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

In the continuation of thes Qperiments the intensity of the So A great number of experiments have also been carried out on the differentiation of pitch. To produce tones of different pitch wind instruments were chiefly employed, and the limit to which we were able to carry our tests was an interval of one-eighth of a tone (800 and 812 d.v.). This interval, as has already been shown in the pre- vious lectures (p. 125), was differentiated with perfect accuracy by the acoustic analyser of the dog. It was not possible to carry our determination of the physiological limit of differentiation of pitch any further, since we could not be sure that our apparatus would accurately reproduce smaller intervals than one-eighth. The re- petition of these experiments with the use of pure tones, produced by telephones with resonators, showed that this differentiation could still be obtained as easily as with the richer tones of wind instruments [experiments of Drs. Anrep and Manuilov].

The upper limit of the auditory range of the dog was demonstrated by Dr. Bourmakin with the use of a Galton’s whistle, and by Dr. Andréev with the use of an apparatus producing pure tones, to be very much higher than in man. Conditioned reflexes were success- fully established to tones of such high pitch as to be quite inaudible to man. It was indeed interesting to observe how sharply and precisely the dog reacted to sounds which were non-existent to the human ear. i

The differentiation of timbre and the differentiation of direction of sounds were also submitted to investigation, bata limits of differentiation were determined in the cases of t pecial qualities. While discussing the functional capacity acoustic analyser we may refer to those experiments in whic Gee de was based. not upon the differences in the Me the sounds but upon differences in the rhythm of succeges pplications of one and the same single sound, The sound in, t ase was produced by a metro- nome beating regularly, but at eer rates in the various experi- ments. Differentiations of this e were quite easily obtained, but the main interest of these ikPstigations lay in the determination of the limit of differentiati ich for the dog was found to be far more subtle than the discri ion which could be recognized inman. The dog was capable Cyry precise differentiation between such rates as 100 and 96 Ce per minute even when applied at a very long

interval after Gage another (t.e. a discrimination of 0024 seconds). A few riments have been conducted with the analysers for N ermal cutaneous stimuli. Differentiation could easily be established for the site of stimulation, and was found to be very precise, but no determination was made as to the actual limit. Differentiations were obtained also for different types of tactile stimulation, namely pressure by smooth and rough surfaces, pressure by blunt points arranged in various patterns, and by scratchings in various directions with a small brush. Furthermore, differentia- tions were also obtained for different degrees of temperature.

The most perfect analyser in the dog is the chemical analyser of smell, but our information concerning it is scanty, mainly on account of instrumental difficulties. It has been exceedingly difficult, if not impossible up to the present, to obtain the same accuracy in gradua- tion of olfactory stimuli as of any other stimuli. It is impossible also to limit the action of olfactory stimuli to any exact length of time. Furthermore, we do not know of any subjective or objective criterion by which small variations in intensity of odours can be determined. On this account only a small number of experiments could be conducted. JDifferentiations were obtained for various odours— camphor, vanillin and many others. ‘Some of these were made into positive conditioned alimentary stimuli or positive conditioned stimuli for acid, while others were given the corresponding inhibitory properties. Experiments upon differentiation were also conducted with mixtures of odours, into which some new odour could be intro-

Lastly, some experimental data have been obtained (Morning the chemical analyser of taste. In this case mat Rey Were more complicated, because the unconditioned stimuli ce usually employed in the experiments (food and reje bstances), both act on this analyser. In order to study oon of taste in a manner similar to that employed for fe lysers, it would be necessary to employ an unconditioned ape to some other analyser and to use various taste sty i es establishing the cor- responding positive and oo. OP ea reflexes. Experiments of this kind have not kae fret We adopted, however, another method in WERE SS Gym ber of conditioned reflexes were established each seo) wn. a different nutritive or rejectable substance (meat mor ead crumbs, sugar, cheese, acid, soda, etc.), and ane inter and mutual inhibition were then observed. The following is a n@ample of such an experiment by Dr. Egorov :

A tactile stj Se of the skin presented a conditioned stimulus to the os lòn of a mixture of meat powder with bread crumbs ; a rotating object was used as a conditioned stimulus to the consump- tion of Dutch cheese. Secretion of Time Conditioned Stimulus Saliva in Remarks drops during 30 secs. Since the conditioned stimulus of the rotating object (3.50 p.m.) was reinforced by administration of cheese, the above form of the experiment does not tell us whether the subsequent diminution in the conditioned reflex to the tactile stimulus (t.e. the conditioned stimulus for meat and bread) was due to the conditioned stimulus of the rotating object or to its reinforcing agent (t.e. cheese). To determine this point similar experiments were conducted with the use of the conditioned stimuli applied without reinforcement. The following is an example of such an experiment.

Tactile stimulation of the skin usually resulted in this dog in a conditioned secretion varying from 5 to 6 drops of saliva during 30 seconds. The stimulus of the rotating object "A: given prior to the application of the tactile stimulus remai Qn inforced. Time Conditioned Stimulus Remarks 3.12 p.m. Rotating object Not reinforced. 320 Tactile 2 Reinforced after 30 secs. 3.00, 7 ; 1 Reinforced after 30 Qy secs. P) | Reinforced simul- JADR 3 — taneously with the AO A N —- beginning of the O 5 tactile stimulus. AE iy Q 2-5 Reinforced after 30 y secs. a T a Se) 7 2 Reinforced after 30

The very considerable and protracted diminution in the reflex — to the tactile stimulus shows that it is the analysing activity of the cerebral part of the chemical analyser of taste which is involved in this case, since the conditioned stimulus to cheese was in itself sufficient without reinforcement to bring about a profound diminution in the reflex to the tactile stimulus. Another set of experiments was performed [by Dr. Savich] in which -“‘ natural ” conditioned reflexes to the appearance of meat powder and to the appearance of granulated sugar were employed. The dog’s usual diet outside the experiment consisted of oatmeal porridge with meat and bread. Later on meat and bread were excluded from the meal, but a large amount of sugar was added to the porridge. After this diet had been maintained for some time the conditioned reflex to the appearance of meat powder was found. to be considerably increased, while the reflex to the appearance of sugar had disappeared almost entirely.

Similar experiments, but with greater precision of detail, were carried out by Dr. Hasen with conditioned reflexes to rejectable substances. He took advantage of two facts which had previously been noticed in reflexes to acid, viz., that in most of the cases the magnitude both of conditioned and of unconditioned reflexes gradually rises towards the end of an experimental day, and that the magnitude of the reflexes also rises, up to a certain maximum, in the course of a series of experiments conducted on smog days. Dr. Hasen modified his experiments in the following er. After the first application of the conditioned stimulus, whi as reinforced, acid was introduced several times in successiog™pthout the con- ditioned stimulus, and finally the condition, imulus was again applied. The effect of the conditioned stj on this last applica- tion was found always to be ee E mpared with the effect on its first application. At a later s in the experiments, which were performed on two dogs, the eas was interrupted by three intervals, of five days each one series of experiments, and of three days each in another g the first interval the one dog received in the form of an 4 and the other dog received through a stomach tube, a co ble amount of dilute acid. During the second interval a solutiewof sodium carbonate was given in a similar manner, and duri e third interval the dogs received no injections at all. The con ned reflexes and the unconditioned reflexes were tested “JS of these intervals. It was found that after the

interval in which acid was introduced the magnitude of the reflexes remained the same or was but slightly diminished as compared with the experiments preceding the pause; after the interval in which nothing was administered the magnitude of the reflexes fell con- siderably ; and after introduction of soda the reflexes diminished still more. In the course of daily experiments a definite amount of acid introduced into the mouth of one of these dogs produced from the submaxillary gland on an average a salivary secretion of 5:1 c.c., while the conditioned stimulus elicited a secretion of 4 drops during 30 seconds. After the interval during which nothing was adminis- tered the figures were 3-8 c.c. and 2 drops respectively; after the interval in which sodium carbonate was-introduced 3-7 c.c. and a secretion of zero were respectively produced; finally, after the interval in which acid was administered, the secretions obtained amounted to 4:5 c.c. and 3 drops respectively. Thus it is evident

that the variations in the chemical composition of the blood which | resulted from excess of acid or alkali were differentiated by the cerebral termination of the chemical analyser, being manifested by an increase or a decrease in the excitability of the central part of the analyser. When the animal absorbed an excess of acid the excita- bility of the “acid ” part of the chemical analyser increased, with the result that on encountering acid from the outer world the organ- ism responded by more vigorous motor and secretory reflexes directed towards the exclusion of the further intrgd\ction of acid. The same, of course, takes place in the case of bive substances, a corresponding increase or decrease being o d in the positive or negative reactions to different substang different quantities of them. It is thus seen that the chen@aVanalyser of taste in its central part forms a connecting JK jetween the internal and external media of the organism, a ad) by regulating their relations one to the other secures a gein constancy of the internal medium.

The experiments whic Gave just been described, and which, unfortunately, were not quently repeated, belong to the earlier period of our work, he novelty and complexity of the subject afforded numero rces of error. Nevertheless, even so, they show definitely the study of the chemical analyser of taste, though co cy can be conducted successfully by the use of con- ditioned r S. With regard to the synthesizing activity of the nervous system, as compared. with its analysing activity, little is known up to the present. It would, indeed, be futile for us to attempt to discuss - the nature of its intimate mechanism : it can only be suggested that, in the future, synthesizing activity will be referred to the physico- chemical properties of synaptic membranes or anastomosing neuro- fibrils. Our immediate task must consist in accumulating experi- mental material concerning the synthesizing activity.

In addition to the formation of the conditioned reflex itself— which is, of course, primarily an expression of the synthesizing activity, and which has constantly formed the starting-point of our investigations—we have also examined the properties of compound conditioned stimuli. Compound stimuli were used with either simultaneous or successive action of their component parts. In the case of compound simultaneous stimuli the following i important relations have been observed : | When the stimuli making up the compound act upon different analysers, the effect of one of them when tested singly was found

very commonly to overshadow the effect of the others almost com- pletely, and this independently of the number of reinforcements of the compound stimulus. For example, a tactile component of a stimulatory compound was usually found to obscure a thermal com- ponent, an auditory component to obscure a visual Pal t, and so on. Thus, in the following experiment by Dr. Pa con- ditioned reflex to acid was established to a simultaneo iOS on of a thermal stimulus of 0° C. and a tactile stimulgWo B the skin. Tests were made both of the compound. ERANA f its individual components applied singly.

Another example or be taken from an experiment by Dr. Zeliony. A he ae alimentary reflex was established to the simultaneous a ation of the tone of a pneumatic tuning-fork, .which was g ably damped by being placed within a wooden box coated with wool, and of a visual stimulus of three electric lamps placed in front of the dog in the slightly shaded room. In the above experiments the action of the thermal and visual components by themselves was ineffective, being completely over- shadowed by the other respective components. It is obvious, however, that the ineffective components in the stimulatory com- pounds could easily be made to acquire powerful conditioned properties by independent reinforcement outside the combination. The true interpretation of the phenomenon which has just been described is revealed by experiments in which both components of a stimulus belong to one and the same analyser. For example, in one experiment there were used as components in a stimulatory compound two different tones, which appeared to the human ear to be of equal intensity. When the conditioned reflex to the com- pound became fully established, the tones sounded separately were found to produce an equal effect. In another experiment, a con- ditioned reflex was formed to a compound in ae two indi- vidual tones were of very different intensities effect of the

tone of weaker intensity when tested singly ow very small or absent altogether. Such a case is ill d in the following experiment by Dr. Zeliony. An alimen eflex was established in a dog to a compound stimulus m Q of the sound of a whistle and the sound of the tone d’ sha neumatic tuning-fork. Both these sounds appeared to the gom ear to be of equal intensity, and both when tested separately elicited a secretion of 19 drops of saliva during one minute Gi addition to this, another compound stimulus was establishe e up of the same sound of the whistle plus the tone a’ of a g-fork of weaker intensity. When tested separately the Kee in this case elicited a secretion of seven drops of saliva during Whitty seconds, and the tone only one drop.

It is ove) from the above experiments that the obscuring of one stimu y another belonging to the same analyser is deter- ` ENS ifferences in their strength, and it is natural to assume that this explanation can be applied also to compound stimuli, the components of which belong to different analysers. On this assump- tion tactile cutaneous stimuli in our experiments should be regarded as being relatively stronger than thermal cutaneous stimuli, and auditory stimuli should be regarded as being relatively stronger than visual stimuli. The natural deductions from such an assump- tion are far-reaching, and it will be necessary at some future date to test its validity by the use of stimuli compounded from different _ analysers and varying as much as possible in their intensities, a very weak auditory stimulus being combined with a very strong visual one, and so on.

The phenomenon in which one stimulus is obscured by another in a simultaneous stimulatory compound, when the two stimuli belong to different analysers, presents several interesting features. The effect of the compound stimulus is found nearly always to be equal to that of the stronger component used singly, the weaker stimulus appearing therefore to be completely overshadowed by the stronger one. If, however, the stronger stimulus is even at long intervals of time, repeated singly without reinforcement by the unconditioned reflex, while the compound stimulus is constantly reinforced, the stronger stimulus by itself becomes completely in- effective, whereas in the stimulatory compound there is no diminution in its effect. It is evident, therefore, that although the efigé of the weaker stimulus when tested singly is invisible, it never s9 plays an important part in the stimulatory compound LexGyiments by Dr. Palladin]. .

Another feature of interest has been alre scribed in the fourth lecture [experiments of Dr. Perelzw. age 56]. If the weak component, which may be even qui éNective when applied alone, is repeated at short intervals wWi¢hott reinforcement—.e. is extinguished below zero—then both tee mpound. and the stronger component undergo secondary inction. In this experiment, therefore, the component whic normally of itself apparently ineffective becomes temp ny ransformed by the process of experimental extinction eS strong inhibitory stimulus.

The following was ¢bse?ved. in a single, but so far as the experi- mental conditions werè”concerned a perfect, experiment. When two stimuli blah to different analysers. were first separately made into congibiohed stimuli, and only afterwards applied simul- taneously sN&rm a compound stimulus, which was repeatedly reinforced, the overshadowing of the one component by the other did not occur. From this it may be concluded that in the usual case where two hitherto neutral stimuli are used to form a compound stimulus, the stronger stimulus at once prevents the weaker from forming a corresponding connection with the centre for the uncon- ditioned reflex. If, however, this connection has been established already, it is not disturbed during the subsequent establishment of the reflex to the compound stimulus. The mechanism on which the predominance of one component of a stimulatory compound over another depends is most probably a form of inhi- bition. This matter will be examined in detail in a subsequent lecture.

The cases mentioned above show that a definite interaction takes place between different cells of the cortex, resulting in a fusion or synthesis of their physiological activities on simultaneous excitation. In the case of a compound simultaneous stimulus made up of com- ponents of unequal strength belonging to the same analyser this synthesis is not so obvious. However, it comes out very clearly that even in these cases there is no summation of the individual reflex effect of each single component, the effect of the stronger component applied singly being equal to that of the compound stimulus.

The phenomenon of synthesis of stimuli belonging to the same analyser is much more evident in a modification o A experimental conditions which was first used in Dr. Zelion periments, then again by Drs. Manuilov and Krylov, and since been widely practised. It was noticed that if a conditi reflex to a compound. stimulus was established as described aA , it was easy to maintain it in full strength and at the sa to convert its individual components, which gave a is ect when tested singly, into

negative or inhibitory stimuli. is result is obtained by constant reinforcement of the compoyndtimulus, while its components, on the frequent occasions w ey are applied singly, remain without reinforcement. The ent can be made with equal success in the reverse directio aking the stimulatory compound into a negative or ins y stimulus, while its components applied singly maintain ir positive effect. We leave the discussion of this pheno to Lecture XVI, and shall pass on to consider the secon 2 of stimulatory compounds, namely, compound

In this second type of stimulus, in which the component stimuli are applied not simultaneously but in succession, the synthesizing function of the nervous system is still more obvious. The compound stimuli employed were of many different kinds. In some cases the compound was made up of successive repetitions of one and the same stimulus. For example, a definite tone was repeated three times for one second with an interval of two seconds between the first and second applications, and an interval of one second between the second and third; this rhythm was repeated after a pause of five seconds, and was now accompanied by the unconditioned stimulus. In other experiments the stimulatory compound was made up of three or four different stimuli all belonging to one analyser; the stimuli were made to succeed one another in a definité order, being each of equal duration, and with equal pauses between them. There were used, for example, in one case the four tones C, D, E, F of one octave ; and in another case the four stimuli were made up of a noise, two different tones and the sound of a bell. Finally, in other

- experiments there was employed a stimulatory compound composed of three or four stimuli belonging to different analysers, each stimulus being of equal duration, the pauses between them being also equal. Conditioned reflexes were readily obtained to all these different compound stimuli, and after a certain amount of practice of the reflexes all the individual components when tested singly were found to exhibit a positive conditioned effect, which vioi in magnitude according to the quality and relative sit of the individual stimuli.

The next step was the introduction of differe odifications of these compound stimuli. In the first case rder of the two pauses between the repetitions of the tongo evened, the longer pause being now made between the seco nd third applications instead of between the first and sxe In the remaining cases the order in which the different stim were applied was changed, either completely by reversing i in the case of the compound consisting of four component li by reversing the order of the two middle ones. These fied compounds were repeatedly applied without reinfo mnt, but when the stimuli were applied in their original orde compound was always reinforced, with the result that aces the original compounds became dif-

modifications, which latter finally lost their positive pond ued effect and acquired an inhibitory one [experi- ments by Drs. Babkin, Stroganov, Grigorovich, Ivanov-Smolensky and Eurman]. The following is an example of the different permutations of four tones (1, 2, 3, 4) which were successfully differentiated by the dog in experiments by Dr. Babkin, the sequence 1234 being the positive stimulus and the sequence 4321 the first stimulus to be differentiated ; this was followed by development of an absolute differentiation of all the remaining sequences. The vibration frequencies of the four tones employed were 290, 325, 370 and 413 d.v. respectively.

In the following experiment [by Dr. Eurman] the positive alimentary conditioned stimulus was made up of the flash of an electric lamp (L), a tactile cutaneous stimulation (C), and,a sound of bubbling water (S), applied in that order, namely, L-C-S. The inhibitory compound was made up in the reverse order, namely, positive conditigngd alimentary stimulus was made up of a hissing sound 2 ), high tone (AT), a low tone (IT), and the sound of a buzzer ( lied in that order, namely H-hT-IT-B. The inhibi- tory § s was made up with the order of the two middle a reversed, namely H-IT-hT-B.

The formation of these inhibitory reflexes usually required a great deal of time: although a relative differentiation could some- times be observed quite early, absolute differentiation was obtained in extreme cases only after more than one hundred repetitions with- out reinforcement. Indeed, in order to obtain a complete differentia- tion it was sometimes necessary to proceed by gradual stages, commencing with differentiation of comparatively simple compound stimuli. It was especially difficult to obtain a differentiation of the stimulatory compound of sound, low tone, high tone and buzzer from the compound in which the tones were reversed. All the differentiations, and especially those exceptionally difficult of for- mation, proved very unstable. On the one hand they suffered con- siderably from frequent repetition (see Lecture XIV), a, other hand any interruption in the work caused them to disappear for a time altogether. So soon as com tion between such compound stimuli had bee ablished, the individual components when tested singly wert fond to have lost all their initial positive conditioned effect.