate of 76 beats per we of a metronome, and from this a rate of 186 beats

per minute is completely differentiated, both with regard to the secretory and motor components of the reflex. Conditioned Secretion in of Time stimulus applied | drops during salivary during 30 seconds| 30 seconds reaction in seconds The positive stimulus tested immediately after the applica- tion of the inhibitory stimulus showed an increase of 30% in the secretory reflex ; the motor reaction was correspondingly intensified, while the latent period of the secretory reflex was considerably shortened.

Another example of positive induction may be given from a research by Dr. Kalmykov: a positive conditioned alimentary reflex was established to strong light, and from this a reflex to weak Ae Salivary Conditioned Secretion i of the Time stimulus applied | drops d Salivary during 30 seconds| 30s reaction i ) in seconds In this oxperima Qo the positive stimulus, tested immediately | after the inhibe Stimulus, showed an increase in the secretory reflex of 40% e latent period was considerably shortened and

the alimen motor reaction was distinctly increased. It m questioned, in the light of the foregoing experiments, me e phenomenon of positive induction does not really represent a form of dis-inhibition. The plausibility of such an interpretation is obvious, since in experiments on positive induction a new condition is introduced in the immediateness of the replacement of the negative conditioned stimulus by the positive one. This new condition might play as potent a rôle as any other fresh change in the environment in evoking an investigatory reaction, so producing dis-inhibition. However, apart from the many points of difference in detail between the phenomena of positive induction and dis- inhibition, this explanation is definitely disproved by the character of the motor reaction of the animal. This never at any time, even from the very beginning, manifests itself as'a general investigatory reaction, but always as a distinctly specialized reaction corresponding to the definite positive conditioned reflex.

The duration of induction varies from several seconds to one or two minutes. The cause of this variation has not yet been sufficiently investigated. After this bare statement of facts, we must now proceed to a more detailed study of positive induction, especially as its occur- rence was by no means so constant as might be inferred from the above account. The subject is not even yet entirely under our control, but a number of conditions on which induction depends can already be indicated.

The first of these conditions was revealed accidentally. In certain experiments with a conditioned alimentary sti flu of 100 beats of a metronome per minute Dr. Kalmykov re ae observed a positive induction on testing the positive sti immediately after the withdrawal of the inhibitory stim 160 beats per minute. However, when Dr. Kalmykov =: demonstrate the experiment in the presence of several vis cluding myself, the results obtained were quite different ey of the customary augmentation, the application of t \ hibitory stimulus immedi- ately before the positive one nowsgaused a pronounced diminution in. the positive reflex. This a deviation was interpreted as follows. Since the do t sufficiently isolated from the sounds produced by the Ore and the visitors, their con- versation must hav as an external inhibitory stimulus weakening the E ition, and at the beginning even distinctly dis-inhibiting aS @)sitive induction could not, therefore, be exhibited at ai pOh e during the experiment. The record of this

: Conditioned stimulus applied secretion in : Time during 30 seconds drops during salivary 30 seconds _Teaction : in seconds It is seen that the differentiation which previously to this experi- ment was complete became disturbed on its first application in the presence of visitors, 4 drops of salivary secretion being now obtained (1.47 p.m.). Later, the inhibitory stimulus applied singly gave the usual zero secretion, but throughout the experiment the inhibitory stimulus never exhibited any power of positive induction; the positive reflex tested immediately after the w inhibitory stimulus underwent a diminution, as therefore in- fluenced immediately by an inhibitory after-e stead of the usual temporary positive induction (2.154 p.m., i ea 2.424 p.m.).

It may be concluded from the fore that the weakening of the inhibitory process leads to t pearance of induction, so that now the inhibitory after-effe gins to develop immediately, without the intervention of a ponines heightening of excitation. In order further to test this ittérpretation Dr. Kalmykov reduced the intensity of the E inhibition by intentionally subjecting it to external inhibiti or this purpose the experiment was preceded by the i ction of a rejectable substance into the mouth of the àn, as in the case just described, no induction could be au er throughout the experiment. It may be con- cluded, thex%ore, that the manifestation of positive induction depends some definite intensity of inhibition.

TheG@eeond condition upon which the development of induction EN was also brought out in experiments on the same dog. Differential inhibition having been established to the metronome beating at a rate of 160 per minute, it was practised for special purposes a great number of times in the course of several months without testing for induction. A more delicate differentiation to a rate of 112 beats per minute was now established (the rate of the positive stimulation being in both cases 100 beats per minute). When this finer discrimination became absolute, tests for induction were performed with the following definite result :

Conditioned Secretion in of Time stimulus applied drops during salivary during 30 seconds | 30 seconds reaction The inductive action of the new and more delicate differentiation resulted in an immediate increase of the positive secretory reflex by 75% and in a considerable shortening of the latent period. When, however, the older and coarser differentiation of 160 beats was now tested, it was surprising to find that no trace of inductive action could be observed. We thought it possible that this regWt might have been due to interruption in the practice of th ha er dif- ferentiation while the new and more delicate diffe@y lation was

being established, and therefore we now practised o differentia- tions alternately for periods of 10-15 days eac is procedure did not, however, change the course of events, as j wn in the following tables : : Latent period Conditioned py of the Time stimulus applie : salivary during 30 sec drops during reaction 8 30 seconds It is seen that the new and finer differentiation produced a distinct though temporary effect of positive induction (11.194 a.m.), whereas the older and coarser one was followed directly by an inhibitory after-effect. On continuation of the experiments the effect of positive induction of the finer differentiation also began to weaken.

In similar experiments conducted by Dr. Frolov a modification was introduced by employing an inhibitory stimulus of greater intensity than the stimulus to the positive conditioned reflex. The tone D of Max Kohl’s tone variator, damped to different degrees, provided the stimuli in these experiments. Three intensities of the tone were employed. The weakest served for tha positive con- ditioned stimulus, while the remaining intensiti ane tone were given inhibitory properties, the strongest tone he first to be contrasted. When this differentiation beca nitely established a test was made of its effect in producin ive induction :

r O Weak tone 17 3i s y On im D, transition from the inhibitory tone to the weak POR the effect of the latter was found to be, increased, by 50% (2.83 p.m.). The differentiation to the strong tone was now repeatedly practised for over a month. When tested at the end of this period it was found to have lost completely its effect of positive | The next stage in the experiment was the formation of a dif- ferentiation to the tone of intermediate intensity. This finer differentiation was already nearing completion on its nineteenth application. The following table gives the results of a test for positive induction : |

older and coarser differentiati d lost its inductive effect after prolonged. practice, while tke nd more delicate differentiation, so soon as definitely estab]; , exhibited an intense effect of positive induction. The natural conclisién to be drawn from these observations is that positive indú on represents a temporary, phasic phenomenon, which is associat@) With the period of establishment of new relations in the nery stem ; induction makes its appearance only with mS Ns

the maximal development of a given cortical inhibition, and dis- appears after the inhibition has become finally stabilized. This general rule, however, is not without exceptions, since in some cases induction is maintained during long periods of time without showing any signs of subsequent diminution. So far as can be judged from the available data, this depends on whether the rapid replacement of the inhibitory, by the excitatory, stimulus affects one and the same cortical area, as in the experiments just described, or whether the two stimuli affect two cortical areas more or less remote, as in the experiments upon the cutaneous analyser. The conditions under which induction assumes a more permanent character can be settled only by further experiments, such as are at present in progress.

It may, however, be added that a considerable number of experi- ments were conducted with differentiation of the rate of rhythmic tactile stimulation of some definite area of the skin. In these experiments a high rate of stimulation acted as the positive stimulus, while a low rate of stimulation acted as the inhibitory one. So far we have been unable to observe any positive induction with these stimuli, on rapid transition from the inhibitory to the excitatory one. In all cases there was only an immediate development of an inhibitory after-effect.

Negative induction—+.e. an intensification oii under the influence of preceding excitation—was also ADxytfved several years ago, but was always wrongly interpret nd it only received adequate recognition in some quite r experiments. In these experiments, which were carried. o r. Stroganov, the pheno- menon of negative induction w Che investigated in many dogs, and can now be timated full value. Negative induction was frst™net with in investigations on the destruction of internal ination. Some experiments of this type conducted. by Dr. Krijs y dealt egpecially with the destruction of conditioned DN A tone employed. singly was given the properties of a pOsitive conditioned stimulus to acid, while in com-

bination with a ile cutaneous stimulus it served as a stimulus in a conditio Gpnhibition. The destruction of the inhibition was attempted reversing the process employed in its formation, the K of the inhibitory combination being alternated, with applications of the positive conditioned stimulus singly, but both being this time reinforced. The results obtained were quite un- expected. In spite of ten applications of the inhibitory combination with reinforcement by acid in the course of three days, its inhibitory properties remained undisturbed. These experiments are repre- sented in the following tables:

Since this methoK filed to produce a destruction of the con- ditioned eens peony technique was used on the day following The combination, which remained always applications of the positive conditioned stimulus singly. Destruc- tion of the inhibition was now rapidly obtained. Conditioned Salivary Secretion Time stimulus applied in drops during 30 seconds during 30 seconds 10.42 a.m. Tone 10 2 inati ; a p Combination 4 EE A Ie 7z 4 all cases by fay 5 6 injection of iar! x g paana.

Although the difference between the results of the two methods seemed. obvious enough, the suggestion offered itself that the destruction which developed. so rapidly with the second method. had been facilitated by the previous reinforcements in the earlier period of alternate application. In order to eliminate this possibility, the inhibitory combination. was re-established and practised during a period of over one year, when the second. method for the destruction was again applied, the result being as follows :

Experiment of 22nd November, 1908. \ Conditioned Salivar cretion onditione ali ary MO These experimen NS early demonstrate an essential difference between the twogmebbAds, but, as has already been said, the apparent anomaly present y the first method remained for many years without any. Gepisfactory explanation, until the question was re- investigate Dr. Stroganov, who worked with differential inhibi- tion. itioned. alimentary reflexes were established in four dogs,

differentiations being established for musical tones of differing pitch and for different rates of metronome beats. A pronounced differ- ence in the rate of destruction of the inhibitory process by the two methods was clearly exhibited in all the dogs, without any exception. In the. case of repeated application of the inhibitory stimulus with reinforcement, destruction of the inhibition was obtained after only a few applications ; in the case of regular alternation of the inhibitory stimulus with the positive conditioned stimulus, on the other hand, both being followed by reinforcement, the destruction of inhibition was very much delayed, and appeared only after a great number of applications. In the different animals employed in this research sometimes the one and sometimes the other method of destruction was used first, and in every case the two methods were. repeated. many times with each animal. The differentiations were, of course, always well re-established before the succeeding . experimental destruction. In addition, several important variations in the experi- | ments were made. The first variation consisted in the application | of the positive stimulus after the differential inhibition had been | almost altogether abolished by the use of the second. method of destruction ; it was found that even one single application was sufficient partially or fully to restore the inhibition (12.6 p.m. in the

table below). A rate of 120\beats of the metronome per minute a for a | positive conditioned alimentary stimulus. A rate of ats per minute was firmly established as a stimulus to a di ial inhibi- tion. During 41 applications within a period of s it did not ions of the positive conditioned stimulus just before wo of the differential inhibition by the second method was begun. ‘This preliminary administration of the posi- tive stimuli caused an unusual retardation of the destruction, which now required an additional five or six applications of the

In another dog the same positive and inhibitory conditioned stimuli were employed, and although the differentiation in this dog was less stable and, therefore, subject more easily to dis-inhibition, a similar delay was observed in its destruction. In a third variation of the experiment, after the destruction had been carried by the method of alternation to such a degree as to be already appreciable, four or five applications of the positive stimulus in succession were sufficient to reverse this effect ablishing the inhibition. XS

Another method of experiment for the de ration of negative induction was employed by Dr. Prorokov, made use of an old observation that the positive condition ex response which was evoked second in an experiment epee showed the greatest secretory effect. This was mos ably owing to an increased excitability of the alimentary he following on the first reinforce- ment with the unconditioned ex; on this account a recently established, but not yet quj Sable, inhibitory stimulus is frequently disturbed when applied ey erreriiiens immediately after the first application of the un tioned stimulus, the reflex being partially dis-inhibited. A everal positive conditioned alimentary reflexes are present, and one of them is associated with a differentiation, it can be obsex$ed that if one of the independent conditioned reflexes is applied in a given experiment, the reflex corresponding to the RS inhibitory stimulus applied second is frequently con-

siderably dis-inhibited. When, on the other hand, an experiment begins with the application of the positive one of two differentiated stimuli, the effect of the negative stimulus when applied second is but rarely, and then only slightly, disturbed. The actual experi- ments of Dr. Prorokov were carried out as follows. A rate of 144 beats of a metronome per minute served as a positive conditioned alimentary stimulus, while a frequency of 72 beats per minute was differentiated from it. The sound of a buzzer served as a further | positive alimentary stimulus. When the buzzer was applied first in the experiment, and the inhibitory stimulus of the metronome second, the reflex to the metronome was dis-inhibited in 8 cases out of 12, the maximum disinhibition being 72%. When, however, the inhibitory rate of the metronome was applied after a preliminary application of the positive rate, dis-inhibition was obtained in only two cases out of 12, and it‘never exceeded 20%. The two procedures were always conducted in different experiments alternately, in order to preclude any effect of an increase in the stability of the differential } . inhibition following on repeated contrasts. The results show | that, at any rate in some cases, the effect of negative induction is } more especially connected with the positive stimulus to which a | differentiation has been established, any other positive conditioned | stimulus, even though it belongs to the same analyser, having but a | small, or even no, effect of negative induction upon the ighibitory | stimulus of the differentiated pair. NN The two forms of experiment just described. leave oubt that | under certain conditions the generation of an excita intensifies a ) succeeding inhibition. | The recognition of the actual existence atre induction naturally led us to inquire whether the i) hich had.

previously | been described under the name of exteyn hibition was in essence | a form of a negative induction, in one the excitation initiated | by extra stimuli falling upon the_coMéx induced a greater or less degree of inhibition in the surro g points. The investigation is, whether the external inhib s a cortical phenomenon, or whether | however, somewhat difficult, e it is necessary to determine . it takes place entirely Ge lower centres of the brain, seeing that | in the case of extern layAibition the centres of two different uncon- ditioned. reflexes sive pb pili From what is known with regard to the functions o ower parts of the central nervous system, it

must undoub e assumed that in the case of external inhibition of conditioned reflexes an interaction of an inhibitory character does occur between the centres of the two unconditioned reflexes. It must be ascertained, however, whether in the case of external inhibi- tion there is not also a similar interaction between the various points of the cortex, and this is where the difficulties appear. An attempt was made to overcome them in experiments by Dr. Foursikov, the actual experiments being performed in the following way. Two conditioned reflexes based on different unconditioned ones were established, the one alimentary and the other a defence reflex to a strong electric stimulation of the skin. The conditioned stimulus to the defence reflex was applied, and as soon as the defence reaction appeared the conditioned defence stimulus was replaced by the conditioned alimentary stimulus. In the majority of cases the con- ditioned alimentary reflex suffered inhibition in a greater or less degree. Since, however, the defence reaction was present, it must be assumed that the unconditioned centre for the defence reaction was in a state of excitation, and therefore, that the possibility of an interaction of the unconditioned centres was not eliminated in these experiments. Although, however, the experiments in this form were inconclusive, they provided occasion for an observation which justified the assumption that in some cases an interaction in the form of a negative induction must also exist between the cortical points corresponding to the two conditioned reflexes. It was noticed that after the establishment of the conditioned defe ex, the older conditioned alimentary reflex when it belonged OF ifferent analyser suffered no diminution in intensity.