Classics in the History of Psychology

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Christopher D. Green
York University, Toronto, Ontario

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By Ivan P. Pavlov(1927)
Translated by G. V. Anrep (1927)

                                                                                                                                            Posted July 2001

[Classics Editor's note: Pavlov used both square and round brackets in his texts. These have been preserved but can lead to confusions as to which insertions are the author's and which are the editor's. Page numbers, reference numbers, and the occasional "sic" have been inserted in square brackets by the Classics editor. All other insertions (e.g., on p. 31) are by Pavlov. -cdg-]


The experimental results obtained with animals in their application to man.

In applying to man the results of investigation of the functions of the heart, digestive tract and other organs in the higher animals, allied as these organs are to the human in structure, great reserve must be exercised and the validity of comparisons must be verified at every step. Obviously even greater caution must be used in attempting similarly to apply our recently acquired knowledge concerning the higher nervous activity in the dog -- the more so, since the incomparably greater development of the cerebral cortex in man is pre-eminently that factor which has raised man to his dominant position in the animal world. It would be the height of presumption to regard these first steps in elucidating the physiology of the cortex as solving the intricate problems of the higher psychic activities in man, when in fact at the present stage of our work no detailed application of its results to man is yet permissible.

Nevertheless, inasmuch as the higher nervous activity exhibited by the cortex rests, undoubtedly, on the same foundation in man as in the higher animals, some very general and tentative inferences can even now be drawn from the latter to the former. In the future it may confidently be expected that a full and detailed knowledge of at least the elementary facts of this activity will be obtained as regards both normal and pathological states. The similarities between the manifestations of this activity in man and animal being more obvious under normal conditions, I shall dismiss these briefly, discussing in more detail certain pathological cases.

It is obvious that the different kinds of habits based on training, education and discipline of any sort are nothing but a long chain of conditioned reflexes. We all know how associations, once established and acquired between definite stimuli and our responses, are persistently and, so to speak, automatically reproduced, sometimes even although we fight against them. For instance, in the case of games and various acts of skill, it is as difficult to abolish all sorts of superfluous movements as to acquire the necessary movements; [p. 396] and it is equally difficult to overcome established negative reflexes, i.e. inhibitions. Again, experience has taught us that a difficult task should be approached by gradual stages. We know also how different extra stimuli inhibit and discoordinate it well-established routine of activity, and how a change in a pre-established order dislocates and renders difficult our movements, activities and the whole routine of life. Again, we know how weak and monotonous stimuli render us languid and drowsy, and very often lead to sleep. We are also well acquainted with different cases of partial alertness in the case of normal sleep, for example a sleeping mother next to her sick child. All these phenomena are analogous to those constantly met with in our animals as described in the preceding lectures, and there is no point in further discussing them in the present lecture. The discussion of pathological cases, however, will prove instructive.

Contemporary medicine distinguishes "nervous" and "psychic" disturbances-neuroses and psychoses, but this distinction is, of course, only arbitrary. No real line of demarcation can be drawn between these two groups: it is impossible to imagine a deviation of higher activities from normal without a functional or structural disturbance of the cortex. The distinction between "nervous" and "psychic" affections is a distinction made on grounds of greater or smaller complexity and subtlety in the disturbance of the nervous activity. Our experiments definitely show the validity of such a distinction. So long as we deal with animals in which the pathological disturbance results from functional interferences including violent changes in the conditions of life (such as our dogs experienced in the great flood in Petrograd), or on account of small operations on the cortex, we can grasp the mechanism of these disturbances more or less satisfactorily and express it in terms of neuro-physiology. Such disturbances would come under the classification of "neuroses." But if the disturbances are the results of extirpation or destruction by scar of large parts of the cortex we encounter great difficulty in picturing the mechanism of the resulting disturbance in the nervous activity, and we depend more largely upon various suppositions which still remain to be verified and controlled. Such disturbances would be classified as "psychoses." Obviously this difference in our attitude is due entirely to the much greater complexity of the disturbance in the latter cases, and to the inadequacy of present- day physiological analysis. We shall not discuss any conjectured subjective sphere of our animals, but shall consider both cases [p. 397] simply as disturbances in the normal cortical activity -- smaller and more elementary in the former and more extensive and more complicated in the latter cases.

In the dog two conditions were found to produce pathological disturbances by functional interference, namely, an unusually acute clashing of the excitatory and inhibitory processes, and the influence of strong and extraordinary stimuli. In man precisely similar conditions constitute the usual causes of nervous and psychic disturbances. Different conditions productive of extreme excitation, such as intense grief or bitter insults, often lead, when the natural reactions are inhibited by the necessary restraint, to profound and prolonged loss of balance in nervous and psychic activity. So, too, neuroses and psychoses may develop as a result of different powerful stimuli, e.g. extreme danger to oneself or to near friends, or even the spectacle of some frightful event not affecting one directly. At the same time we know that the same influence may produce a profound disturbance in some individuals and show no trace of effect on others, according to the power of resistance of the nervous system in each case. Exactly the same difference is observed also in dogs, which show a great variation in regard to the production of pathological disturbances. We had dogs in which one of the most efficacious methods of evoking nervous disturbances, namely, a direct transition from an inhibitory to an excitatory rate of stimulation of the same place of the skin, failed to produce the slightest effect after a great number of repetitions on many days. In others disturbance occurred eventually after many repetitions, while in some it was produced by a single juxtaposition of the stimuli. In the same manner the great flood, which, as was mentioned previously, led to a profound disturbance, obviously analogous to traumatic neurosis in man, produced this effect only in some of the dogs, namely, those of an extremely inhibitable type.

It has been seen that the above-mentioned method may lead to different forms of disturbance, depending on the type of nervous system of the animal. In dogs with the more resistant nervous system it leads to a predominance of excitation; in dogs with the less resistant nervous system, to a predominance of inhibition. So far as can be judged on the basis of casual observation I believe that these two variations in the pathological disturbance of the cortical activity in animals are comparable to the two forms of neurosis in man -- in the pre-Freudian terminology neurasthenia and hysteria -- [p. 398] the first with exaggeration of the excitatory and weakness of the inhibitory process, the second with a predominance of the inhibitory and weakness of the excitatory process. There are grounds for considering the first type as having a more resistant nervous system which (at least in some cases) is able to perform a large amount of coordinated activity, while the weaker type of nervous system is quite incapable of adaptation to the ordinary conditions of life. The first type also goes through periods of weakness, and this can easily be understood, since for the most part such individuals are continuously excited, active, profligate of nervous activity -- and the nervous exhaustion must, of course, be made good. This type may be regarded as having a longer period in the sequence of activity and rest of the nervous system as compared with the normally balanced brain, the periods of excitation and inhibition being more accentuated. Though the second type may exhibit violent attacks of excitation this does not imply greater vigour of their nervous system: the excitation is generally without aim and without result -- so to speak, crudely mechanical. In the observations made on dogs we obtained, I believe, some indication as to the origin and character of this excitation. We had one dog [experiments of Dr. Frolov] of a very inhibitable type, or, as it would be more commonly described, a very cowardly and submissive animal. This animal served for experiments upon gastric secretion, and in the course of the experiments it had to remain in the stand for many hours in succession. It never went to sleep while in the stand: though remaining very quiet it preserved a fully alert posture, only moving slightly and sometimes carefully shifting its legs. This state of the animal was not semi-cataleptic, since it invariably responded to the call of its name. When it was taken from the stand and freed from the loops and leash, this dog invariably entered into astonishing fits of excitation, howling, throwing itself vigorously about, sometimes upsetting the stand and falling off the table. This excitation (which by the way was not caused by desire for micturition or defaecation) could not be stopped in any way, whether by shouting, petting or by striking the animal, which became absolutely unrecognizable. A few minutes of exercise in the yard restored it to its normal state, the animal leading the way of itself into the experimental room, jumping up on the stand and again standing motionless. The same behaviour was sometimes observed in other dogs, but never in so exaggerated a form. These wild attacks of excitation may possibly [p. 399] be regarded as a brief outburst of positive induction following a prolonged and intense inhibition. A similar explanation may also be suggested for the fits of excitation in neurosis of the second type in which the inhibitory tendency prevails. The possible participation of another cause also is suggested by experiments [by Dr. Podkopaev] on another dog. This dog was a quiet animal with a well-balanced nervous system, not very alert, which did not jump into the stand of itself, but when placed in the stand stood quietly and never slept. The positive and negative conditioned reflexes were very constant and precise. The dog had several conditioned reflexes established to stimulation of places along one side of the body, a stimulus on a definite place on the hind leg being a positive alimentary stimulus and all the rest negative. All these reflexes had developed rapidly and were very precise. During the application of the tactile stimuli the animal had always remained quiet, not making any local or general movements; even the positive motor alimentary reaction was very weak, and the dog was slow in taking the food. The development of the negative reflexes had been begun at the front paw -- the most remote from the positive place. Suddenly and quite unexpectedly the stimulation of the front paw began to be accompanied by a motor reaction in the form of rapid twitching of the stimulated extremity. Sometimes the twitching assumed the rhythm of the tactile stimulus. Such local motor reactions began now to appear on the successive stimulation of other inhibitory places in closer and closer proximity to the place of positive significance, the reaction at the same time becoming more vigorous, more extensive and involving all extremities. The head and neck, however, remained motionless, not participating in the activity of the extremities. Salivary secretion was of course absent. When, however, the place on the thigh nearest to the positive one was now also made positive the motor reaction to the stimulation of this place vanished entirely. The same happened also to the motor reaction for other places when they were transformed from negative into positive ones -- with the exception only of the two most remote places which, though acquiring the positive secretory effect, continued to evoke the local motor reaction in a much weakened form. The fact that this phenomenon made its appearance not during the establishment, but only after the complete development, of the differentiation -- this and its localized form make it probable that the disturbance was of spinal origin, occurring on account of a partial functional disconnection of [p. 400] the cortical cutaneous analyser from the lower centres. A similar explanation may be advanced for analogous cases in man.

We have a number of further observations which recall some more or less well-known forms of nervous disturbance in man. I shall remind you of the dog [experiments of Dr. Rickman (p. 302)] which was brought into a state in which it could not withstand any strong conditioned stimuli -- immediately entering into an inhibitory state so that a conditioned activity could be elicited only by the use of very weak stimuli. It is permissible to draw a parallel, of course only as regards the mechanism, between the case of this dog and the cases of many years of sleep in human patients -- for example, of a young girl described by Pierre Janet and of an adult man as observed in one of the Petrograd hospitals for nervous disorders. The patients in both cases were lying in a continuous sleep, entirely motionless, did not speak a word and had to be fed artificially and kept clean. Only during the stillness of the night, when the daily bustle of life with its strong and varied stimuli quieted down, had the patients a chance of exhibiting some activity. The patient of Pierre Janet was observed to eat and even write during the night. It was reported of the Petrograd case that sometimes during the night he got out of bed. When this patient, at the age of 60, after nearly twenty years of continuous sleep, began to improve and could speak, he recounted that he often heard and saw everything occurring around him, but had no strength either to move or to speak. Both these cases obviously presented an extreme weakening of the nervous system -- especially of the cortex -- which quickly led under the influence of any strong stimuli to a development of complete inhibition, i.e. sleep.

In the same dog we observed also another symptom of pathological nervous activity which has often been described, in the neuro-pathological literature, for man. This dog had a narrowly localized chronic functional lesion of the cortical part of the acoustic analyser, any stimulation of the deranged part of the analyser by an appropriate agent leading to inhibition of the entire cortex. We are aware of many states of the nervous system in man in which a perfectly normal activity can be maintained only so long as the man is not affected by any, sometimes almost a negligible, component -- even the remotest hint -f those strong stimuli which originally evoked the nervous disturbance.

Finally, I want to remind you of the case, described in the nineteenth lecture, of periodical visual illusion in one of our dogs (p. 327). [p. 401] This was probably due to distortion of the effect upon the cortex of the external visual stimuli by local, internal stimuli originating in the extending scar. Many similar cases of illusions in man are probably due to the interference of similar cortical stimuli of internal local origin.

Though our research abounds in cases of pathological disturbances which are comparable to those observed in man, I do not feel either safe or justified in proceeding in my comparison beyond the above observations, and these should not be taken as in any sense explaining the incalculably complex symptoms observed in man, but only as showing that a comparison of a general nature can even now be made. Similar comparisons between experimental animals and man can be made also in respect to therapeutic measures -- general and pharmacological. It has been stated already that rest and interruption of experiments in many cases helped in the restoration of normal conditions. Several interesting details must, however, be described. One of our dogs was brought into an extremely excitable state by a clash of the inhibitory with the excitatory process [experiments of Dr. Petrova]. All forms of inhibition were disturbed, all negative conditioned stimuli acquiring positive properties. On application of any of the conditioned stimuli -- those formerly positive as well as those formerly negative -- the animal entered into a state of pronounced excitation which, as generally happens, was accompanied by severe hyperpnoea. The disuse of negative conditioned reflexes did not improve the condition of the animal. Hyperpnoea continued and the positive reflexes remained excessive. It was then resolved to use only those of the positive stimuli which were physiologically weak, i.e. the visual and tactile, and to discard the auditory, which as a rule in our experiments were strong. The beneficial result of this treatment was immediate. The animal became quiet, hyperpnoea disappeared and the magnitude of the salivary effect returned to normal. After some time it became possible gradually to introduce again the stronger positive stimuli without upsetting the result of the treatment. Furthermore, after several days a pre-established differentiation of the tactile stimuli according to place (one of the easiest forms of internal inhibition) spontaneously reappeared in full vigour, and this without any signs of excitation on the part of the animal. This is an instructive case, showing how a diminution in the strength of stimuli affecting the hemispheres led to a diminution of the excessive excitability of the [p. 402] cortical elements. Of course, in the treatment of neurotic conditions in the human subject similar therapeutic measures are very widely adopted.

I shall describe also another case which seems to me very instructive from the point of view of therapy. In this instance we are concerned with a dog which was entirely out of the ordinary run and which had an obviously abnormal reaction to cutaneous stimuli, a reaction associated with a strong excitation of the cortex [experiments of Dr. Prorokov (p. 183)]. On application of the usual tactile stimulus to the skin of the thigh the animal immediately began to wriggle its hind quarters, stamp its hind legs, throw up its head in a peculiar manner and make peculiar little noises, sometimes yawning. On administration of food and while it was being eaten the reaction disappeared. Contrary to our expectation the presence of this reaction did not in any way interfere with the development of a, conditioned reflex to the tactile stimulus, a phenomenon which usually occurs in the presence of some extraneous motor reactions in animals, e.g. retraction of the extremities or local twitching of the platysma muscle. In the case under discussion, however, a conditioned reflex developed very quickly, and, what was quite exceptional, this tactile cutaneous salivary conditioned reflex was even stronger in intensity than the reflexes to the most powerful auditory stimuli. Similarly, the motor alimentary reaction -- which usually replaced the peculiar special reaction somewhere towards the middle of the isolated action of the cutaneous conditioned stimulus -- was considerably stronger than the motor reaction observed with any other conditioned stimulus. Furthermore, the usual period of "alimentary" excitation observed as an after-effect following reinforcement with food was the most intense and the most prolonged in the case of the tactile cutaneous stimulus. In the experiments in which the tactile stimulus was used the dog showed signs of a general excitation: at the slightest sound from the experimenter's room the animal immediately responded by the peculiar motor reaction. Obviously the tactile cutaneous stimulation in this dog brought about a vigorous and widely irradiated excitation in the cortex. The nature of this excitation remained, however, unknown. It did not seem to be associated with any sexual reflexes since it was not accompanied by erection of the penis. It seemed to be something like the common reaction to tickling. At any rate it was a sufficiently interesting nervous phenomenon to study, and we determined to overcome it. For [p. 403] this purpose we began to develop internal inhibition in the form of differentiation of tactile stimuli according to their place of application. On account of initial generalization the application of the tactile stimulus to the shoulder gave some conditioned secretion, and this also was accompanied by the special motor reaction. On repetition of the stimulus without reinforcement the motor and the salivary components of the conditioned reflex disappeared (8 repetitions), and this was followed by the disappearance of the special motor reaction (40 repetitions). The stimulation of the place on the thigh continued, however, to evoke the special motor, and the alimentary motor, reaction in succession. A differentiation was now was developed to stimulation of a place on the side of the animal nearer to the thigh. The different stages repeated themselves in the same way as for stimulation on the shoulder, but again the special reaction to stimulation of the thigh did not diminish. Finally a differentiation was developed to a tactile stimulation on the hind paw, and now the special motor reaction in response to the stimulation on the thigh first began to weaken, and then disappeared altogether. At the same time the strength of the salivary reflex to the tactile stimulus took up its usual position in the series of conditioned reflexes as regards the relative strengths of the stimuli producing them, falling from its predominant place to a position below the conditioned reflexes to auditory stimuli.

In this manner we see that the development of several Inhibitory areas in the cortical part of the cutaneous analyser abolished the special cutaneous reflex, at the same time preserving, and even rendering normal, the alimentary cutaneous conditioned reflex.

This example and other observations suggest that a gradual development of internal inhibition in the cortex should be used for re-establishment of the balance of normal conditions in cases of an unbalanced nervous system. The method is being used at present on the dog, described in the eighteenth lecture, which had a narrowly localized functional injury of the acoustic analyser. Since this region was specially related to the beating of a metronome we resolved to develop a differential inhibition of other auditory stimuli related to normal areas of the acoustic analyser. We hope that irradiation of the inhibition to the defective metronome-point will have a beneficial effect, restoring this point to its normal excitabiiity and normal activity. I do not know whether similar therapeutic measures [p. 404] (not counting of course general sedatives such as hot baths) are applied in human neurotherapy.

We shall now attempt a discussion of borderline states of the nervous system in our dogs, states merging from a normal into a pathological character which, on the analogy of corresponding human states, should in some instances be described as psycho-pathological. These are different hypnotic phases, such as the transition phases between the alert state and sleep, and the passive defence reflex. We have seen in the sixteenth lecture that the transition of animals from the alert state into sleep is based upon the development in the brain of an inhibitory process which, under the influence of definite stimuli, is initiated in the cortex and reaches different stages of intensity and extensity during the different phases of the developing sleep. Undoubtedly, even at present, the observations made upon animals allow in part of a physiological interpretation of the fundamental aspects of hypnotism in the human subject.

We shall consider first the conditions under which hypnotic states develop. In animals, as we already know, they appear, as the result of monotonous stimuli of a small and medium intensity repeated for a long time (the most common case in our experiments), more or less gradually, while in the case of stimuli of a considerably greater intensity they appear quickly -- a form of animal hypnotism which has been known for a very long time. The stimuli directly initiating these states, both weak and strong, can also be signalled by any other stimuli which have conditioned properties in respect to the first. In this connection the special mode of formation of conditioned reflexes described in the sixth lecture will be remembered where the neutral stimulus repeated several times in conjunction with the pre-established inhibitory stimulus acquired inhibitory properties of its own [experiments by Dr. Volborth (p. 106)]. The method of inducing hypnosis in man involves conditions entirely analogous to those which produced it in our dogs. The classical method consisted in the performance of so-called "passes " -- weak, monotonously repeated tactile and visual stimuli, just as in our experiments upon animals. At present the more usual method consists in the repetition of some form of words, describing sleep, articulated in a flat and monotonous tone of voice. Such words are, of course, conditioned stimuli which have become associated with the state of sleep. In this manner any stimulus which has coincided several times with the development of sleep can now by itself initiate sleep or a [p. 405] hypnotic state. The mechanism is analogous to the inhibitory chain reflexes, which are similar to the positive conditioned chain reflexes, i.e. reflexes of different orders which have been described in the third lecture [experiments of Dr. Volborth]. Finally, hypnosis in the case of hysteria (in the sense of Charcot) can be obtained by the application of strong and unexpected stimuli, as in the old method of initiating hypnosis in animals. It is obvious that in this respect physiologically weak stimuli may act in the same manner if, on account of a coincidence in time, they have acquired signalling properties in respect to the strong ones. Most of the procedures producing hypnosis become more and more effective the more frequently they are repeated.

One of the first expressions of hypnosis in man is the loss of so-called voluntary movements and the development of a cataleptic state, i.e. maintenance by different parts of the body of the position given to them by external forces. This may best be regarded as an isolated inhibition of the motor analyser which has not descended to the sub-cortical motor centres. Other areas of the cortex may continue to function quite normally. A man in a state of hypnosis may understand what we tell him, may realize what kind of unnatural posture we have given him and may attempt to change it, but is incapable of doing so. The outward signs of the hypnotic state are similar in men and animals. It has already been described in the sixteenth lecture how some animals retain their alert posture while all conditioned reflex activity disappears -- obviously a case of inhibition of the entire cortex without descent of the inhibition into sub-cortical regions. Other dogs continue to react to all the conditioned stimuli by the secretory component of the reflex only, exhibiting no motor reaction and not touching the food -- obviously a case of an isolated inhibition of the motor analyser. Finally, in animals hypnotized by the old method it could often be observed that the body and extremities remained motionless while the animal yet continued to follow everything with its eyes, and even accepted the food. This is obviously a case of a still more localized inhibition within the motor analyser. The local " tonic" (i.e. spinal flexor and extensor) reflexes, which are observed in man and animals in some cases, become understandable only if we postulate complete inhibition of the motor analyser of the cortex.

When we come to deal with more complicated forms of the hypnotic state, it obviously becomes, for several reasons, difficult, or [p. 406] even impossible, to draw a parallel between man and animals. As already stated, we know only a few of the phases of the hypnotic state, especially as regards their relative intensity, and we have no definite idea as to the sequence of their development. We are not familiar with the manner in which these phases manifest themselves under natural conditions of life in animals, since the transition phases were observed not in the natural individual and social sphere of life, but only in the restricted sphere of a laboratory environment. In man, however, we become familiar with these phenomena under more normal conditions of life and we can evoke and investigate them with the help of the most valuable signalling medium -- speech. Of course, on account of the extraordinary complexity of the behaviour of man as compared with the higher animals, the latter may not exhibit some of the phases of the hypnotic state seen in man at all. It is, therefore, only those crude and elementary results obtained in the animal which can be used for an attempt at a physiological interpretation of the different hypnotic phases in man. Let us consider the automatism of the hypnotized subject who repeats in a stereotyped fashion the movements of the hypnotist, being able to perform even difficult movements such as balancing along a difficult path. Obviously we deal with a certain degree of inhibition of some parts of the cortex -- a state in which the more complicated forms of normal activity are excluded and replaced by responsiveness to immediate stimuli. This partial inhibition allows of or even favours the establishment and reinforcement of the physiological connections between certain stimuli and certain activities, e.g. movements. In this manner, in hypnosis all activities based on "imitation" are accentuated and we see revealed the long-submerged reflex which in all of us in childhood forms and develops the complicated individual and social behaviour. Similarly, some change in the environment, which in former days had repeatedly evoked certain movements affecting certain analysers, in hypnosis calls forth without fail and in a stereotyped manner the old response. It is a common occurrence that, being predominantly preoccupied with some one activity, we can simultaneously perform some other activity which has long been practised, i.e. those parts of the cortex involved in this older response, although in a state of partial inhibition through negative induction, still continue to function in a normal manner. That this interpretation is somewhere near the mark I become more and more convinced, through observing the [p. 407] diminution in the reactivity of my own brain through my advancing age(my diminished memory of recent events). Moreover, with time I progressively lose the faculty, when busy with one activity, of performing correctly another also. Apparently the concentrated excitation of some definite point induces, on account of diminished excitability of the cortex, such a strong inhibition of the rest of the cortex that even the conditioned stimuli of the old firmly established reflexes are now below the threshold for excitation. The above described phase of hypnosis in the human subject may perhaps be compared with what I have termed the narcotic phase of transition in dogs, when strong and old reflexes persist while the more recent ones disappear.

Among the various aspects of the hypnotic state in man attention may be drawn to "suggestion" so-called and its physiological interpretation. Obviously for man speech provides conditioned stimuli which are just as real as any other stimuli. At the same time speech provides stimuli which exceed in richness and many-sidedness any of the others, allowing comparison neither qualitatively nor quantitatively with any conditioned stimuli which are possible in animals. Speech, on account of the whole preceding life of the adult, is connected up with all the internal and external stimuli which can reach the cortex, signalling all of them and replacing all of them, and therefore it can call forth all those reactions of the organism which are normally determined by the actual stimuli themselves. We can, therefore, regard "suggestion" as the most simple form of a typical conditioned reflex in man. The command of the hypnotist, in correspondence with the general law, concentrates the excitation in the cortex of the subject (which is in a condition of partial inhibition) in some definite narrow region, at the same time intensifying (by negative induction) the inhibition in the rest of the cortex and so abolishing all competing effects of contemporary stimuli and of traces left by previously received ones. This accounts for the large and practically insurmountable influence of suggestion as a stimulus during hypnosis as well as shortly after it. The command retains its effect after the termination of hypnosis, remaining independent of other stimuli, being impermeable to them, since at the time of primary introduction of the stimulus into the cortex it was prevented from establishing any connection with the rest of the cortex. The great number of stimuli which speech can replace explains the fact that we can suggest to a hypnotized subject so many different [p. 408] activities, and influence and direct the activities of his brain. It could be questioned why does suggestion carry in itself such a commanding influence as compared with dreams, which are usually forgotten and only have a very small vital significance? But dreams are due to traces, generally of very old stimuli, while suggestion is a powerful and immediate stimulus. Moreover, hypnosis depends upon a smaller intensity of inhibition than sleep. Suggestion, therefore, is doubly effective. Still further, suggestion as a stimulus is brief, isolated and complete, and therefore vigorous, while dreams are generally linked up into chains of various, sometimes inconsistent or antagonistic, traces of stimuli. The fact that it is possible to suggest to a hypnotized subject almost anything, however little it may correspond to the physical reality, and to evoke a reaction in opposition to the actual reality -- for example, the reaction appropriate to a bitter taste when the reality is a sweet taste -- this fact, I believe, can be compared with the fact observed in the paradoxical phase of transition in the dog, that weak stimuli have a greater effect than strong ones. The real stimulus from the sweet substance going directly to the corresponding cortical cells should be expected to be much stronger than the substituted verbal stimulus of "bitter," which goes through auditory cells to the chemical analyser of taste for bitter -- just as a conditioned stimulus of the first order is always stronger than one of the second order. The significance of the paradoxical phase is not limited to pathological states such as those previously observed, and it is highly probable that it plays an important part in normal men too, who often are apt to be much more influenced by words than by the actual facts of the surrounding reality. I hope to be able to produce a phenomenon in animals analogous to "suggestion" in man during hypnosis.

The fact that certain phases of the hypnotic state in man remain more or less stationary repeats itself in dogs. Similarly, under certain conditions and in dependence on the individual condition of the nervous system the hypnotic state in man, as in animals, passes more or less quickly into complete sleep.

The passive defence reflex stands in a definite connection with the hypnotic state. As I suggested previously (p. 312) the old form of hypnosis in animals may be regarded with reason as a reaction of passive self-preservation, occurring when the animal meets with some very powerful or extraordinary external stimulus, and consisting in a more or less profound immobilization of the animal by means [p. 409] of an inhibition, beginning in the cortical cells representative of all the skeletal muscles (motor analyser). This reflex was often observed in our experimental animals, of course in different degrees of intensity and in somewhat different forms, always, however, retaining its fundamental inhibitory character. The variations consisted in a smaller or greater diminution of the movements, in a smaller or greater weakening, or in the disappearance, of conditioned reflexes. The passive defence reflex was usually evoked by relatively unfamiliar and powerful external stimuli. The relative strength of a stimulus will of course depend on the state of the given nervous system, on its inherent properties, the state of health or disease, and on different periods of life. Animals which have been employed many times in front of a large audience remain quite normal under such conditions, while those which are exposed for the first time enter into a state of powerful inhibition. The exceptional dog described before (p. 402) behaved towards even the slightest changes in the environment as if to strong stimuli, and its activities became deeply inhibited. Some of the dogs which passed through the extraordinary stimulus of the flood entered into a chronic pathological state and now became inhibited under the influence of strong conditioned stimuli, which previously had produced a specific conditioned effect. Finally, some other dogs become as easily inhibited but only in some definite phases of hypnosis. The following is a particularly striking case. A dog which under the conditions of our experiments remains fully alert accepts the food following the conditioned stimuli quickly and with avidity. By repeated application of weak conditioned stimuli a certain stage of hypnosis is induced in the animal, which soon becomes practically motionless. When a strong conditioned stimulus is now applied the animal first turns in the direction from which the food is administered, then turns away without touching it. To a casual observer the animal looks frightened. A weak conditioned stimulus is now applied and the animal immediately approaches the plate and quietly takes the food. On dispersing the hypnotic state all the conditioned stimuli again give their normal effect. Obviously in the special phase of hypnosis of the animal the old and usual stimuli now produce the effect of very strong ones, evoking an inhibitory reflex. In a similar manner, in our exceedingly inhibitable dog "Brains" as soon as the pitch of the excitability of the cortex was raised by the special method described on page 381, there was observed a considerable [p. 410] weakening of the otherwise almost continuous reflex of passive self-preservation.

In all the cases which have just been described what is most striking is the extremely characteristic passive self-protective postures of the animal. When I recall a large number of experiments performed one after another and year after year it is hardly possible not to conclude that at least in most cases what is known in psychology under the names of "fear," "cowardice" or "caution" has a physiological substrate in a state of inhibition of the nervous system, which varies in intensity and so produces different intensities of the reflex of passive self-protection. Developing these conceptions further we are bound to regard the obsession of fear, and different phobias as natural symptoms of inhibition in a pathological and weakened nervous system. There are, of course, certain forms of fear and cowardice, as for instance flight and panic, and certain postures of servility, which apparently do not conform with the idea of an underlying inhibitory process, having a much more active aspect. These types must, of course, be subjected to experimental analysis, but it is perhaps not impermissible to regard them provisionally as developing in co-operation with, and as a result of, inhibition of the cortex. We have even now a, few observations which conform with this point of view.

I would like to turn briefly to the experiments described at the end of the preceding lecture. If on repeating them in different variations the preliminary results should find full confirmation, these results will throw some light upon one of the darkest points of our subjective self -- namely, upon the relations between the conscious and the unconscious. The experiments if confirmed will have demonstrated that such an important cortical function as synthesis ("association") may take place even in those cortical areas which are in a state of inhibition on account of the existence at that moment of a predominant focus of strong excitation. Though the actual synthesizing activity may not enter our field of consciousness the synthesis may nevertheless take place, and under favourable conditions it may enter the held of consciousness as a link already formed, seeming to originate spontaneously.

In concluding this series of lectures I want to repeat that all the experiments, those of other workers as well as our own, which have set as their object a purely physiological interpretation of the activity of the higher nervous system, I regard as being in the nature only of a preliminary inquiry, which has however, I fully believe, entirely justified its inception. We have indisputably the right to claim that our investigation of this extraordinarily complex field has followed the right direction, and that, although not a near, nevertheless a complete, success awaits it. So far as we ourselves are concerned we can only say that at present we are confronted with many more problems than ever before. At first, not to lose sight of the main issue, we were compelled to simplify, and, so to speak, schematize the subject. At present, after having acquired some knowledge of its general principles, we feel surrounded, may crushed, by the mass of details, all calling for elucidation.