Practically nothing is known with regard to localization of function in the association-zones, with the exception of the localization of the centres for words; but this exception is so remarkable as to suggest that if there were any other faculties, interference with which caused defects as distinct as those which characterize disorders of speech, it would be found that the association-zones are made up of definite centres. As the evidence stands with regard to the broadest continental divisions, we can merely state that it points, although not very clearly, to the connection of the frontal zone, the region in front of the kinæsthetic area, with ideas of personality, of other zones with ideas of environment. Injury to the frontal region has in certain cases resulted in the victim’s losing his knowledge of himself, his name, and his relation to his family. On the other hand, gunshot wounds and other definite injuries have in a large number of cases destroyed portions of the cortex behind the forehead without causing any recognizable intellectual change. It is quite certain that this part of the brain performs no functions which are of a different, or, as it is often called, higher order than those of other association-zones. It has been stated that disease of the zone which intervenes between the visual and auditory areas is more likely to cause hallucinations, disease of the frontal zone delusions. A patient fancies in the one case that he sees things that are not there, or hears voices when no one is speaking; in the other case he imagines himself a king; but evidence connecting localized disease with mental derangement is very scanty. The functional disturbance which causes lunacy is usually of a general character; or, if local to begin with, it becomes general before the death of the patient makes possible the examination of his brain.
Fig. 25.—The Surface of the Left Cerebral Hemisphere, Cerebellum,
and Medulla Oblongata.
Sensory areas are enclosed by broken lines; certain centres in the association-zones are marked by dots. The sensory area of smell is on the inner aspect of the brain; so also is the area of vision which borders the calcarine and retrocalcarine fissures, and only rarely extends on to the external surface, as shown in the diagram. The sensory area of hearing is largely hidden within the fossa of Sylvius, the opening into which is indicated by the dark line above it. The kinæsthetic-sensory areas for the various muscles of the body occupy the territory between the dotted line in front and the bottom of the fissure of Rolando behind. They do not extend on to the posterior wall of this fissure. It is impossible at present to define the boundaries of any of the centres in the association-zones.
Derangements of speech throw a flood of light upon the organization and manner of working of the association-zones; and, owing to the accident of the continuation of the line of the carotid artery by the middle cerebral artery, which supplies the speech centre, there is no other spot in the cortex so likely to be thrown out of gear. A little plasma coagulates on one of the cardiac valves, or about an atheromatous spot in the aorta. Detached by the blood-stream, it is shot into one of the branches of the middle cerebral artery, which it plugs, causing apoplexy. A larger or smaller number of muscles on the opposite side of the body are paralysed. If the plugging occurs on the left side of the brain, it is accompanied by aphasia; but only if it occurs on the left side, owing to the fact—perhaps the most remarkable in connection with the localization of speech—that only on the left side is the cortex trained to utter words. In course of time the patient may recover the power of speaking, but not until he has, with almost as much labour as in childhood, educated the right side to do the work. There are four speech-centres, quite distinct one from the other. Near the visual area is the centre for seeing words, or rather the centre for seeing the meaning of words. If this centre be diseased, a written word is merely a crooked line. Behind the auditory area is the centre for recognizing the meaning of words heard. If it is interfered with, the most endearing or commanding phrases produce no more impression on the hearer than a bird’s song. In front of the hand-area—its localization is less certain than that of the other three—is the centre for writing. In it are associated words heard or seen, with the movements necessary for the making of letters. In the centre first referred to, as being the one most often thrown out of gear, which lies in front of the area for the mouth and throat, words heard or seen are translated into movements of the parts which give them sound. No other actions illustrate so clearly the “law of neural habit.” In the infant’s brain sounds of words are distinguished from other sounds. They are associated with the objects which they name. Movements of the mouth and throat, made at first ineffectively, blunderingly, succeed after a time in securing the thing of which they sound the name to the child’s satisfaction. Thus, two centres are gradually established in his mind. Sounds and ideas of things are associated in the one; words and ideas of the movements necessary to their pronunciation in the other. Either of the four speech-centres may be placed out of action without the others suffering. A man may be able to write without being able to read what he has written. He may read aloud, although apparently deaf to speech. He may be unable to write or unable to speak, although understanding what he reads or hears. Aphasia, when partial, illustrates still further the law of neural habit. The ability to remember nouns, especially proper names, is most easily lost. Few are the people who, as age advances, do not suffer from this failing. Even the names which are most familiar elude the memory. From one point of view this is strange. Nouns-substantive are the words first learned. Of all words they have the most definite objective association. But it is just their definiteness which makes them difficult of approach when the apparatus of mind is working badly. There are so few paths by which they can be reached. Their mental associations are limited. A patient who is recovering from the effects of a lesion which has rendered him partially aphasic may be able to recall adjectives when he cannot recall nouns. He may say, “Give me the black,” when he wants ink, and “Give me the white,” when he needs paper. Or he may retain control of verbs. “Where is the—— what I put on—what I think with?” may be the circumlocution for hat.
Psychologists explain the voluntary production of a movement as the setting flowing of a sensori-motor current. Everyone agrees that it is impossible to think of the impulses which produce movement as originating without sensory antecedents. Hence psychologists picture the nerve-current as originating on the sensory side. Kinæsthetic images of the sensations which will result from the movement are described as being called up in the mind by the agitation of the part of the brain which, by association, is linked with the neurones which discharge impulses to the appropriate spots in the grey matter of the spinal cord. The idea of movement flows over to the muscles. But this conception of the relation of mind to body assumes too much. It postulates an existent mind in which the images of movement-sensations—the memories, that is to say, of the sensations which previously accompanied movement—are stored. The study of the apparatus of mind does not warrant this assumption of an existent mind. It finds nothing in the nervous system but apparatus. There is no mind existent in the brain during sleep. It would appear to be sufficient to describe the origination of a voluntary movement as the opening of the channels which convey the afferent impulses which are ceaselessly pouring into grey matter from nerve-endings in and about muscles into efferent channels. Our conception of the number of sensations which reach the realm of consciousness is ludicrously restricted by our inability to pay attention to more than one sensation at a time—a restriction, it is needless to remark, which is imperative in the interests of consistency of behaviour. Two personalities paying attention to different sequences of sensations would give incompatible orders. One would command the muscles to cause the body to recline; the other would direct them to make it stand up. From myriads of sense-organs impulses are continuously rippling through the cortex of the brain. The term “impulse” is too heavily weighted by its association with the idea of currents which are strong enough to prove effective without the intervention of consciousness; but no other is available. They ring the bell of consciousness, however little may be the attention which their summons secures. Attention cannot be directed to two things simultaneously. It moves, as it were, on a succession of points. On some it rests longer than on others. They make an impression which can be recalled; the rest being passed by so rapidly that they are not remembered, it is as if they had never been perceived. They blend, as a succession of moving lights blend, in producing a background to consciousness. Not recognizing their separateness, we interpret them as fused. A good deal of misleading metaphor has been used, as it seems to the writer, in accounting for the effect upon the mind of impressions which make but a weak demand upon attention. They are spoken of as “marginal” perceptions, from the analogy of the ineffectiveness of impulses generated at the periphery of the retina, as compared with those which give rise to direct vision. A “subconscious,” or even “unconscious,” self is evoked. The self cannot be less than conscious. Self is the passage of attention from sensation to sensation. Its relation to the not-self is temporal, not spatial.
Every sensation which is called up into consciousness, though it occupy attention for the shortest possible time, tends to give rise to movement—is, indeed, in its very nature an impulse flowing through a sensori-motor arc. The circuit for the voluntary execution of a movement is represented as flowing through kinæsthetic-movement arcs. This may be necessary for volitional actions, but it is not essential for reflex actions. A spinal frog will remove an irritant from its back with its hind-leg, after the roots of all the afferent nerves of the hind-leg have been cut. In this case the reflex is direct, from injured skin to muscles of the leg. It is not double—muscular sensations from the leg, liberated into efferent leg-muscle-nerves by skin-sensations originated simultaneously in a part anterior to the segments in which the roots have been cut.
The unit of sensation to which attention can be directed has yet to be defined. Like sensations—sensations which are correlated in experience, that is to say—seem to fuse in consciousness. A sequence of similar sensations appeals to attention. Unlike sensations interfere one with another. The apparent fusion is not a composite neural effect which consciousness views as a single unit. Not even identical images simultaneously formed on the two retinæ produce a superimposed effect upon a particular spot in the brain. Different brain-spots receive the two separate images which the mind views as one. This raises a doubt as to whether perceptions are, properly speaking, fused. It suggests that they are separate points upon which attention rests in rapid succession; but such a hypothesis does not preclude the conception of the production of a composite sensation by impulses coming simultaneously from the same sense-organ—e.g., a unified neural effect as the result of several musical tones.
Every neural agitation which attracts attention has an effect upon the growth of the nerve-strands in which it occurs. Memory is not an existent. It is the repassage of the same strands. There is no such thing as memory. It is the neural apparatus which responds in a similar way to a similar agitation. It is difficult to speak of association and neural habit, the phenomena upon which not only all mental life, but all co-ordinated activities, are based, without using such expressions as “the broadening of the path” or “the thickening of the conductor” by the impulses which pass through it. Apparently these analogies may with safety be pressed curiously far. Chaotic response to stimulation is unknown. Thanks to the nervous system, action exhibits an ordered relation to stimulation. This relation is determined by education, giving the term a connotation wide enough to cover all experience. Nerve-tissue adjusts itself to experience; and since the nerve-matter which takes the pattern is not labile, the process of organization is consecutive and the result permanent. One pattern is not destroyed as another is impressed. Hence temporal associations are formed. What has been thought once will be thought again, if the circumstances in which it was thought recur. What has been done once will be done again under the influence of a similar sequence of stimuli. The conductors are widened every time that they are used. But, so far as concerns the mind, a reversed influence comes into play. The wider the conductor, the less appeal to attention is made by the impulses which pass through it. It is as if currents which have to overcome resistance in a narrow path acquire a higher potential than those which find an open road. And since the making of the road depends upon attention, the limit of broadening is reached when a volitional act becomes a habit. The first time that a piece of music is played consciousness is alert. Marks on the page and movements of the fingers are felt intensely. With each repetition the need for attention subsides.