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Christopher D. Green
York University, Toronto, Ontario
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By William H.R. Rivers (1920)
Posted March 2000
SUPPRESSION AND INHIBITION
The examples of the unconscious and of its instrument, suppression, which have been given in the last two chapters, have been taken from aspects of experience which belong clearly to the domain of psychology, and involve mental processes of a relatively high order. I propose now to consider the relation between the suppression of psychological experience and certain physiological processes. I will begin with an example drawn from the borderland between the psychological and the physiological, one dealing with the sensory concomitants of nervous process in a case of experimental interference with the integrity of the nervous system.
Observations on the sensory changes which accompany the regeneration of a divided and reunited nerve have led Head and his colleagues to distinguish two different kinds of mechanism on the afferent side of the nervous system. Prolonged observations after the division of nerves in Head's own arm brought out clearly the existence of two definite stages in the return of sensibility. In one of these, the protopathic stage, the sensations are vague and crude in character, with absence of any exactness in discrimination or localisation and with a pronounced feeling-tone, usually on the unpleasant side, tending to lead immediately, as if reflexly, to such movements as would withdraw the stimulated part from contact with any object to which the sensory changes are due. At this stage of the healing of the reunited nerve there are present none of those characters of sensation by which we recognise the nature of an object in contact with the body. The sensations are such as would enable one to know [p. 23] that something is there and that it is pleasant or unpleasant. It is also possible to distinguish between mere contact or pressure and stimulation by heat or cold, but within each of these modes of sensation there is no power of distinguishing differences in intensity nor of telling with any exactness the spot where the processes underlying the sensory changes are in action.
The second stage of the process of regeneration is characterised by the return of those features of normal cutaneous sensibility, such as exact discrimination and localisation, by means of which it becomes possible to perceive the nature of an object in contact with the skin and adjust behaviour according to this perception. The modes of reaction which make this exactness of discrimination and power of external projection possible are grouped together under the heading of epicritic sensibility. In interpreting these observations two chief possibilities are open. Epicritic sensibility may be only a greater perfection of protopathic sensibility, experience gradually enabling an exactness of discrimination and localisation which were not at first possible. The other alternative is that the two kinds of sensibility represent two distinct stages in the development of the afferent nervous system. According to this second view the special conditions of the experiment revealed in the individual two widely different stages in the evolution of cutaneous sensibility. Many features of the experiment point strongly to the truth of the second of these alternatives. The way in which epicritic sensibility returns and the fact that it is possible to annul it by treatment affecting only the peripheral factors, without influence on such central processes as would be set up by experience, go far to show that the two modes of sensibility represent two stages in phylogenetic development.
All that we know of the protopathic stage is consistent with its being the representative of the sensibility of an animal which possesses only the power of becoming aware of changes of a crude kind and, according as these changes are pleasant or unpleasant, of reacting at once by such mass-movements as would take it nearer to, or remove it from, the source of the stimulation. If [p. 24] there is only capacity for such mass-movements, there will be no necessity for the discrimination which would enable the exact perception of the nature of the object. There would be a definite correlation between the crude nature of the sensibility and the limited capacity for behaviour possessed by the animal.
Epicritic sensibility, on the other hand, is adapted to behaviour of a far more complex and delicate kind. The sensations do not merely tell the animal or man that something is there and set up the crude mass-movements of approach or withdrawal, but they enable the many forms of reaction which become possible when the exact nature of the stimulating object is recognised.
It is not necessary for my present purpose to consider these two possibilities fully. Whatever be the interpretation of the two stages, there is no doubt as to their existence, and my present object is to point out certain special features of the relation between the two. When epicritic sensibility returns, the earlier protopathic sensibility does not persist unaltered side by side with the later development, but undergoes certain definite modifications. Some of its elements persist and combine with elements of the epicritic stage to form features of normal cutaneous sensibility. Thus, the cold and heat of the protopathic stage blend with the modes of temperature sensibility proper to the epicritic stage, and form the graded series of temperature sensations which we are normally able to discriminate. The crude touch of the protopathic system blends with the more delicate epicritic sensibility of this kind, while protopathic pain, with its peculiarly uncomfortable rather than acute quality, forms a much larger element in the normal sensibility to pain. In this process of blending or fusion certain aspects of the earlier forms of sensibility are modified to a greater or less extent, and in some cases this modification involves the disappearance of certain characters. This disappearance is especially striking and complete in the case of the spatial attributes of protopathic sensibility. In the protopathic stage (when the sensibility of deeper structures is excluded) there is no power of exact localisation. When a point of the skin is stimulated the sensation radiates widely and is often localised at a [p. 25] considerable distance from the actual place of stimulation. These two characters of radiation and distant reference disappear with the return of epicritic sensibility, and afford examples of a process of suppression comparable with that considered in the last chapter. Moreover, these spatial features of protopathic sensibility do not disappear entirely, but persist in a latent form ready to come again into consciousness if the appropriate conditions are present. This was well illustrated by certain experiments made shortly after the return of epicritic sensibility. At this time, when radiation and reference were completely absent at the ordinary temperature of the room, they could be again brought into existence by cooling the limb. Characters of sensibility which a few minutes before had so disappeared from consciousness so they could not be elicited by any kind of stimulus, were again brought to consciousness when the controlling epicritic system, but lately returned and not having yet attained its normal stability, was put out of action by the application of cold. When the limb was warmed the radiation and reference disappeared, but were again made manifest when the limb was once more cooled. In this state of enhanced vulnerability of the lately recovered epicritic sensibility, it was possible to produce suppression experimentally. Moreover, if the whole process of regeneration is interpreted as the manifestation of an early stage of the development of the nervous system, it will follow that the potentiality for the radiation and reference of localisation had been present throughout the life of the subject, but in so complete a state of suppression that we only became aware of their existence through a special experimental procedure. The experiment revealed a feature of primitive sensibility which had been so successfully suppressed that its existence had not been suspected until the beginning of the twentieth century, though radiation and reference in other parts of the body, where they have not been so completely suppressed, might have led students to its recognition.
Another interesting and rather more complex example of suppression became evident in Head's experiment. On the normal skin stimulation by a temperature of 40°C.-44°C. produces a pleasant sensation of heat free from any element of' pain, and this effect was present on the dorsum of Head's thumb after epicritic sensibility had returned. The index knuckle lingered in its recovery behind the thumb, so that at one stage of the experiment when epicritic sensibility was present on the thumb, it was still absent on the index knuckle. At this time stimulation of the knuckle by cold produced a referred sensation of cold on the dorsum of the thumb. When the two regions were stimulated simultaneously, the thumb by a stimulus of 40°C.-44°C. and the index knuckle by cold, the two temperature sensations on the thumb neutralised one another and a third mode of sensation, one of pain, appeared. The observation is most naturally interpreted by the supposition that when a temperature sensation is present, any painful element is suppressed. Though the pain was localised on the thumb, it may have belonged either to the heat sensation due to direct stimulation of this region or to the referred cold, and observations on another part of the body point to the former alternative. When a part of the glans penis which is free from heat-spots is stimulated by a temperature of 40°C.-44°C. there is no sensation of heat, but only one of pain. This part of the body is normally devoid of epicritic sensibility, and the occurrence of pain on the glans, as the result of stimulation by heat, points to the pain on the thumb having been the result of the direct stimulation of that region by heat rather than an element of the referred cold. In either case the special conditions of the experiment allowed the emergence of a mode of sensation which in the normal state is kept in a state of suppression, though it is manifest on the glans penis in the absence of heat-spots and of epicritic sensibility.
The special interest of the case is that there is no such suppression for temperatures of 45°C. and upwards, where pain is the normal result of stimulation. It is only in the case of [p. 27] temperatures less than 45°C., where the presence of pain would conflict with the pleasurable character of the heat sensation, that suppression has taken place.
These examples of suppression have been taken from the physiology of thc nervous system. Though they became manifest through the changes in consciousness we call sensations, they are nevertheless the expression of purely physiological processes in the peripheral nervous system.
There is reason to believe that the two forms of cutaneous sensibility, which I have described, represent two different stages in the evolution of the nervous system with their associated varieties of consciousness. The facts seem best to fit with the hypothesis that the manifestations of protopathic sensibility which are suppressed belong to a crude form of nervous system which has been superseded by a later and more efficient mechanism. If now we pass to the central end of the nervous path by which the impulses subserving cutaneous sensibility reach the brain, Head working in conjunction with Holmes has discovered a relation between thc cerebral cortex and the optic thalamus very similar to that existing between protopathic and epicritic sensibility. In this case the special modes of activity they have studied are associated with structures which belong to widely separated stages of the development of the nervous system. The optic thalamus represents the dominant part of the brain of lower vertebrates, while the cerebral cortex or neo-pallium developed far later. When by injury, disease, or operative procedure, the cortex cerebri has been put out of action, stimulation of the skin produces sensations characterised by a peculiar quality such as would be produced by over-weight of the affective aspect of sensation, very similar to that shown by protopathic sensibility. Moreover, there is an absence of objective character very similar to that of this form of sensibility. When the cortex is in action the affective over-response of the thalamus is largely suppressed under ordinary conditions, but the process of suppression does not come out so strongly as in the case of the peripheral nervous system because some of the [p. 28] primitive features which most need suppression have already suffered this fate. Thus, removal of cortical activity does not produce radiation and reference of localisation because the suppression of these characters is still being maintained at the periphery.
Similar examples of suppression have been observed in the reflexes. In reflex action a movement takes place in response to stimulation which depends on a highly-organised and strictly-determined physiological mechanism. The whole process is immediate and incapable of modification. With a given stimulus and an intact nervous system, the effect follows the cause with a simplicity and definiteness far more obvious than in the case of activity which is accompanied by consciousness. In the normal state most of the reflexes, at any rate those in which the limbs and exterior of the body are concerned, are of a kind which, were they accompanied by consciousness, would imply accuracy of localisation and other forms of discrimination.
Experiments on animals, in which there has been interference with the integrity of the nervous system, have shown the exaggeration of certain forms of reflex action, pointing to the existence of some degree of suppression, but it has been reserved for injury of the nervous system in Man to show this process in its most characteristic forms.
Head and Riddoch have observed a number of patients in whom the spinal cord has been completely divided, and in these cases have been able to study the functions of the lower end of the spinal cord when isolated from the rest of the nervous system. In such cases they find a peculiar form of reflex with characters unknown when the nervous system is intact. The reflex shows itself in movements, chiefly of flexion, involving mainly the stimulated side of the body, but far more widespread than is the case with the reflexes of health. The reflex can be produced by stimulating almost any part of the limbs or trunk below the site of the injury. The nature and extent of the movements does not vary with the locality of the stimulus as in [p. 29] normal reflexes, but is of much the same nature whatever the stimulated part. Moreover, the movements of the limbs and trunk muscles are accompanied by sweating and contraction of the bladder. This form of reflex has been called by Head and Riddoch the "mass-reflex." They note that such a mass-reflex would form an excellent answer to noxious stimuli in the lower animals. Owing to the necessary conditions of their observations the movements are limited to part of the body, but similar movements of the whole body would tend to remove an animal from noxious stimulation. They point out that this kind of reflex would be useless for the purpose of discrimination. The "mass-reflex " has a generalised character and shows an absence of discrimination and localisation which reminds us at once of the characters of protopathic sensibility. The special feature of interest from our present point of view is that this diffused and generalised reflex is wholly suppressed in the normal human being, the suppression having taken place in favour of reflexes delicately regulated according to the locality and, to some extent, according to the nature of the stimulus. Here, as in the case of protopathic sensibility, the suppression has been so complete that the presence of the mass-reflex is only revealed by disease or injury. It has been so successful that it needed the vast scale on which injuries of the central nervous system have been produced during the war to enable Head and Riddoch to discover the presence in Man of these old and long-suppressed processes.
In this case, and in the case of protopathic sensibility, we are not dealing with the suppression of individual experience, but with the suppression in the race of experience belonging to the earlier phases of its history. Through a special experimental procedure, or through the accidents of war, it has been possible to follow the suppression of this experience in the individual. The fact that this is possible suggests that the racial suppression is repeated in every individual as part of the recapitulation of the racial history. If this be so, however, the suppression takes place at so early an age that its detection is impossible. It would never have been suspected if the experiment and the [p. 30] clinical observations of Head had not pointed to the way thereto.
The special importance of suppression on the reflex and sensori-motor levels is that it reveals clearly the biological significance of the process. The exact localisation of fully developed cutaneous sensibility would be impossible if the early radiation and distant reference of the protopathic stage persisted. These features would furnish elements of vagueness and confusion wholly incompatible with the exact power of localisation which developed later and enabled the animal to modify its behaviour according to the nature of the external object by which the sensations were being produced. It is essential that reactions founded on the exact discrimination and localisation rendered possible by the epicritic system shall be prompt and definite. This would not be possible if the properties of the new order of sensibility were continually being complicated by sensations characterised by the old vagueness and the old inexactness of spatial reference.
Similarly, the uncomfortable feeling-tone of protopathic sensibility and the strongly affective aspect of the reactions of the thalamus need suppression when it is necessary to discriminate with exactness and to adjust behaviour to the more complicated conditions of life, made possible by the development of epicritic and cortical activity. In these cases, however, the suppression is less complete and only occurs when the affective accompaniments would interfere with the perfect adjustment of behaviour to the needs of the situation which the animal has to meet. The affective reactions lie ready to spring into activity whenever the situation calls for an emotional rather than an intellectual response.
In the case of the reflexes the need for suppression is imperative. The essence of reflex action is its immediacy and perfection based on the thorough organisation of the physiological mechanisms. The perfection of one of the higher reflexes would be hopelessly prejudiced if there were even a trace of the activity of the older mass-reflex with its diffuse character and implication of visceral processes. The movements of the [p. 31] localised reflex could not perform their task properly if at the same time they were involved in mass-movements of a wholly different kind. Suppression is here even more essential than in the case of conscious activity.
The argument of this chapter has been directed to show that the process of suppression by which elements of conscious experience pass into the "unconscious" is of the same order as the suppression which takes place on the sensori-motor and reflex levels. A number of processes have been found which form intermediate links connecting the suppression of highly complicated mental process at one end of the series with the suppression necessary for the perfection of reflex action at the other end of the series. In all cases we have to do with the means by which behaviour, whether of human being or animal, is adjusted to the needs with which man or animal is confronted. The suppression of conscious experience is only one example of a process which applies throughout the whole of the animal kingdom and is essential to the proper regulation of every form of human or animal activity. This suppression is only an example of processes even more fundamental in the animal economy. Every living process of the animal involves, not only activity devoted to the special end the animal has to meet, but also the inhibition of tendencies to activity of other kinds. The suppression which I have been considering in the last two chapters is only one aspect of the universal physiological property of inhibition. It is now recognised that the activity of every functional unit of the nervous system is of two kinds. Every unit forms part of a hierarchy in which it controls lower, and is itself controlled by higher, elements of the hierarchy. Control or inhibition belongs to the essence of nervous activity, and the lesson suggested by the study of sensation and reflex action is that the suppression by which experience becomes unconscious is only a special variety of the process of inhibition, common to every phase of animal activity.
There is one aspect of the psychological processes I have been considering which I should like especially to emphasise. If I am right in my interpretation of the facts revealed by the [p. 32] observation of cutaneous sensibility during the regeneration of a divided and re-united nerve, the earlier and cruder kind of sensibility undergoes two different kinds of fate. Such elements as are serviceable are utilised when the later and higher forms of sensibility come into existence. These useful elements of protopathic sensibility become fused with epicritic elements to form the fully developed sensibility which is possessed by the normal skin. Utilisation by means of the process of fusion is the fate of the greater part of the complex body of processes which make up protopathic sensibility. It is only the smaller part which undergoes the other fate of suppression. It is only those features of early sensibility which are incompatible with later developments which are suppressed. Thus, the wide diffusion and distant reference of protopathic spatial sensibility are suppressed because, if they persisted, they would prejudice in the most serious manner the exact localisation and spatial discrimination of fully developed cutaneous sensibility. Again, the suppression of the painful element when the normal skin is stimulated with an object at 40° C.-44°C. takes place because this painful quality would interfere with the pleasant character of the normal heat sensation and with the discrimination which is normally possible at that temperature. In the case of cutaneous sensibility there has taken place a differentiation of treatment according as the earlier material could or could not be utilised in the interests of the higher purpose offered by the possibility of discrimination and graduation. Corresponding with this distinction two kinds of elements in the unconscious might be recognised -- those which have only disappeared from consciousness in their original form, but continue to exist in the different form they have assumed through the process of fusion, and those whose disappearance has been more complete so that they do not enter into consciousness even in an altered form under normal conditions, though their continued existence is shown by their reappearance under peculiar conditions such as those which accompany the regeneration of a divided and reunited nerve.
There is little doubt that a similar twofold possibility is also [p. 33] open in the case of other kinds of early experience. There is reason to believe that al kinds of early experience undergo transformations similar to those undergone by protopathic sensibility. According to this view certain elements of early experience are utilised and form, by fusion with other elements, the products which make up the experience of any later period of life. It is only elements of experience and modes of behaviour which are incompatible with these later developments which are suppressed.
It now becomes necessary to reconsider the sense in which we shall use the term "the unconscious." It would be possible to use it to include not only the fully suppressed elements, but also those which might be regarded as unconscious because they no longer exist in their original form but in the form they have assumed through their fusion with later products of mental development. It will, I believe, be convenient to limit the use of the term "the unconscious" to the former category, to those earlier forms of mental activity and mental experience which have not been capable of utilisation by the process of fusion, but have required the more drastic measure of suppression.
 H. Head, W. H. R. Rivers, and J. Sherren, Brain, vol. xxviii. (1905), p. 99; H. Head and J. Sherren, ibid., vol. xxviii. (1905), p. 116; W. H. R. Rivers and H. Head, ibid., vol. xxxi. (1908), p. 323.
 Brain, vol. xxxi. (1908), p. 396.
 Brain, vol. xxxi. (1908), p. 396.
 H. Head, "On disturbances of sensation with especial reference to the pain of visceral disease," Brain, vol. xvi. (1893), p. 1; vol. xvii. (1894), p. 339; vol. xix. (1896), p. 153.
 Brain, vol. xxxi. (1908), p. 445.
 H. Head and Gordon Holmes, Brain, vol. xxxiv. (1911-12), p. 102.
 H. Head and G. Riddoch, Brain, vol. xl. (1918), p. 188; G. Riddoch, ibid., p. 264.