Popular Science Monthly/Volume 25/September 1884/Where and How We Remember
|WHERE AND HOW WE REMEMBER.|
IF you examine the brain of a dog, or an ape, or a man, you will see that it is made up of two kinds of substance, gray and white. The gray substance, which is formed of round bodies of nervous matter called nerve-cells, is spread out in a thin layer over the entire surface of the brain. The white substance constitutes the center and body of the organ, and consists of white threads or nerve-fibers which pass in various directions through the brain and end in the cells of the gray matter. It is the office of the white fibers to convey messages; it is the office of the gray cells to dispatch them, or to receive and register them.
If a brain be properly torn apart, it can be shown that many of the white threads are collected into bundles. These bundles, each of which contains many thousand threads, can be separated from one another and followed to their terminations. It will then be found that each bundle, or tract, as it is called, connects some one organ of the body with some one region of the gray matter on the surface of the brain. For example, one tract joins the muscles of one half of the body with the lateral part of the opposite half of the brain; another ascends from the surface of the body, being made up of many fibers, each of which comes from one little area of skin, and this tract ends in the surface of the brain just behind the first one; another bundle comes from the eye and goes to the posterior part of the brain. So too the ear, the nose, the tongue, send in their bundles, and each of these goes to a definite and separate region of the surface. And thus, as every part of the body is connected by its own tract with its own part of the gray matter, we can imagine upon the surface of the brain a map of the entire body laid out, and can say, as Meynert does, that the surface of the body is projected upon the surface of the brain.
Each of the little white threads, like an electric wire in a cable, is insulated from every other by a sheath. It is therefore impossible for a message sent from one end of the thread to leave it; the message must go to the other end of the thread. Therefore, an irritation set up in any organ of the body is always transmitted to that part of the brain with which the organ is joined, and can not reach any other part directly, although it may do so indirectly, by means of association fibers which join the different regions with one another. The anatomy of the brain, thus studied, gives a clear indication that the different regions of its surface govern different organs of the body, and that each region has a distinct function to perform.
It is an admitted fact that an irritation set up at one end of a sensory nerve and sent to the brain produces a change of state in the gray cells which receive it. That change of state is known to us as the conscious perception of a sensation. The conscious perception does not occur in the organ irritated, nor in the nerve which carries the irritation. It occurs in the brain. The perception of an object seen does not take place in the eye, nor in the optic nerve, but in the posterior part of the brain where the tract from the eye terminates in gray cells. In like manner each sensation is consciously perceived in that part of the brain with which the sensory organ is connected whose irritation produced the sensation.
Being perceived, the sensation is in some way registered and preserved, so that when a second similar irritation is sent inward we not only perceive it, but recognize it as a matter of former experience. But, independently of a second perception, we have evidence that the first is preserved in the fact that we can call it up to consciousness by a voluntary effort, and make it, by means of memory, an object of thought. In both these processes the same part of the brain is in action which originally perceived the sensation. But, as sensations are perceived in various regions, it becomes evident that memories are stored up in various regions. If this is so, our various kinds of memory must be independent of each other, and one may be lost while others remain. We shall soon see that this is the fact.
If you lay bare the brain of a dog, and carefully cut out all the posterior part of both halves or hemispheres, you will find, when the dog recovers from the operation, that it is totally and permanently blind. It can smell, and hear, and taste. It can run about, and can perceive sensations of all kinds except those of sight. If from the brains of other dogs you cut out other parts, but leave the posterior part untouched, sight will not be affected in any case. These physiological experiments show that perceptions of sight occur in the posterior parts of the brain, the parts to which we have already traced the white threads from the eye.
If, instead of cutting out the whole of the posterior part of the brain, you select the central portion of the posterior part, leaving a ring of tissue about it uninjured, the result is more interesting. (See Fig. 1, A.) After a few days, when the wound is healed, you will find that the dog's hearing, smell, taste, motion, and general sensation are in no way affected. The animal runs about the room, and, unlike the first dog, either avoids or jumps over any obstacle which may be put in his way. He can therefore see the obstacle. But the sight of other dogs, or of men, whom he used to recognize with signs of pleasure or dislike, no longer affects him at all. However hungry or thirsty he may be, he no longer looks for his food and water in their usual places, and when they are put before him he does not seem to know them as food and water until his nose is put into them, when he recognizes them by other senses than sight. The sight of the whip, which used to make him run into a corner, does not frighten him any more, though he jumps when he hears it snap. He used to give his paw when the hand was held out for it. Now he will not do so until the word paw is spoken, when he holds it up as before. The dog is not blind, but he has lost the power of recognizing objects formerly recognized by sight. He has been deprived, by the operation, of his sight-memory pictures, or sight-imaging power. He has been put back, as far as one sense is concerned, into the condition in which he was when born—that is, destitute of knowledge acquired by sight-perception. He acts just like a puppy; for he soon begins to smell and lick objects in an inquiring way, and to run to and examine curiously things with which he was formerly familiar. He sees these things, he learns again to know them; in a word, he begins at once to lay in a new store of memory-pictures. It is only necessary to put his nose into water a few times; after that he looks for and finds it when he is thirsty. Then he begins to know his master. The whip soon becomes again a dreaded object. And in the course of two or three months he has gained a new set of memories and recognizes objects just as before the operation.
In the first dog, the entire posterior part of the brain was removed, and the dog was made permanently blind. In the second dog, a portion of this part of the brain was cut out, and the dog was deprived of his sight-memory. He was, however, able to recover. And, if by successive operations the experiment be repeated on the same dog, it will be found that recovery is always possible until the entire posterior part of the brain is removed, when, like the first dog, he becomes permanently blind. The recovery then was possible because around the area cut out there was left a ring of gray matter which was in connection with the eye; and in this ring of gray matter, which formerly contained no memory-pictures, the new memory-pictures were stored. All the posterior part of the brain in the dog is, therefore, a potential area for sight-memories. The actual area of sight-memories occupies only a part of the potential area. If the actual area is cut out, but a part of the potential area remains, the dog is temporarily deprived of sight-memory, but can recover. If the potential area is entirely extirpated, the dog remains blind, and can never regain his memories. The distinction between actual and potential memory is important, as we shall see when we come to similar phenomena in man.
The experiments just described were first made by Hermann Munk, Professor of Physiology in the University of Berlin, and they have been confirmed by many other experimenters. What has thus been proved of the location of perception by sight, and of sight-memories in the posterior part of the brain, has also been proved of other senses and their memories. The perception of sounds and sound-memories are destroyed when the lower lateral part of the brain (the temporal region) is injured, but are not affected as long as this region remains intact. When the anterior portion of this region is destroyed, the animal becomes deaf to sounds of a low pitch; when the posterior portion is injured, high notes are no longer heard. If the region is entirely extirpated, the animal is totally and permanently deaf. If it is only partly extirpated, the animal loses all memory of words or commands formerly recognized, attaches no meaning to the cry of its puppies, to the snap of the whip, or to its master's whistle. The perception of touch and its memories are destroyed when the upper lateral portion of the brain is injured (the parietal region). Voluntary motion is suspended when the antero-lateral portion is destroyed. If the destruction of this part is complete, the paralysis is permanent; if not, recovery is possible. In all these regions the distinction between actual and potential obtains: if the actual area only is cut out, the acquisitions already gained are lost; but, as long as some of the potential area remains, the power to acquire is present and recovery is possible.
|Brain of a Dog||Diagram of Munk.|
A. Visual area; potential area of sight-memnories
A1. Visual area; actual area of sight-memories before operation.
B. Auditory area and potential area of sound-memories. B1. Actual sound-memories.
C. Area governing motion and sensation in hind-leg of the opposite side.
D. Area governing motion and sensation in fore-leg of the opposite side.
E. Area governing motion and sensation in head or the opposite side.
F, G. Area governing motion of the muscles of the eye and ear respectively of the opposite side.
H, I. Area governing motion and sensation In neck and body of the opposite side.
Thus, by experiment, a number of regions are mapped out on the surface of the brain and the function of each is determined. When the results of the physiologists are compared with those of the anatomists, they are found to agree. The area of the brain which the physiologist has shown to govern sight has been shown by the anatomist to be connected by means of insulated white nerve-fibers with the eye. The area which, one says, governs touch, the other says is connected with the skin. The area which one proves to be concerned with voluntary movements, the other finds to be joined to the muscles. Thus the two independent lines of evidence unite in indicating that each region of the brain has its own work to do, its own memories to preserve.
While the anatomical evidence in favor of the localization of memories is as strong in the case of man as it is in that of the dog or ape, the physiological evidence is wanting. Physiologists lament that they can not experiment upon man, and psychologists are slow to admit that these experiments throw any light upon man's mind and its action. Just here, however, the study of disease comes in to help out our knowledge. Disease may be regarded as an experiment of Nature to satisfy both physiologists and psychologists, and its results are the more satisfactory, since man is an animal who can describe his sensations during the experiment, as no other animal can. The nature and value of the evidence for the localization of memories to be derived from the study of disease will be clear after the blood-supply of the brain in man is understood. Every artery divides and subdivides as it passes outward from the great central artery of the body—the aorta—so that the vascular system may be likened to a tree, with trunk, boughs, branches, and twigs. Each terminal division of an artery supplies with blood a little cone-shaped mass of brain, the base of the cone being the gray surface of the brain, and its apex being the point of entrance of the little artery. In the brain the terminal branches of the arteries do not run into each other, as in some organs, so that each little cone, like the leaf on the tree, is independent of adjacent cones and hangs upon its own arterial twig. Now, it is evident that anything which plugs up the artery is going to cut off the blood, and therefore the nutriment from the little cone of brain, and then the little cone will wither and die. The larger the artery plugged, the greater the surface of brain destroyed. This is the process of disease known as embolism or thrombosis. But such a destruction of brain-tissue in man corresponds to the artificial destruction of brain-tissue in the dogs experimented upon, with this advantage in the case of man, that the shock of the operation is avoided. The experiments of Nature and of the physiologist are therefore parallel. The only difference is in the order of the observation. The physiologist cuts out a definite part and observes the result. The pathologist observes the result of Nature's experiment by watching the symptoms of his patient, and, after the patient's death, he can ascertain the position of the part diseased. Now, if the old theory be true, according to which the brain acts as a whole, and its various parts do not possess distinct mental functions, a limited area of disease in one part may impair the mental powers but will not produce a loss of one function. If, on the contrary, the new theory be true—the one to which the anatomical arrangement and the physiological experiments point—that each part of the brain has its own work to do, a limited area of disease will interfere with the work of the part diseased—will produce a loss or impairment of one function, and will not affect all the powers.
The following instance shows that pathology supports anatomy and physiology, and that the localization of functions and memories is no longer a matter of question among scientists:
Not long ago a man was brought into Bellevue Hospital, in this city, suffering from fever, headache, delirium, and stupor, which had developed after a blow upon the head. In addition to these symptoms, he had a paralysis of the muscles on the back of the fore-arm, so that he could not raise his left hand. The general symptoms indicated the presence of an abscess in the brain. To the surgeon, familiar with the anatomy and with the physiological experiments upon animals, the paralysis of the arm-muscles indicated that the abscess was situated in that part of the brain whose function it was to raise the hand. He therefore sawed through the skull over the supposed site of the abscess, and, although the hole which he made was only large enough to admit his little finger, the abscess was found lying just beneath it, and was emptied.
Such a case shows that the study of localization may aid in saving life. The following cases of loss of a definite kind of memory, occurring suddenly, and accompanied by symptoms which indicated the situation of the disease in the brain, remind one very forcibly of the physiological experiments described, and afford positive proof that powers of sensation and memory, as well as the power of motion, may depend upon the integrity of definite regions of the brain:
An intelligent gentleman, while playing billiards, suddenly became aware of the fact that he could see but one half of the ball at which he was aiming. He had become blind in the right half of both eyes. Soon after, on attempting to read, he found, much to his surprise, that he could not read. He could see the letters and words, but they conveyed no meaning to his mind, and appeared to him as so many forms—just as a set of Chinese letters do to us. He had lost the power to recognize written and printed language. Singularly enough, he could write as well as ever, but it was impossible for him to read what he had just written. The memory of the motion involved in producing a letter remained, the memory of its appearance was gone. The memory of the motion served to take the place to some degree of the lost memory-pictures, for, when asked to read a word, he would bring up his hand to the page and with his finger trace the form of the letter, and then name it. It was evident that the only means he had of recalling a letter was by going through the motion necessary to write it—in other words, by calling into play his motor-memories. As he was more accustomed to trace written than printed letters, it took him a longer time to recall by tracing printed than written words. But this was not bis only defect of memory. He found that many objects formerly perfectly familiar were no longer recognized by sight. He was well acquainted with the streets of Paris, but on going out he now looked at the houses and streets as at those of a new, unknown city, and he was unable to find his way about. The loss of memory did not consist simply in a failure to recognize objects seen, it involved his power to call up to his mind objects formerly well remembered, places well known, faces, scenes of his childhood, etc. The blindness in the right half of both eyes indicated that the disease was situated in the posterior part of the left half of the brain, for this has been found diseased in nearly thirty cases of similar defects in vision in which an examination of the brain was made. The loss of memory of objects seen indicated that in that part of the brain in which the perception by sight occurs were located nerve-cells whose integrity was necessary to the existence of the sight-memories lost. The case demonstrated conclusively that sight-memories lie in the posterior part of the brain. The mental vigor of this man was good. His other faculties, his other perceptions and memories, were not affected. He was not paralyzed; but, as far as reading was concerned, he had been put back into the exact condition in which he was when as a boy he began to learn to read. And when the writer saw him last, in the wards of Charcot's great hospital in Paris, he was studying away at his alphabet like a school-boy of six years.
This is not an isolated case. In the same hospital, at the same time, was another gentleman who had been remarkable for the excellence of his memory. It had always been possible for him to acquire easily, and he had only to read a passage carefully in order to remember it verbatim. He also had considerable talent in sketching, and was in the habit of drawing any figure or view which pleased his eye. His memory of music, or of things heard, was less active and reliable than his visual memory. One day he suddenly noticed a peculiar change in his power of mental action, which alarmed him very much. He found that everything about him seemed strange and unfamiliar. His visual memory was entirely gone, so that he no longer recognized objects or faces, and could not call up to his mind the forms or colors of well-known things. The town in which he lived seemed an unknown place. Ho looked at its streets, its houses, its statues with curiosity, as at those of a strange city. The same was true of Paris, to which he came for medical advice, and where to his surprise he could not find his way about. At the same time he lost the power to sketch, being unable to remember the object to be drawn long enough to draw it, and being unable to recall the appearance of lines and shading in a picture. He found that he could not recall the faces of his wife and children, and when they came to him he only recognized them by the sound of their voices. He even forgot his own appearance, and, being in a large public gallery, and seeing, as he supposed, some one in a doorway barring his passage, he stepped forward to ask the stranger to let him pass, when by the motions he realized that it was his own figure seen in a large mirror. This loss of visual memory extended to memories of his childhood as well as to those acquired recently. It interfered much with his power of reading. In reading a book or in adding a column of figures it was necessary for him to have recourse to movements of articulation of his tongue and lips in order to understand what he read or in order to add. While formerly he could remember easily what he read, he now was obliged to read aloud anything he desired to commit to memory, and thus to learn it by impressing his auditory memory. An interesting detail of the affection was the fact that in his dreams he no longer saw objects, but merely heard sounds or words.
Thus he had been deprived entirely of one class of memories, while all others were still at his command. As a consequence, there had come about a complete change in his character, which can easily be understood when one considers how largely one's thinking is made up of the comparison of one set of memories with another, and how frequently the whole circuit of one's thoughts and actions centers about one group of memories. This man was an artist, and in a moment all the powers, the result of long study and labor, which enabled him to perform and enjoy his life-work, were taken away. In this case, as in the first one related, the disease must have been situated in that part of the brain where visual memories are stored, viz., in the posterior part.
Such a loss of visual memories may be temporary, as is well illustrated by the case of a city district messenger-boy, who found on several occasions that he suddenly lost his way and could not recognize streets with which he was usually familiar, so that he was obliged to ask a policeman to take him to his home; where, however, in the course of a few hours he recovered his memory of places and of faces which he had lost. In this case, which may be regarded as one form of epilepsy, the loss of memory can be explained by the hypothesis that a spasm of the arteries occurred in the posterior part of the brain, just as such a spasm in those of the face gives rise to a sudden pallor.
Visual memories are not the only ones to be temporarily or permanently lost. There is another class of cases whose study gives unmistakable evidence of the localization of memories in that part of the brain in which the original perception occurred. It has been stated that the auditory nerve sends a tract to the lower lateral portion of the brain (the temporal region), and that destruction of this region in animals gives rise to deafness. When this part is injured by disease in man, a peculiar condition is observed, known as word-deafness. This can be readily explained by a review of what occurs in answering a simple question. When you answer a question, the following processes have taken place: 1. You have heard the words of the question. 2. The words have been recognized as known words, and have awakened a corresponding concept. 3. The concept has started a train of thought which has led you to a conclusion. 4. You have formulated your conclusion in words. 5. You have voluntarily set in motion a mechanism consisting of your throat, lips, and tongue, to speak the words. 6. This mechanism has responded to the effort, and has produced the sound of your reply. Now, any one of these processes may be interfered with, in which case you will not answer the question. If you are deaf, you may not hear it. If it is spoken in a language which you do not understand, the words will fail to be recognized, as the sounds will not awaken any memory or concept.
But the words addressed to a child are at first mere sounds to him, and it is only by repeated reiteration of the word in connection with the object or act indicated by it that the child has acquired a knowledge of its meaning. If these acquired bits of knowledge stored up in the memory in childhood are blotted out, the meaning of the word will be lost, and the effect will be the same as if the word had never been learned, or as if it were spoken in an unknown language. This is the condition known as word-deafness, or loss of memory of the sound and meaning of words. It is not an uncommon form of brain-disease, and the symptom and the location of the disease have been connected in so many cases that it is now possible to state that in right-handed persons such a condition is due to disease of the left temporal region, and in left-handed persons to disease in the right temporal region. But such a defect will not only prevent one from recognizing a word when spoken, it will blot out the memory of words, and the power of recalling the words which you desire to use. Therefore you will be unable to answer the question, not only because you do not understand it, but because, if you did understand it—as you might be made to do by appropriate gestures—you could not find words in which to reply. Here, then, another special class of memories, to the exclusion of all others, is blotted out by a localized disease.
But let us follow the process a little further. Suppose you have heard the question, and understood its import, and the concept awakened has set in motion a train of thought which has led you to a proper conclusion, since it is to be supposed that you are neither an idiot nor insane, both of which conditions might interfere with this part of the process; and suppose that your conclusion is formulated in words in your mind. You have still to speak the words before your reply is heard. We will pass by a paralysis of the muscles of the throat or tongue, which would, of course, prevent your speaking, and consider the process of setting in action the voluntary centers which govern speech. You have learned to speak by repeated efforts, at first with imperfect success, later witb proper regulation of the tone and effort necessary; which, when acquired, is remembered, so that the words are now clearly pronounced. But this learning to talk is simply the acquiring of memories of definite combinations of muscular action; in a word, of motor-memories. Other examples of motor-memories are the memory of the motions made in playing a musical instrument, swinging Indian clubs, writing, or using various implements of trade. These are all distinct memories, and any one of them alone can be blotted out by disease. But, if the memory of the motions necessary to pronounce the words of your reply is affected, it is evident that you will be as powerless to answer the question as though you did not understand it. If this is the case, the disease will be in a different part of the brain from that affected in the first case. It will lie above and in front of the temporal region, in what is known as the third frontal convolution of the brain. This, too, is established by hundreds of examinations of persons who died with loss of speech.
It thus becomes evident from the study of brain-disease that our visual memories, our auditory memories, our memories of motion, and our memories of speech may each be lost while other memories are unaffected; and further, that a loss of any one of these memories is always due to disease in its own appropriate part of the brain.
One other set of facts remains which confirms in a remarkable manner the theory of the localization of functions. It is well known that organs which are constantly used grow in strength by use. The blacksmith's arm is the favorite example. It is no less true that an organ which is not used withers away. If one carries his arm in a sling for several weeks, it grows thin. Now, a sensory organ, like the eye, is simply a mechanism for the reception and transmission to its corresponding part of the brain of appropriate impulses. Suppose the organ to be destroyed. It is evident that the part of the brain with which it is joined is no longer called into action; it is no longer used, and the result is that it withers. If from a new-born animal you remove an eye, the tract to the posterior part of the brain and that part of the brain will never be called into use, and hence they never develop to a normal size. If a child is born blind, or loses his sight in infancy, the same is true; so that, when in old ago he dies, the posterior part of his brain will be found small and shrunken. It is probable that the examination of the brain of a deaf and dumb person would show an atrophy of the speech-centers, although this has not yet been investigated. It is known that if a limb be amputated and the individual lives for twenty years or more, the part of the brain which formerly governed the movements of that limb, and which received sensations from it, will be found shrunken and withered. So that from this class of facts important evidence is derived regarding the parts of the brain which preside over various functions and which preserve their appropriate memories.
If, now, we take models of four brains, and on the first mark out the location of the various areas connected with the various sensory-organs as determined by the anatomical connection of the white nerve-threads; on the second mark out the location of the various areas which physiologists have shown to govern various sensory organs; on the third mark out the various areas whose disease produces disturbance of action in the various sensory organs, and loss of memories of perceptions by those organs; and on the fourth mark out the various areas which wither after disease of the various sensory organs—we shall find that upon all four brains the areas belonging to any one organ coincide. We may therefore conclude that each class of sensations
|Fig. 2.—Outline of Human Brain, Side-View. (After Ecker.)|
|1.||Area of sight and its memories.|
|2.||Area of hearing and its memories.||upper one third, leg|
|3.||Area of motion and its memories,||middle one third, arm|
|4.||Area of touch and its memories,||lower one third, face.|
|5.||Area of motor speech-memories.|
|The areas of motion and general sensation coincide to some extent.|
and each class of memories has its own definite area of the gray matter on the surface of the brain. Memories of objects seen are located in the posterior part in the occipital region. Memories of sounds heard are located in the lower lateral part in the temporal region. Memories of motions in the limbs, and of touch in those limbs, are located side by side in the central lateral region. Memories of speech are located in the frontal region. It is therefore a mistake to speak of memory as a single faculty of the mind. It is really an assemblage of distinct memories which we possess, each kind of memory being as different from the others both in its nature and in its location as are the different organs of sense through which the original perception came. These various memories are associated with each other, and this association is secured by means of fibers passing between and joining these different areas. It is also a mistake to give memory as a whole a location in one place as the phrenologists do. Our various memories are scattered over the brain in different regions, being distributed at the time of the perception of the sensation remembered in accordance with the anatomical connection of the percipient organ. It is, finally, a mistake to speak of a good memory or a bad memory. The degree of power to remember differs in our various kinds of memory. One man can remember things seen; another can remember things heard; a third is skillful in the performance of certain motions, and may be said to possess a good motor-memory. A fourth acquires languages readily. So each of us has a stronger and a weaker kind of memory, and it is important to recognize this, in order to train and educate the weaker memory up to the level of the stronger one. The memories which we possess are our actual memories. But around these are areas of gray matter still unoccupied by memory-pictures, and in these potential areas new memories can be stored up. The actual area is always extending, the potential area is diminishing, as we acquire new facts. The wider and more varied our knowledge, the greater the actual area of any one memory, and the more complete our command over our inherent brain-power.
- This case was fully reported by Charcot, in the "Progrès Médicale," May, 1888.
- This case is reported in the "Progrès Médicale," July 18, 1888.