Instinct and the Unconscious. W.H.R.Rivers
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.[3] 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.[4]
Another interesting and rather more complex example of suppression became evident in Head's experiment.[5] 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[6] 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[7] 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