§ 3. Motor Sensations.—If we close our eyes so that we cannot see our own body, we are none the less distinctly and accurately aware of the position of our limbs. If we move a limb, for example the arm or the finger, we are distinctly and accurately aware of the amount and direction of the change and of the new position which it produces. Similarly, if instead of merely moving a limb we push against a wall, or lift a weight, we are aware of the kind and degree of tension produced by the resistance opposed to our efforts. If, instead of actively initiating movements ourselves, we allow the position of our limbs to be shifted in various ways by another person while we remain passive, we are still almost equally capable of appreciating position and change of position. If our muscles are contracted by the application of an electric current, the experiences which mark position and change of position as well as amount and kind of resistance, continue to be present.
How do these experiences originate? In any movement a great many changes take place in a great many tissues. In moving the arm, the skin is in various ways crumpled and pressed at every stage of the process. There are varying degrees and kinds of tension in the tendons: the joints slide over one another; the muscles pass through various stages of contraction. All these tissues appear to be supplied with sensory nerves; it is therefore possible that each and all of them contribute to determine the experiences which mark position and change of position. As a matter of fact, it is probable that all of them contribute in some degree. The skin appears to be the least important. Our discrimination of position, movement, etc., is not "notably diminished by temporary anaesthesia of the skin; if, for instance, the skin of the arm be rendered for a while anaesthetic, we do not find any marked change in our power of judging weights or resistance, or in appreciating, with the eyes shut, the position of the limb."* Joints, on the other hand, constitute a very important factor, so far at least as concerns appreciation of position and change of position. This is shown in a series of experiments carried out by Goldscheider. "This patient observer caused his fingers, arms, and legs to be passively rotated upon their various joints in a mechanical apparatus which registered both the velocity of movement impressed and the amount of angular rotation. No active muscular contraction took place. The minimal felt amounts of rotation were in all cases surprisingly small, being much less than a single angular degree in all the joints except those of the fingers."+ Anaesthesia of the skin made no difference in the result. Anaesthesia of the joints themselves greatly decreased the power of discrimination. In the perception of resistance, the tendons are probably the most important factor. They are organs especially adapted for the appreciation of strain or tension. "Let your arm hang down loosely by your side. Attach a fairly heavy weight by a string to the forefinger. The weight pulls the surfaces of the elbow and other joints apart; so that there is no pressure or friction of one surface against another. But you soon get the sensation of strain throughout the arm."+ Sensations due to the states of the muscles themselves undoubtedly seem to exist; but it is very difficult to estimate their importance, as marks of varying position, movement, and tension.
* Op. cit., p. 1436. +James, Principles of Psychology, vol. ii., pp. 192193. +Titchener, An Outline of Psychology, p. 61.
The distinction between position-sensations and movementsensations is important. The former are due to the particular form of the tension of the organs when quiescent, the latter to change in this form.
The sensations we have so far considered are peripheral in their origin. They are produced by impressions proceeding from outlying portions of the body to the nervous system. They are equally present when we move by our own volition, and when we allow our limbs to be moved by another person, or our muscles to be thrown into contraction by such artificial means as the electric current. But it has been maintained that besides these sensations due to the actual state and changes of state of muscles, joints, tendons, and skin, there is also a sensational experience, directly connected with the initiation of movement, with the discharge from the nervous centre independently of any effect produced by it on the muscles and the tissues connected with them. Thus, according to Bain, there is a direct sense of energy put forth which is independent of any results the putting forth of energy may produce. This peculiar modification of sensory consciousness has been called the sense of effort, or the innervation-sense. At the present time it is the fashion wholly to deny its existence. The denial is mainly founded on the fact that we can appreciate position, movement, and tension as well when the limbs are passively moved as when we move them by our own volition; and that, on the other hand, when in consequence of nervous diseases the sensibility of the joints, tendons, etc., is impaired or destroyed, there is a corresponding incapacity to appreciate position, movement, and tension. But this argument lacks logical cogency; for if there be an innervation-sense it cannot, from the nature of the case, inform us of the actual effects of the motor impulse. It can only tell us what we are attempting to do, not what we are actually doing or have done. Thus a patient may will to move an anaesthetic limb with his eyes shut; and he may suppose that the movement has actually taken place, although the limb has all the time been held in its original position by another person. The patient does not know whether the limb has changed its position or not; but he none the less knows that he has made an attempt to change its position. Hence the argument does not positively disprove the existence of an innervation-sense. But it must be admitted that it throws the onus probandi on those who maintain this peculiar mode of sentience. We have said that the patient would in such a case be aware that he had made an attempt; but this only shows that he is conscious of his volitions. But volition is by no means the same as innervation-sense: it is not, in fact, a sensation at all, any more than a belief is a sensation; it is a peculiar mode of conation. It is true that in order to will a movement a person must in some way be able to think of this movement. But for this, ideal representation is enough: the ideal representation may involve ideal reproduction of motor sensations proper, or it may mainly consist of a visual image. In neither case can it be regarded as a peculiar sensation immediately accompanying the motor discharge.
There is certainly a vital difference between the experience of having a limb passively moved, and that of moving it by our own initiative. But it is very far from clear that the active movement involves a peculiar sensation which is absent in the passive movement, a sensation comparable with those which arise from joints and tendons. In passing from a state of doubt to a state of belief there is a peculiar change in consciousness, but it is not a sensation. Similarly, in passing from a state of indecision to one of voluntary determination, there is also a change in consciousness; but it is in no way comparable to sensations such as those of redness or greenness, of heat or cold.
There are, however, certain facts which lend support to the assumption of an innervation-sense. The patient who attempts to move a paralysed limb not only knows that he is making the attempt, but is also aware of differences in the amount of effort which he puts forth. This may be explained, at least in part, by the fact that the motor impulse proceeds not only to the limb which is to be moved, but also to other parts of the body which preserve their sensibility, and especially to the organs of respiration. But, as Wundt insists, there are certain cases of paralysis of the muscles of the eye which are harder to deal with. If the muscle which moves the eye to the right is completely paralysed, so that an outward movement of the eye is no longer possible, the effort to move it produces an apparent movement of the object looked at. The muscle may be only partially disabled, so that it is still capable of a lateral rotation of twenty degrees and of no more. In this case, the patient, although he has moved his eye only through the angle of twenty degrees, refers the objects seen to the same position which they would occupy if they were seen by the normal eye turned as far as possible in an outward direction. This seems to show that the patient measures the amount of movement by the amount of his own effort, independently of any peripheral sensations which this effort may produce. It seems impossible to explain the illusion as due to sympathetic movements in the other eye: for when both eyes are open, either the illusion does not occur, or the two eyes see double, and the illusion is confined to the image presented to the diseased eye. The illusion is constantly present when the normal eye is closed.
It seems that the case for innervation-sensations cannot be regarded as completely disproved by its opponents. It is probably best at present to suspend judgment and wait for further evidence. In conclusion, we must note one very important point. There are two forms of the theory, one advocated by Bain, and the other advocated by Wundt. According to Bain, the innervation-experience is primarily occasioned by the motor discharge itself: it is a unique kind of sensation correlated with the active initiation of movement. According to Wundt, on the contrary, its specific quality is ultimately derived from a peripheral source. The area of the cortex from which the motor impulses are discharged is also the area in which motor sensations in general are localised. Hence, the excitement of this area in the process of motor discharge involves a reproduction of experiences more or less similar to those which arise from peripheral sources in the actual execution of the movement; but the reproduction assumes the character rather of an actual sensation than of an idea.* There can be no doubt that, if we are to accept the theory of innervation-sensations at all, we must accept it in the form in which it is propounded by Wundt, and not in that which Bain has given it.