Popular Science Monthly/Volume 18/March 1881/Cerebral Localization; Or, The New Phrenology
|CEREBRAL LOCALIZATION; OR, THE NEW PHRENOLOGY.|
"WHEN the one who listens does not comprehend, and the one who speaks understands as little, you have metaphysics," says Voltaire. Taking this as a true definition, we may say that there has been, and yet remains, much metaphysics in the treatment of the functions of the brain. But the difficulties in cerebral physiology are great. There is divergence of hypotheses, the facts themselves are not settled, and contradictions abound. The foundations of the science are yet deficient. But it does not follow that experiments are useless. For half a century important researches have been carried on; and more recently facts have been discovered to which we would here draw attention.
From an anatomical point of view the brain is composed of two symmetrical halves, right and left, united by a voluminous commissure, which probably puts into communication the homologous parts of the two hemispheres. Each hemisphere consists of a central mass, with its envelope of convolutions. The central mass, partially separated from its outer covering by the lateral ventricles, is composed of two round bodies, formed of gray nerve-cells—the active part of the nervous system. The office of these rounded ganglia seems to be to strengthen the impressions that come from without, or from stimulated parts of the brain itself, and they may take part in automatic actions. They are in relation, on the one hand, with the spinal cord, and perhaps, more or less directly, with most of the motor and sensitive fibers of the body; on the other hand, they are connected by fibers with the gray matter which is spread out in layers over the convolutions of the brain. In other words, the nerve-cells forming the periphery of the convolutions give out white fibers, which penetrate the central ganglia, probably connecting themselves with their cells. From these cells other fibers proceed toward the cord and extremities of the body. The central masses are on the line of the cerebral fibers between their origin in the gray cells of the convolutions and their termination in the cord and body generally.
This anatomical arrangement seems to indicate that the peripheral substance, and not the central mass, is the point of motor stimulation and the seat of sensitive impressions. It is everywhere admitted that the brain is the organ of thought and will; but for a long time it was believed that the central mass was the important portion, and the convolutions were disregarded. Hippocrates thought they were only a gland. Malpighi and Vieussens thought the same. Ruysch, from their vascularity, considered them as a simple sanguinous network, and Boerhaave and Haller adopted this conclusion. Vic d'Azyr was the first to examine their structure; then came Baillarger, Ehrenberg, Purkinje, Meynert, Luys, Betz, and Charcot, who revealed their precise anatomy. As to their physiology. Gall taught that intelligence is a function of the convolutions; Desmoulins added that the degree of intelligence is in proportion to their number and depth; and Broca, taking the ideas and facts of Dax and Bouillaud, announced the first discovered localization—that of articulate language in the third left frontal convolution.
In 1870 two German scientists, Fritsch and Hitzig, passed a current of electricity across the head and behind the ears of a living subject, which caused movements of the eyes. They referred these movements to stimulation of the gray matter of the convolutions, and set about the verification of their hypothesis. From their experiments they drew the three following fundamental propositions: 1. There are in the head convolutions that may be excited by electricity, and this excitation is followed by the production of determinate movements—depending upon the point excited; other parts being excited without producing movements. 2. The points, which under stimulation induce action in such and such muscular groups, occupy a very limited portion of the cerebral surface. 3. The extirpation of such a point of the surface, known to be a center of distinct movements, paralyzes these movements.
The theory of cerebral localization assumes as proved that there is in the brain a peripheral portion devoted to the production of movements—a motor region; and another where stimulation is not followed by movements—a non-motor region. It further assumes that the motor region may be subdivided into a certain number of small tracts, definitely circumscribed, each of which presides over the movements of a certain muscular group and this group alone. Ferrier, stalling with these conclusions, proceeded with the research, and seems to have established—that the convolutions, in man as well as in the lower animals, may be separated into three regions: the anterior, devoted to intellectual functions; the middle portion, charged with the motor innervation of the body; and a sensitive region where are received the impressions made upon the sense-organs by the external world. To show how Ferrier reaches and justifies these conclusions is the object of this article.
Two methods of investigation are open to the physiologist—the experimental and the clinical. In the order of time, the first and the most practicable is the experimental method, which consists in observing the brain through openings in the skull and exciting the convolutions or removing them according to the end proposed. This method is applicable only to animals; and the monkey, from the likeness of his brain to that of man, is best suited to these investigations. But experiments upon other animals are very useful. They show the homologous regions in different cerebral types; and the inequality in relative importance of the central masses, the region of automatic functions, and the convolutions, the region of the will. The great advantage of this method is that the operator can repeat the experiments indefinitely upon all sorts of animals, varying the conditions, and choosing his own time and place. Lesions can also be better circumscribed, and the autopsy can be made at will. On the other hand, animals can not tell their sensations, and we have to judge as best we can as to affections of the intellectual and sensational regions. And the preliminary operations may cause general symptoms that mask the phenomena to be studied. Still, this method is in merited favor, and it will yet yield answers to many questions if we know how to put them.
All the processes of experimenters may be reduced to two categories, according to their effects upon the functional activity of the convolutions. These are irritating lesions and paralyzing lesions. Irritating lesions provoke spasms when applied to the motor region, subjective sensations when applied to the sensitive region, and delirium when they affect the intellectual region. Paralyzing lesions in the motor region paralyze the normal action, and in the sensitive and intellectual regions bring on anæsthesia and mental depression. But this division applies only to immediate results, for it is not rare to see a lesion, whether experimental or in clinic, provoke symptoms of irritation in the beginning, followed by paralytic symptoms, and reciprocally.
Experiments that produce prompt paralyzing effects are more numerous than those which excite the functions of the convolutions. But the most preferable one, and that adopted by Ferrier, is the method of limited ablation. By it lesions can be more circumscribed, it affords a sure means of control, and hence it has come largely into use. Electricity is the only process for exciting the functions to greater activity; and since it has been much opposed, and is the sole support of the theory of cerebral localization, we must defend its legitimacy.
It may be remarked that the convolutions of the brain are in the form of long, round, gray swellings, separated by furrows. Each of these swellings has a direction, relations, and a situation peculiar to itself, and identical for all animals of the same species; and the principal ones have their homologues in all animals. They have also their special names. When the excitant is moved about over the different convolutions, the existence of the motor zone is made manifest by the successive movements of the animal; the sensitive zone is known by signs of sensation, while the indications of an intellectual zone are not thus positive. Observe what passes when we irritate the so-called motor region. We excite a certain point of the convolutions, and a movement is produced. Groping about with our electrodes, we find the effect limited to a small tract where at all the points the same movement is provoked, and that alone. Next to this zone, we can in the same way find the limits of others presiding over other movements. The influence appears greatest at the center of the zone: exciting the periphery sometimes produces a light supplementary movement belonging to a neighboring zone. The special centers or zones being very near together, our means of electrizing them are not perfect enough to prevent a slight diffusion of the current into neighboring centers and faintly exciting them. To get precise separate effects we must excite the center of the special zone.
As the same movement invariably follows the stimulation of a special center (the name of a special zone), we conclude that the center in question has charge of this movement, and, as a great number of these centers, placed side by side, preside over most of the movements of the body, we infer the existence of a motor region of the brain, the seat of voluntary control of the body. But, since, as the galvanometer shows, the electricity is diffused over neighboring centers, what right have we, it may be asked, to assert that the movement observed, when a certain center is excited, is due to that excitation, when neighboring centers are also excited? We reply that to these facts we can oppose other facts. On removing the electrical conductors scarcely a centimetre, we excite perfectly definite unlike movements, although the current is diffused as before. Why are not the movements last produced the same as those observed at the first position? Why such distinct localization in spite of diffusion '? Although the occurrence of diffusion is proved by means of sensitive apparatus, it is physiologically insufficient. Besides, it can be prevented by proper precautions.
Again, it is asked if there may not be such a thing as diffusion beneath the surface? This is a grave question, for, under the motor region, there is a large ganglion containing motor fibers going to the muscles. If the current diffuses downward as far as this ganglion, we can not assume that the convolution alone is excited. There are three answers to this objection: 1. The excitation of the convolutions nearest to this ganglion gives the least results; sometimes no movement at all. Unless we admit that the effects are directly as the resistance, which is absurd, downward conduction can not be affirmed. 2. Braun has demonstrated that a section of the white fibers below the excited points, interrupting the physiological continuity, does not arrest electrical conduction sufficiently to prevent movements. 3. Ferrier has shown that the direct excitation of this ganglion produces a general muscular contraction of the other side of the body, and not special isolated movements. So that we again conclude that, though electrical diffusion toward the corpus striatum (the name of the ganglion in question) may exist, it is physiologically insufficient.
But, it may still be asked if electrical diffusion does not excite the white fibers interposed between these convolutions and the ganglion beneath. This objection borrows a character of probability from the slight thickness of the layer of gray matter of the convolutions, and also from the alleged unexcitability of the gray substance, which has been proved in the case of the spinal cord, but conclusive proof in regard to the brain is yet wanting. Many physiologists claim that these cells are only excitable by the will. For the theory of motor centers they substitute that of psycho-motors. Reserving this discussion for another time and place, we may say that, whether we excite the cells or the fibers that arise from them, the result is the same.
All stimulation, whatever its nature or origin, acts upon a nerve according to its functions. Excite a motor nerve at any point of its course, and you produce movement; excite a sensory nerve, and the subject will feel a sensation which will vary with the nature of the nerve. Compress the eyeball, you excite the optic nerve and get the sensation of light; auditive nerves give sensations of sound, and so for all the nerves of special or general sensibility. If, then, in the brain we excite the motor region, the origin of the motor nerves is irritated, and we get movements; excite the sensitive region, and in place of movement we have sensation owing to the connection of these nerves with sensitive cells. So that the electrization of the gray matter of the convolutions acts in the same way as the electrization of a nerve on any point of its track; the only difference is that in one case we excite them at a point near their origin. From the anatomical relations that exist between the white fibers and gray cells, we infer that the cells play the role of center to the nerves.
We come now to the details of Ferrier's experiments. Ferrier operated chiefly on monkeys, because in them the will is in the ascendant, while in lower animals automatism preponderates. As the result of his experiments, he affirms the existence of three zones—the intellectual, motor, and sensitive—into which the surface of the brain can be divided. The one best known, and to which least objection has been made, is the motor zone. In passing the electrodes over its surface, we soon find the little, well-defined centers that preside over particular groups of muscles. Under the microscope there is seen at these points a limited mass of large cells, called nests by Betz. The functions of these centers are best shown when they are electrized at the central point, the current being then less likely to spread, and so produce more complicated movements that mask the true function of the center under experiment. Here electricity provokes movements of the leg on the opposite side, owing to the cross-action of the hemispheres. It moves as if to go forward; or the movement may be limited to the foot, or even the great-toe. Sometimes the motions are still more complex, and involve many muscles, as if the animal would scratch its breast or press against it some object which it had taken from the ground. Again, it is the arm, forearm, or hand that moves in various ways, and to different ends; sometimes there is a combination of movements, like those required for swimming or prehension; the fingers may lock together with force, as if to retain an object, or extend themselves with a lively movement, as if to scatter something. It is probable that, if our instruments were more perfect, we should find in each center a number of subordinate centers for the execution of single movements, or the moving of a single muscle.
The general idea of the relations of the brain and spinal cord is that the brain commands and the cord obeys. The brain requires such or such a movement, and the cord, working unconsciously, coördinates the elementary and individual movements required to produce the desired effect. Electrizing the centers is the same as issuing an order to the cord, not of directly exciting movement.
Monkeys being difficult to obtain in our climate, to satisfy the constant needs of experiment, Ferrier operated upon dogs, jackals, Indian pigs, rats, pigeons, frogs, fishes, anything that was going. These experiments fully confirmed the results otherwise obtained, and showed, besides, that the action of the hemispheres is of less importance as we descend in the animal scale, while automatism rises.
Ablation of a limited portion of gray matter of the brain leads to the same conclusions as electrization. It produces paralysis, and monkeys rarely or imperfectly recover from these lesions, while inferior animals, as the rabbit and Indian pig, recover; thus showing that the voluntary centers are more important in monkeys than in lower animals.
Such are the facts on which we base the existence of a motor region in the brain. Putting aside all questions of interpretation, it is undeniable that there is a region in the brain where stimulation produces movements varying with the zone or center excited.
Behind this region we find the sensitive centers which receive impressions made upon the sensory nerves, and form perceptions from them. As the terminations of the nerves of sensation are specialized, in exciting the centers we produce, subjective sensations, like those caused by cerebral maladies, in which the intellect refers to the outward world the origin of sensations, of which the cause is in the brain; and, whether voluntarily or by reflex action, the animal operated on by electricity shows by evident signs the nature of the sensations he experiences. By means of counter-verification we determine the existence and topography of a certain number of centers which Ferrier has studied with great care.
Take, for example, the visual center: its stimulation caused disordered reflex actions, indicating unpleasant visual perceptions. This alone is insufficient proof; but we can control the region in question by ablation, which brings on unilateral or bilateral blindness, according as we operate on one or both of the visual centers. Stimulation of the auditive centers, in the same part of the brain, provokes movements of the ears, eyes, and head, showing astonishment or terror, just like those caused by a violent and unexpected noise. Ablation causes deafness; the animal remains indifferent to all sounds. Excite the centers of touch, and the signs indicate disagreeable or painful tactile impressions. Ablation brings on complete anæsthesia of the same parts; you may prick, cut, and bruise the animal, and he remains insensible.
Some experiments seemed to indicate the existence of centers of taste and odor, but it is difficult to trace their limits. They are intermingled like the gustative and olfactive sensations. Electricity causes movements indicating unpleasant tastes and odors, and extirpation of the parts ends these sensations. The animal will respire odors, or taste savors that in the normal state would make him fly about the laboratory, and it all passes unperceived. Still more hypothetical are the centers of the organic needs of hunger and thirst, and more yet those of sex, but Ferrier's arguments are strong in favor of their existence.
The presence of a third, or intellectual region, is proved, as far as it can be, by experiments on the lower animals. It is difficult to understand the mental action of a dog, Indian pig, or even of a monkey. Ferrier observed numerous facts tending to establish the intellectual function of the anterior region of the brain. Electricity could hardly be employed in these researches; but ablations, when performed with caution, brought on notable changes in the habits of animals. The monkeys chosen by Ferrier were remarkable for their vivacity and intelligence, prying about right and left, and observing everything. After the operation they became stupid and apathetic. But these indications are not convincing. The clinic alone can decide whether physiology sustains the doctrine of cerebral localization.
We know in what the clinical method consists. Applied most often to man, it amounts to this: to observe the symptoms of cerebral disease, and at the autopsy to connect the lesions, discovered by the naked eye or the microscope, with the symptoms, as cause and effect. It is true that in cerebral pathology there is great difficulty in separating the essential from the accidental, and distinguishing cause and effect among a plurality of causes. Besides, it frequently happens in cases of cerebral disease that at the autopsy no appreciable lesions can be found. The question is still further complicated by the solidarity that binds together the different parts of the cerebro-spinal system and which makes it probable that a simple local trouble will produce general functional perturbation. The brain is like a complex machine, in which, if a screw loosens, or a nut gives way, or a rod bends or breaks, at once all goes wrong. It is not that the screw, nut, or rod in question is the immediate cause of the movements of the machine, but that the failure of these accessories may, for the moment, produce accidents as grave as would be caused by disturbance in much more important parts. Again, cerebral lesions tend to spread and become general. And yet, we have to accept the lesions caused by disease, for we can not produce them at will.
With these reservations, the clinical method is still of the first importance. By means of it we verify in man the hypotheses of experiment, and assure ourselves of the existence of the intellectual and sensitive regions of the brain. Neither medicine nor physiology opposes the use of the clinical method in cerebral localization. But only circumscribed lesions that have little or no tendency to become general, or to act at a distance by compressing the brain, or otherwise, can come to the aid of our theory. When there is a lesion of the cortical region of the brain which fulfills these conditions, the resulting symptoms may be of two orders—either stimulative or paralytic of the true function. These are the two opposed symptoms that we produce experimentally by electrization and ablation of the substance of the convolutions. It goes without saying that the symptoms vary with the locality of the lesion: the intellectual region gives delirium; the motor region, spasms; the sensitive region, subjective sensations. The symptoms of functional paralysis are also diversely represented by mental feebleness, motor paralysis, and anæsthesia limited to one sense. A lesion frequently presents both orders of symptoms, which succeed each other, or alternate, according to its nature. This fact is as important as the division of the symptoms into two great classes. We will now consider the facts in the same order as before.
The middle region of the superior face of the brain appears to be the motor region. In fact, limited lesions of this region bring on marked troubles in the motor innervation of the body, such as monoplegia, or limited paralysis, or equally limited spasms. Putting aside those cases where the lesions cause general trouble, and regarding those where the symptoms are limited, we come at a constant relation between certain lesions and certain troubles. In ocular monoplegia, the eye can not be controlled by the will. Brachial and crural monoplegia are more frequent; sometimes a single member, arm or leg, sometimes both; but successively, because of the extension of the lesion to both centers, which are near together. In this case the lesion advances slowly and invasively; at the autopsy we can often appreciate the differences of age of the extreme points of the diseased spot. Not far from the brachial and crural centers is the center that presides over the muscles of the face. As before, this center may be affected alone or simultaneously with the other. It depends upon the nature of the lesion whether the invasion is sudden, or slow and progressive, ending in feebleness rather than in paralysis. The proximity of the brachial and facial centers explains their apparent solidarity in the normal state, as shown by the grimaces that often accompany vigorous use of the arms. It seems as if the second center were stimulated by the activity of the first. We close our enumeration of the centers which the clinic has proved to exist by reference to the center of articulate language, discovered long ago by Broca, which attends to the coördination of phonetic movements.
We have seen that lesions of the motor region of the brain may be manifested by spasms as well as by paralysis. These monospasms have been long known, but it was Hughlings Jackson who first attributed them to lesions of the motor region of the brain. Prior to this they had been described by Bravais, but he did not seek for their origin or signification. They are localized convulsions, or partial epilepsy, and Hughlings Jackson thinks they are due to nervous tension. Any new excitement added to those already stored up will produce discharge or spasm. Like monoplegia, they may be limited to an arm or leg, or even the face; but these phenomena are seldom noticed, and we have few observations upon them. When the spasm involves several parts of the body, it always begins at the same point and follows the same order. Dr. Maragliano has made a very interesting study of partial epilepsy, and explained its causes and signification. Both monospasms and monoplegia indicate the same localization of power.
The sensitive region is found, by experiment, behind the motor centers. While limited lesions of this region often manifest themselves externally as circumscribed anæsthesia, it sometimes happens that they remain latent when they are seated on only one hemisphere. There is no sign of pathological perturbation, and in this case we seem forced to admit functional substitution, or the possibility of the regular action of two sensitive, homologous regions, notwithstanding the absence of one of the two corresponding cerebral hemispheres. What does this signify? Must we abandon the doctrine of localization as regards the sensitive centers? A single center suffice for the two parts of the body? This anomaly is probably due to insufficient observation. Disease of the cerebral centers may give no further symptom than enfeebled sensibility, which might pass unperceived. Lesions of the motor region often result not in total paralysis but in slight paralysis—a feebleness and not an abolition of the functions. But there are cases where lesions of the sensitive region are accompanied by less equivocal symptoms, and it is from these that we affirm localization.
Symptoms may be of two orders, according to the nature and the phase of the disease: symptoms of excitation, which produce subjective sensations responding to nothing external, and symptoms of anæsthesia manifested by an abolition of the perceptions belonging to the affected part. These two classes of symptoms may also alternate in the same disease, as in the motor and intellectual regions.
A characteristic case, confirming this theory, is that of a child who fell on his head and buried a portion of the parietal in the surface of the brain. He became blind in the eye of the opposite side. He was trepanned, and the blindness ceased immediately. Soon inflammation arose at the wounded part; blindness returned and lasted till the inflammation disappeared. Compression of the visual center, first by the bone and then by the products of inflammation, was the evident cause of this intermittent blindness. Other cases establish the possibility of abnormal stimulation of the visual center.
The centers of hearing, taste, odor, and touch are localized in the same way, and, if the observations are not very numerous, they make a strong presumption in favor of the localization actually adopted. It sometimes happens that several centers are affected at the same time. In these cases, if the lesion is an irritating one, there occurs from time to time a simultaneous discharge producing a singular mixture of sensations. One such patient, observed by Ferrier, said that he had the sensation of a horrible odor and green thunder. We admit that the clinical arguments in favor of the localization of the sensitive centers are not so numerous or conclusive as could be wished. But only lately have they been sought, and each day brings its contribution, which, considering the rarity of limited lesions of the brain, can not be very considerable.
The localization of intelligence in the frontal region of the brain was thought of long before our day. Gratiolet used the expression frontal races for intelligent races; and those of least intelligence have been called occipital races. The frontal region is greatest in man along with the predominance of reason and logic, while in women, who are dominated by their sensibilities, the occipital region prevails. We may cite to the same point the researches of Bordier on the skulls of assassins, of Luys on the brains of fools and idiots, of Bénédikt on the brains of criminals, of Lombroso on the characters of habitual criminals. Their conclusions are analogous, and favor more than they oppose the popular idea.
But these arguments are not precise and positive. Happily there are others more scientific and more conclusive. Take the celebrated crowbar case, where a young man, who was blasting, had a pointed bar of iron about three quarters of an inch in diameter and weighing three pounds, driven, by a sudden explosion, upward through his head. It entered at the angle of the under jaw, passed behind the nose and eyes, penetrated the skull, and cutting the cerebral substance of the frontal region passed out at the top of the head, above the forehead and to the right of the median line. The wound was frightful. All one part of the brain was disorganized, without counting the multiple fractures of the skull and face. He was alone, but, in less than an hour after the accident, without help, he walked to the surgeon's, went up the steps, and related the circumstance clearly and intelligibly. He recovered, but died of epilepsy some twelve years later. His physicians and friends observed that his character and intelligence changed notably after the accident. From being intelligent and active he became capricious and unsteady, and had to retire from his post of overseer. Analogous cases to the same purpose might be cited, but we have no space. We need new facts, but those we possess strongly favor the theory of Fritsch, Hitzig, and Ferrier.
We have passed in review the experimental and clinical arguments in support of this theory; and there are others of not less importance drawn from pathological phenomena. But we have no space for their consideration.
M. Brown-Séquard has shown the greatest hostility to this theory. His chief argument is that lesions and symptoms are not coextensive. An insignificant lesion causes general trouble; a considerable lesion remains latent in the matter of symptoms. This is true, but it is the exception; whereas he ought to show that it most frequently happens. The real question is, Does the seat of a lesion signify nothing, and may we have identical symptoms with two very different lesions? And we have demonstrated that this can not occur. The facts cited in opposition to the theory of Ferrier may be embarrassing, and at present inexplicable; but such facts would be far more abundant if we admitted the theory of M. Brown-Séquard.
Again, there is the theory of Vulpian, who thinks that the stimulation of the gray cells by electricity is not possible. For motor centers he substitutes psycho-motor centers. In his view, stimulation of the convolution acts, not on the cells, but on the white fibers which proceed from them. But his mode of interpretation does not alter results, nor set aside the centers.
Goltz made a curious experiment, showing clearly the office of the centers. He took two dogs of the same species, one having the education common to all dogs, and the other some supplementary accomplishments, and among them that of giving the paw. In both dogs he removed the center which presides over the movements of the forepaw of one side—the one given by the knowing dog. They soon recovered, and could run about. Running is a reflex act, that does not require the intervention of the centers. But, while the learned dog could use his legs, and go and come, he could not give his paw. This was a superior, voluntary act, which could not be performed in the absence of the corresponding center. It is in this differentiation of the organs of voluntary activity from those of automatic activity that we find the explanation of so many singular facts which at first sight seem to contradict the theory of localization.
It is undeniable that there is yet much to be done in this domain. But the results obtained by Ferrier are so encouraging that we hope this new way of studying cerebral physiology will be followed and explored with more care than ever.
- Translated and abridged from the "Revue des Deux Mondes" by Miss E. A. Youmans.