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

rates of develop of a precise differentiation by these two methods. For ezampS i begin with the first method we generally find that the differentiation does not become established even after a con- siderable number of contrasts of the two very closely allied stimuli ; but if we proceed to establish a differentiation of a remoter stimulus, working up gradually through finer differentiations until the very closely allied stimulus is again reached, it is found that this differen- tiation is now very rapidly established. The following experiments of Dr. Gubergritz serve to illustrate these relations :

A circle of white paper provided a conditioned alimentary stimulus from which it was required to differentiate a circle of grey paper of similar size made of No. 10 in Zimmermann’s scale (50 shades from white to black). Seventy-five applications of the grey circle No. 10 without reinforcement, contrasted. with frequent applications of the white circle which always remained reinforced, failed to produce the slightest sign of differentiation. A much darker circle No. 35 was now contrasted with the white, and a differentiation was quickly established. Differentiation was now carried out for grey circles Nos. 25 and 15, after which the attempt to differentiate circle No. 10 was made again, with the result that complete differentiation was established after a total of only 20 applications, in all, of the four different circles.

A similar experiment, also with a visual stimulus, but in a modified form, was carried out on another dog. In this case the conditioned alimentary stimulus was again a circle, while the stimulus to be differentiated from it was an ellipse cut from the ga pope and of equal surface, with the semi-axes in the ratio of Although at the beginning 70 applications of the pre Ed Sy e with the method of contrast, no differentiation was obtaine cecessive differentia- tions were now obtained in stages fo a with ratio of the semi-axes 4: 5, 5:6, 7: 8, and fin h the ellipse of the ratio 8:9. A precise differentiation ok t tter was finally established after a total of only 18 Er in all, of the four ellipses.

In ai up a differentiation by stages, beginning with a nt of the first crude differentiation an absolute, or absolute, differentiation has been obtained, the succeeding stagés of progress towards the finer differentiation are passed. Deh with increasing rapidity, becoming, however, somewhat rded as the limit of the analysing activity is A white circle of a given surface area was used for a conditioned stimulus, while ellipses of the same area and whiteness but with different ratios of semi-axes provided the stimuli undergoing differen- tiation. In order to obtain a pronounced differentiation of the first ellipse, in which the ratio of the semi-axes was 4:5, twenty-four applications were required, with, of course, frequent contrastings by the circle. At this stage the circle elicited a secretion of 34 divisions of the scale in 30 seconds, whereas the effect of the ellipse was measured by only four divisions. The next ellipse, with a ratio of 5:6, required only 3 applications in contrast to the circle before it became fully differentiated. Three repetitions were required also for the next ellipse, in which the semi-axes were in the ratio 6: 7. |

It should be noted that irregularities in the curve of development of differentiation do not depend always on the disturbing influence of the investigatory reflex due to external stimuli ; in all probability they are sometimes caused. by variations in the intensity of the underlying nervous activity. The stability of differentiation of a given stimulus can be measured by the length of time reckoned from the last application of the positive stimulus during which differentiation is fully maintained. When differentiation has only recently been established, the length of time during which the differentiated stimulus without intermediate practice will yet give a full zero on its next applicati short ; this length of time increases, however, as the iret becomes more firmly established. For practical purposes e a differen- tiation as being fully established when it is ined for not less than 24 hours, still giving a zero reflex when ied as the very first stimulus in an experiment. ` (e)

Our repeated experiments have Qe strated that the same precision of differentiation of various étyhuli can be obtained whether they are used in the form of negatége or positive conditioned stimuli. This holds good in the case of Oftone trace reflexes also. The following experiment from Ky y Dr. Frolov gives an illustration of the differentiation of N stimulus : A rate of 104 beafS minute of a metronome was established as a conditioned alinterftary stimulus. The conditioned trace in- hibitor undergoiti erentiation was given by a definite tone of an organ pipe (N Gg which was sounded for 15 seconds and followed after a NS ne minute by the stimulus of the metronome which

I| remained in this case without reinforcement. A combination of the I metronome with a trace of the tone of the next organ pipe (No. 15, I | an interval of one tone from the first) was contrasted with the first, | being reinforced so that it became an excitatory stimulus. The Ij differentiation of the trace inhibition is illustrated. in the following I experiment : Time Stimulus Duration of divisions of the scale Stimulus during successive It | It should be added that the above differentiatiga\was obtained Hl | by passing through a long series of crude differenMg@tionis, beginning I | with traces measured by seconds, and with wi Catervals of tones ; | | but once developed the differentiation could peated from day to

| day. x5 | | With regard to the nature of th us process by which the I | initially generalized conditioned ulus comes to assume an.. extremely specialized form, we hake a undant experimental evidence i} | * Differentiations of trace-coyedjtioned inhibitors are very easily subjected | to dis-inhibition and are very, able. It can be noticed in the above ex- periment that the first app} of the organ pipe No. 16, the after-effect of which should have ZAN the secretory action of the metronome, failed to do so, when applie in this experiment, as the first stimulus after an | interval of 24 hou fremi the preceding experiment. The second application q of the organ pipe Nw 16 exerted a powerful inhibitory after-effect, giving a || secretion of ones divisions with a latent period of over 15 seconds as com- r

| pared with etion of 90 divisions with a very short latent period with | the RS n pipe No. 15. that it is based upon internal inhibition ; in other words, we may say that the excitatory process which is originally widely spread in the cerebral part of the analyser is gradually overcome by internal inhibition, excepting only the minutest part of it which corresponds to the given conditioned stimulus. This interpretation of differentia- tion as based upon internal inhibition rests upon evidence to be described now.

A differentiation is established, between two closely allied stimuli, so that one of them which is reinforced gives a constant positive conditioned effect, while the other, which remains unreinforced, gives no secretory effect. If, however, the positive stimulus is applied a short time after the differentiated one, there is found to be a considerable diminution of its secretory effect. An illus- tration of such an experiment can be given from a research by Dr. Beliakov :

A definite tone of an organ-pipe has been given properties of an alimentary conditioned stimulus, and an interval of jth lower has been firmly differentiated from it by the usual method of contrast. Salivary ; Stimulus applied Secretion Time during 30 secs. in drops Rem during It follows that after applicatio the differentiated. tone there remains in the nervous system @ate of inhibition which is for some time sufficiently powerful t ken the excitatory process set up by the application of tne stimulus.

The inhibition w if) s exhibited in differentiation must be recognized as S eA the fourth type of internal inhibition, which may be cąllgd differential inhibition. It would t mind be quite appropriate to bring conditioned inhibition al(ONthder the heading of differential inhibition, since in both cae deal with a removal by means of internal inhibition of an excitatory effect of simple or complex stimuli which acquired their excitatory properties spontaneously in virtue of their partial | resemblance to the original positive conditioned stimulus.

The inhibitory after-effect in differential inhibition corresponds exactly with the inhibitory after-effect in conditioned inhibition, both becoming shortened by repetition. At the beginning they may persist upwards of an hour, but they become restricted finally to a matter of a few seconds. It is necessary to emphasize in this place the fact that the finer the degree of differentiation the greater is the intensity of the inhibitory after-effect. The following experiments of Dr. Beliakov serve to illustrate this point :

A definite tone represents the conditioned stimulus in an ali- mentary reflex ; intervals of one-half and one-eighth were used for differentiation. Salivary Stimulus applied | Secretion Time during 30 seconds in drops Remarks during 30 seconds conditioned inhibition and rential inhibition, the latter provides Apart from the close ential already mentioned between a close parallel in al spects to the three types of internal inhibition which ha en dealt with in previous lectures. Thus the inhibitory si pe in differential inhibition, similarly to other forms of interngl ibition, undergoes summation on repetition of the stimulus: fe following experiments are again taken from the

researche r. Beliakov : YX dog is taken in which a conditioned alimentary reflex is established to a definite musical tone, while a semitone lower is firmly differentiated as an inhibitory stimulus. Salivary Stimulus applied Secretion Time during 30 ana in drops Remarks during 30 secs. In differentiation as in the other types of internal inhibition the intensity of inhibition stands in direct relation to the strength of the excitatory process on the basis of which it was establi And can therefore be disturbed by any increase in the intensity e Stimulus which developed the inhibitory properties, or by general or local excitability of the central s system. To illustrate this last condition we may take aos of differential inhibitions established on the basis of Era reflex. If, for example, the dog has been kept ie thout food for a much

longer period than usual before th eriment is conducted, the increase in excitability of the whole wimentary nervous mechanism renders the previously estab] differential inhibition wholly inadequate. Again, if the xcitability of the central nervous system has been inorease Ne example by an injection of caffeine, the previously establis QO erentiation similarly becomes disturbed. This effect of an alt on of the general nervous excitability is experiment of Dr. Nikiforovsky :

ation of the fore-paw serves as a positive con- y stimulus, while a tactile stimulation of the back fferentiated from it. Salivary Secretion in drops during ; Stimulus applied successive Time during 1 minute minutes Remarks from the beginning of the conditioned stimulus 12.52 p.m. Tactile stimulation of back | 0, 0, 0 Not reinforced. 1 ©; Ta Tactile stimulation of fore- paw 5 Reinforced. Subcutaneous injection of 5 c.c. of 1% solution of caffeine — — Bis; Tactile stimulation of fore- paw + Reinforced. i PS Dae Tactile stimulation of back | 3, 3, 2 Not reinforced. 1.45 5 Tactile stimulation of fore- | paw. 7 Reinforced.

Lastly, in common with the other three groups of internal in- hibition, differential inhibition is subject to dis-inhibition, becoming temporarily removed under the influence of mild extra stimuli belonging to the group of external inhibitors, so as to reveal the underlying excitatory process. Two experiments Dr. Beliakov carried out on the same animal are given in lyse A tone of 800 d.v. served as a condition mentary stimulus, and an interval of one-eighth (812 d.v.) wa roughly differentiated. from it. A sound of bubbling water o) odour of amyl acetate served as mild extra stimuli which mselves did. not evoke any

Salivary : Stimulus ied Secretion Time during 3 nds in drops Remarks per 30 It is interesting to note that dis-inhibition can also be obtained when mild extra stimuli influence the hemispheres while the after- effect of differential inhibition is still persisting. The following is an experiment carried out by Dr. Beliakov on the same animal. . The extra stimulus is given by the sound of a metronome which by Among the extra stimuli which haveSteen employed there were some which evoked, not an ordinary<Dvestigatory reaction, but specific reflexes of greater intensity Qa complexity ; in these cases the dis-inhibitory after-effect wa, O much prolonged. An example of the use of such an ENI us can again be taken from an experiment by Dr. Beli erformed on the same animal as

before. A strong extra stingylus was provided by the blare of a toy trumpet which produced v thous and exceedingly discordant noises. The dog reacted by ng wildly, trembling and trying to break away Time Stimulus applied during Salivary Secretion in successive The experimental evidence advanced in this lecture leaves us in no doubt but that the establishment of differentiation is based upon the development of internal inhibition in respect to the differentiated agent.

On the evidence of our experiments we are also forced to the conclusion that there is an important difference between the cruder form of differentiation depending upon external inhibition, and the finer form of differentiation depending upon internal inhibition. The former and more generalized inhibition is brought about by the intervention of an excitatory process, in most cases in the form of an investigatory reflex, and this has only a secondary inhibiting or dis-inhibiting effect upon the conditioned reflexes; the latter is brought about by a primary development of an inhibitory process, resulting, so to speak, from a conflict between excitation and in- hibition. This supremacy of the inhibitory Bey is sometimes gained only with considerable difficulty, and i e cases it is even beyond the power of the nervous system olve the conflict in favour of either process. In the latter c e antagonism between the excitatory and inhibitory processe not always bring about a full utilization of the results of s of external stimuli for the general benefit of the organism. eing so the study of the analys- ing activity of the nervous syadem by the method of conditioned reflexes will also have its Ghitetions—a fact which in itself

The analysing and synthesizing activity of the cerebral hemispheres (continued) : (c) Examples of the analysis of stimuli. (d) Synthesis and analysis of compound simultaneous stimuli. (e) Synthesis and analysis of compound successive stimuli. It was shown in the preceding lecture that the animal at first general- izes any definite individual stimulus of the outer world, but that with repetition the stimulus becomes more and more specialized as a result of the development of an inhibitory process (differentiation). In this final form conditioned reflexes provide a reliable method for an experimental study of the scope and limits of the activity of the different cortical analysers. Our knowledge of the different analysers in the dog has recently been considerably advanced through the study of conditioned reflexes,—a fact which affords a striking example of the practical utility of this method of research. It should, moreover, be mentioned with regard to these particular experiments, that while considerable difficulties were sometimes enco red in the course of the work, these did not arise in the physiokNcal part of the technique, but derived from instrumental limit@fions, since in many cases it has been exceedingly difficult to oR or construct suitable physical apparatus. The main requing is for instru- ments which are capable of producing a xeNéctly isolated and unvarying elementary stimulus of a deffnit} legree of intensity. This, however, is very often a practical Mnpossibility. For example, it is exceedingly difficult to find an apkadatus for tactile stimulation of the skin which will not prod some slight sound during its application. It is also not eas D tain an alteration in the pitch of a tone without simultan. ffecting its strength. Indeed, it seems to me that future) rimentation upon the analysers of animals will exhibit an(intp esting competition between the delicacy of the nervous anes es and the skill of the instrument maker.

We shall turn, to a consideration of the experiments at our disposal, takin those dealing with the visual analyser of the dog. In AS of discrimination of luminosity this analyser was found to be greatly superior to that of man. Thus, for example, a conditioned reflex was established in a dog to the presentation of a black screen perfectly uniform in shading, without any traces of graining or spots. A white screen of identical shape and size, and also of uniform luminosity, was differentiated from the black screen by the usual method of contrast. The experimenter was provided with a number of screens of different shades (50 numbers of Zimmer- mann’s collection) ranging from white through different shades of grey to black. After the differentiation of white had been firmly established the same method of contrast was used to obtain finer degrees of differentiation of grey screens approximating more and more nearly to black. It was found in this way that the visual analyser of the dog was capable of distinguishing between the neigh- bouring shades Nos. 49 and 50, while to the human eye there appeared not the slightest difference between them, whether they were examined successively at different intervals of time or simultaneously. This was also true for several other shades separated further from one another on the scale, which could not be discriminated by the human eye but which the dog differentiated perfectly. The following experiment shows an evident, though not absolute, differentiation of the screens Nos. 49 and 50 [experiments by Dr. Frolov] :

Secretion Conditioned of Saliva Time Stimulus applied in drops EAN marks during 30 seconds during 30 seconds Q 3.13 p.m. Screen No. 50 1 Q Reinforced. rer Screen No. 49 Q Not reinforced. It thus becomes evident t s regards the analysis of intensity of illumination the visual ied er of the dog is so highly developed. that we were unable KO rmine the limit to which this activity In the case of NA of various colours the results obtained were quite diff Dr. Orbeli in a first series of experiments was unable to pie differentiation of colours on the part of his dogs.

In a seco ries of experiments, however, positive results were obtained @) ne dog, but only with great difficulty, and even in this ANN were still open to criticism. The results obtained by other investigators, both Russian and foreign, lead to the con- clusion that colour vision in dogs, if present, is only of a very rudi- mentary form, and that in most dogs it cannot be detected at all. Dr. Orbeli studied also the differentiation of figures. Examples of figures for which an absolute differentiation was obtained, are given in Fig. 6.

Experiments with regard to differentiation of shapes were con- tinued by Dr. Shenger-Krestovnikova. An alimentary conditioned reflex was established in a dog to a luminous circle which was pro- jected on to a screen placed in front of the dog. After the reflex had attained a constant strength the animal was made-to differentiate from the circle a number of ellipses of equal surface and luminosity. In the first of the ellipses the ratio of the semi-axes was 2:1, and differentiation was established with ease. This was followed up by a series of ellipses which gradually approximated to the circle in shape, and so required a finer and finer differentiation. The ellipse with ratio of the semi-axes 9:8 proved to be the limit at which differentiation just failed. Some indication of differentiation appeared, at first, but on repetition it gradually disappeared, and with it disappeared also all the previously established coarser differentiations. To renew these it was necessary to work up care- fully from the very beginning, starting with the first ellipse with a ratio of the semi-axes 2:1. When all the coarser discrim Ga ined

had again been obtained, the ellipse with the ratio 9: tried once more. Its first application showed a complete di ination giving a zero secretion of saliva. Further tests, ho r, led to the differentiation again (if the first trial can arded as a real differentiation at all), but all the earlie ser differentiations disappeared, as well. In this phenome sa e have a clear repro- duction of the case referred to at the of the preceding lecture. When the stage of minute differeyges between stimuli is reached, analysis of itself appears still Le but the relations existing

between the excitatory and itory processes seem to present an insurmountable obstacl © continued. and permanent utiliza- tion by the animal for æn ropriate responsive activity. The investigati irection of motion of figures and points was also investigattCD our laboratories, but the limit of discrimina- tion was not de ned, in these cases. The ans activity of the acoustic apparatus in the dog was FIG. IN ples of different figures which were successfully differentiated in experi- ments by li. The letter T, shown in the upper left-hand corner of the figure, served e cuca the other black figures and the white letter T where differentiated

for the from << itive stimulus. ther dog the white cross was the positive stimulus from which the other white investigated in especial detail and in various directions. In the first place we shall consider the analysis of different intensities of the same sound. It was found that any definite degree of intensity of a sound could easily be made into a stable conditioned stimulus and could be differentiated from slightly higher or slightly lower inten- sities of the same sound [experiments of Dr. Tichomirov]. A tone of 1740 d.v. was sounded by an organ pipe into which air was blown at a constant pressure of 3-6-3-8 cms. of water by means of a spiro- meter. The organ-pipe was fitted in the centre of a wooden board. covered by a thick layer of cotton wool. Above this board and over the pipe was suspended a wooden box, open below and also coated By raising or lowering this box to different heights over the pipe definite dampings of the sound were obtaingd. The limit of differentiation to the intensities of a given sound could now be determined for the dog and compared roughly with that of human beings. Thus it was found that an intensity very closely approaching the one employed as a positive conditioned stimulus could be differentiated by the dog with an absolute precision even when a pause of 17 hours was made between the two stimuli. The experimenter found himself able to detect a difference between these two intensities of the sound only when they succeeded each other immediately. The following is an example taken from these experi-

different intensity K Not reinforced. 4.49 ,, Usual intensity of tone O 3 Reinforced. inhibitory tone was brought s rer to the intensity used for the positive conditioned. stimu d an absolute differentiation was obtained even after a Pp AN three hours between the stimuli. Un- fortunately these expeNmgnts were conducted in our old laboratory where the effect of inhibitory stimulus was easily disturbed, and it must be left t uture to repeat these experiments under more perfect conditi n our new laboratory.