Speech and Brain Mechanisms 9.95%

THE ORGAN OF THE MIND

A. The organ of the mind

At the beginning of the 19th century the accepted teaching was simply that the brain was the organ of the mind. But it operated, as a whole, like other organs—the liver, for example, and the heart. Men spoke of the brain as children speak of a clock, saying, “It tells the time”—children who have not yet looked into the works.

At the beginning of the century phrenologists, led by Gall and Spurzheim in Vienna, were placing their hands upon men’s skulls and drawing fantastic conclusions. They subdivided the great organ inside the head into little organs. In the little organs there were, they said, representations of different feelings and intellectual faculties such as love of children, sexual passion, acquisitiveness, benevolence, wit, language.

The more orthodox teachers in the Universities ridiculed this first effort by medical “quacks” to introduce a conception of functional localization. But academicians and phrenologists alike were making the mistake that still haunts our thinking on this subject. The former said function dwells in the brain as a whole. The latter said separate functions dwelt in separate compartments, operating as autonomous units.

This misconception lingers on in the minds of modern neuro-physiologists and clinicians. It is so easy to say that the “representation” of movement is here and sensation is there in the cerebral cortex, as though such things could have independent dwelling places.

At about the same time that Gall and Spurzheim were publishing their treatise on phrenology (they gave it the presumptuous title of “The Anatomy and Physiology of the Nervous System”!), the first steps toward an explanation of the working of the brain were being made in Italy. It was then that Galvani and Volta were initiating the study of electricity and the conduction of electrical currents along nerves.

The illuminating discovery by the French surgeon, Paul Broca, was that a patient had lost the power of speech without other serious defect. He showed the cause at autopsy: a restricted lesion in the left hemisphere of the brain. It was located in the general

FUNCTIONAL ORGANIZATION OF THE HUMAN BRAIN

region of the third frontal convolution. That clinical observation in 1861 was of great importance in the history of neurology.

The next momentous event came when Fritsch and Hitzig (1870) applied an electric current to the cerebral cortex of a lightly anaesthetized dog. The limbs on the opposite side of the body moved. Here, under the experimenter’s electrode, was the place of movement control—a place, they thought, where mind might enter body.

Let us consider the clock again. It is not enough to recognize that the second hand, the hour hand, and the chimes all have partially separable mechanisms of their own, and that the chimes, for example, may be paralyzed without stopping the hands. He who would understand the clock-works must analyze the interrelationship of its mechanisms and the transmission of forces that makes the clock tell time. Today we are beginning to see into the works within the head. We perceive that the secret of functional activity in the living brain is the movement within it of ”transient electrical potentials, travelling the fibers of the nervous system” (Sherrington’s phrase). We surmise that it is the changing pattern of these travelling potentials, as they flash into this or that portion of the total mechanism, that makes possible the ever-changing content of the mind.

It is now almost a century since Broca showed that speech had some degree of neuronal localization in the brain. He demonstrated that what he called aphemia, and what we now call aphasia, was produced by a relatively small destruction of a certain area of cortex in the dominant hemisphere of a man. This meant, of course, not that speech was located there in the sense of a phrenologist’s localization, but that the area in question was used as an essential part of a functional mechanism employed while the individual spoke, wrote, read, or listened to others who spoke. It showed further that a man could still think and carry out other forms of voluntary activity while the speech mechanism was paralyzed. The clock still ran, although the chimes had been silenced.

B. Parts of the central nervous system

In recent decades, the action of the brain has been analyzed with great industry, and parts of its many integral functions have

PARTS OF THE CENTRAL NERVOUS SYSTEM

been mapped out. For those who are unfamiliar with this field, a preliminary discussion of some present concepts of the physiology of the central nervous system of man may serve the purposes of orientation. Chapters II and III will be devoted to this purpose, and in Chapter X we will reconsider our observations on speech mechanisms against this background.

Some portions of the human central nervous system are outlined in Figure II-1. The spinal cord contains primitive reflex arcs

q R A L H e 

Midbrain

Pons

\Medulla oblongata \-/~ Spinal cord

ebellum pulled down and back

Fig. II-i. The central nervous system of man. The higher brain stem, including thalamus, midbrain, and part of pons are shown within the brain. The lower brain stem composed of pons and medulla emerges below with the cerebellum. related to muscle movement and tone. The medulla oblongata, at the upper end of the cord, contains the primary reflex mechanisms of respiration as well as cardiovascular and gastro-intestinal reflexes. The cerebellum is the head ganglion of the proprioceptive system. It receives information from the vestibular labyrinths regarding body position and movement, and from the muscles as to their position and movement. It influences muscle tone and coordinates the movements which may be initiated elsewhere in the central nervous system.

The cerebral hemispheres, taken together with the higher brain

FUNCTIONAL ORGANIZATION OF THE HUMAN BRAIN

stem that unites them, are related to these activities and also to acquired reactions, voluntary activities, and states of consciousness. The midbrain is well named since it plays an intermediary role. Together with the adjacent brain stem below, it contains centers for reflex regulation of muscle tone and body posture and movement. Thus it has a close functional relationship to the cerebellum and pons, medulla, and cord below it. But it is also oriented in the other direction. It includes an important part of the brain stem reticular system, and it provides neuronal mechanisms which seem to be essential to consciousness and the integration of function in the cerebral hemispheres.

It is not possible to point to any sharp functional frontier. There are no sharp functional levels between the higher brain stem, which includes the thalamus, and the lower brain stem which is primarily involved in the coordination of more primitive activities. Nevertheless, it is of interest to consider the result of transection through the midbrain. When an anaesthetic is given to a cat, for example, the cerebrum seems to be “put to sleep,” and so the animal is unconscious. If, then, a transection is made at the upper level of the midbrain and the cerebrum is thus removed, the animal does not wake up when the anaesthetic is withdrawn. It has become a decerebrate animal—an automatic motor mechanism. Sherrington showed that the decerebrate “preparation” could be “touched” into action in certain ways. It pulled its limb away from the thorn that pricked it. Milk in the mouth was swallowed; acid was rejected. It might stand and even walk a little, might vocalize and even purr. But the decerebrate animal has no “thoughts, feelings, memory, percepts, conations.” It moves and adjusts to an immediate environment. “The mindless body reacts with the fatality of a multiple penny-in-the-slot machine to certain stimuli.”* It is an automaton that seems to satisfy the conception of the most exacting materialist.

* The quotations are from Sir Charles Sherrington. Bazett and Penfield (1922) showed that the “chronic decerebrate preparation,” kept alive for weeks, maintained the general characteristics described by Sherrington in the decerebrate animal during the shorter experiments that he carried out.

THE CEREBRUM

C. The cerebrum

“It is then around the cerebrum, its physiological and psychological attributes, that the main interest of biology must ultimately turn.”* The mammal’s brain, in contradistinction to its lower brain stem, cerebellum, and spinal cord, has evolved with the distance receptors: the organs of smell, vision, and hearing. To some extent, the hemispheres are built upon the distance receptors. But in the case of man, as contrasted with other animals, vast new additional areas of cerebral cortex have made their appearance, crowding the sensory and the motor areas of the cortex down into the deep fissures. His olfactory areas, which bulk so large in the brain of lower animals, have shrunk to a position of negligible importance in olfactory bulb and temporal lobe.

There remain three important sensory areas in the human cortex (Fig. II-2 and Fig. II-5), which receive projected streams of nerve impulses as follows: 1) from the eyes, through lateral geniculate nuclei, to the primary visual sensory areas about each calcarine fissure on the mesial surfaces of each occipital lobe (see also Fig. II-9); 2) from the ears to the buried transverse gyri of Heschl, which form part of the first temporal convolution on each side and run deep into the lateral fissure of Sylvius; 3) from face, arm, leg, and body to the somatic sensory areas in the postcentral gyrus of each side.

There are also the primary somatic motor areas (Fig. II-2 and Fig. II-5), one in each hemisphere, which pass on the streams of nerve impulses that are projected downward through the ganglionic junctions in medulla and spinal cord to the muscles of face, limbs, and body. These outflowing impulses, thus, produce the action we call voluntary.

These are the four major areas of cortex that have what may be called trunk lines of communications through to the outside world (Cobb, 1944). Three bring in information; one sends out the stream of impulses that determines voluntary action, an important part of which is talking and writing.

Removal of the calcarine area of one occipital lobe produces

* This was the final sentence written by Sherrington in his Silliman Lectures, delivered at Yale University in 1904, and published as “The Integrative Action of the Nervous System.” Scribners, New York, 1906.

FUNCTIONAL ORGANIZATfON OF THE HUMAN BRAIN

MOTOR TRANSMITTING

SOMATIC SENSORY TRANSMITTING

(Postcentral 3

AUDITORY TRANSMITTING 

VISUAL TRANSMITTING

Fig. II-2. Projection areas of cortex. Transmitting areas on the lines of communication with environment. Three important sensory transmitting areas (stippled) pass afferent informative streams of impulses through the cerebral cortex to brain stem. The voluntary motor system carries impulses in a planned pattern through the motor transmitting area (lined) on the pre-central gyrus, out to the muscles that control voluntary movement. The functional contribution of the cortex to the sensory and motor streams that pass through it is not clear.

blindness in the opposite field of vision. Removal of one postcentral gyrus results in loss of discriminatory sensation from the opposite side of the body. Removal of one transverse gyrus of Heschl, on the other hand, affects hearing little, if at all, perhaps because the incoming auditory impulses pass from each ear to both auditory areas. Removal of the precentral gyrus results in permanent paralysis of skilled or delicate movements in the opposite hand and the foot. But voluntary movement of the proximal joints of arm and leg are not lost, and there is no more than a minor interference with movement of face and mouth after cortical removal of face area.

It must be added that there are additional motor and sensory areas (see Fig. II-10). The supplementary motor area projects its

THE CEREBRUM

impulses, outside the brain, down the spinal cord also (Bertrand, 1956). There is also a second somatic sensory area which receives impulses directly from the periphery. There are also secondary visual areas which surround the primary calcarine fissure. These secondary and supplementary areas can be removed, at least on one side, with little or no resultant deficit. There is also a probable secondary auditory area adjacent to the primary auditory area. The vestibular areas may well be nearby.

The other vast areas of the human cortex, such as those in the temporal, the anterior frontal, and the posterior parietal lobes, have connections within the brain itself which enable them to carry out functions that may be described as psychical rather than sensory or motor. But all areas of cortex are united with subcortical gray matter by means of two-way specific or non specific nerve fiber projection systems* (see Fig. II-3).

CEREBRAL CORTEX

■SSffiflfe 

LOWER J BRAINSTEM

Fig. II-3. Diagram of connections between brain stem and cerebral cortex corresponding with Fig. II-4. This illustrates the hypothesis that each functional area of cortex forms a unit with some portion of the diencephalon of which it is the developmental projection. After Penfield and Jasper, 1954.

♦Jasper: Subcortical Interrelationships, Chapter IV of Penfield and Jasper, 1954.

FUNCTIONAL ORGANIZATION OF THE HUMAN BRAIN D. Centrencephalic integration

It has long been assumed that the cerebral cortex was the summit, functionally speaking, and that the business of the mind was somehow transacted there. The transactions were somehow to be carried out there by means of “association” areas of cortex and the transcortical fiber systems. According to this conception, the visual information that came from the right half of the visual field to the left hemisphere would have to be made available across the hemisphere to the precentral gyrus, so that appropriately patterned impulses could be sent out to the right hand. But what about the other hand, and the other field of vision, and what about the plan of action?

It is obvious that the brain must have a central coordinating and integrating mechanism. If this “machine” is at all like other machines, there must be a place toward which streams of sensory impulses converge. There must be a place from which streams of motor impulses emerge to move the two hands in simultaneous, planned action. There must be neuronal circuits in which activity of both hemispheres is somehow summarized and fused—circuits the activation of which makes conscious planning possible.

From certain philosophical points of view the foregoing assumption might be denied at once. Since no one knows the nature of mental activity, it is as easy to conceive of it in relation to the surface of the two hemispheres in simultaneous neuronal action (and even in the peripheral nerves too) as it is to believe that it depends on a centrally placed zone of neurone circuits where neuronal activity is summarized and finally integrated.

But a neurophysiologist may not listen to such objections, especially when the evidence before him seems to indicate that such central integration is actually taking place. And a clinician, who is forced to take action in order to deal with patients, must construct a working hypothesis.

In 1936, in a paper entitled “The cerebral cortex and consciousness,”* the clinical evidence was examined with the following conclusion:

There is “evidence of a level of integration within the central nervous system that is higher than that to be found in the cerebral * The Harvey Lecture, New York Academy of Medicine, Penfield, 1938.

CENTRENCEPHALIC INTEGRATION

cortex.” There is a “regional localization of the neuronal mechanism involved in this integration” which is “most intimately associated with the initiation of voluntary activity and with the sensory summation prerequisite to it. . . . All regions of the brain may well be involved in normal conscious processes, but the indispensable substratum of consciousness lies outside the cerebral cortex, . . . not in the new brain but in the old . . . probably in the diencephalon.”

In 1946 it was pointed out* in a paper on “Highest Level Seizures” that Hughlings Jackson’s “highest level” of integration was located not in the frontal lobes, as he had suggested, but “in the diencephalon and mesencephalon.”

In 1950, in a report on “Epileptic automatism and centrence-phalic integrating system” (Penfield, 1952), it was proposed that, because of criticism of the term “highest level” as suggesting separation of one level from another, the word “centrencephalic” might be used to identify that system within diencephalon, mesencephalon, and probably rhombencephalon, which has bilateral functional connections with cerebral hemispheres. “The centrencephalic system” was then defined as “that central system within the brain stem which has been, or may be in the future, demonstrated as responsible for integration of the function of the hemispheres.”f

It has been suggested by our associate, Professor Herbert Jasper, that the definition should be enlarged to include “integration of varied specific functions from different parts of one hemisphere.” We are forced to agree with him. The subcortical coordinating centers which will be described for speech in this monograph are integrating areas within one hemisphere. Thus, although the centrencephalic system would not include the cranial nerve nuclei of the brain stem, it would include all those areas of subcortical gray matter (together with their connecting tracts) which serve the purposes of inter-hemispheral integration and intra-hemi-spheral integration.J

* Penfield and Jasper, 1947. Hughlings Jackson (1890) described “highest level fits” in his Lumelian Lectures. f The brain stem, as defined by Herrick, includes the thalamus on either side but not the cerebellum nor the cerebral cortex and their dependencies.

X It would seem that the corpus striatum, or basal mass of gray and white matter in each hemisphere, forms an extra-pyramidal motor mechanism and is probably not to be considered a part of the higher centrencephalic integrating system.

FUNCTIONAL ORGANIZATION OF THE HUMAN BRAIN

For the past twenty years this hypothesis has been tested by practical application in the problems of patients crowding through the Montreal Neurological Institute—men and women, conscious and unconscious, with lesions and local epileptic discharges in many different areas, patients seen in the consulting room, in the operating room, and also, alas, in the autopsy room. This experience taken with evidence from many other sources 1 * seems to leave no other hypothesis tenable and establishes it as a working-theory. The brilliant work of Morison, Magoun, Jasper, and many others on the reticular system with its “non specific” connections is a most important beginning of anatomical confirmation.

We will continue to assume, therefore, that there is a central integrating system situated within the higher brain stem. Integration reaches the highest level there, in the sense of Jacksonian philosophy, but it is presumably never divorced from the activity of some areas of the cortex and especially certain areas of the temporal lobes and the anterior portions of the frontal cortex.

Under normal circumstances, consciousness accompanies this combined activity. Consciousness disappears with interruption of function in the centrencephalic system.

Bits of evidence which support this thesis are many. Any area of cortex may be removed on either side without loss of consciousness. All areas except those devoted to speech have been removed in our clinic at one time or another during the treatment of focal cortical epilepsy or for the control of involuntary movements. Whether or not the centrencephalic system could function at all, if all areas of cortex could be removed at once in a single individual, is a question which cannot be answered. It must remain a matter of speculation as to the way in which the subject would still be considered “conscious.” On the other hand, any lesion, such as a tumor exerting pressure or some agent that interferes with the circulation in the higher brain stem, is accompanied by unconsciousness.

* Our very great indebtedness to other writers and associates is acknowledged in other publications, e.g. Penfield and Rasmussen, 1950; Penfield and Jasper, 1954; etc. The attempt in this monograph to present neurophysiological concepts to those who have little experience in the field might well be defeated if it bristled with such references.

SOME FUNCTIONAL SUBDIVISIONS OF THE CORTEX

E. Some functional subdivisions of the cortex

Each functional subdivision of the cerebral cortex of man may be looked upon as an outgrowth or projection outward of some area of gray matter in the older brain stem (Fig. II-4). Thus, the projected area in the newly formed cortex presumably serves to amplify and enlarge a function already being served in some sort of rudimentary manner by the old brain of more elementary animals.

For example, the anterior frontal cortex might be thought of as an elaboration from the dorso-medial nucleus of the thalamus, and much of the temporal cortex as an outward projection of the pulvinar and posterior part of the lateral nucleus of the thalamus (Fig. II-3). This is in many ways a surer guide by which to predict functional subdivision than the cyto-architectonic panellation of cortex (e.g. Brodmann’s areas, Fig. II-5).

As a preliminary to our discussion of speech it may serve a useful purpose to describe in general outline some of the functional areas of the cerebral cortex of man. These areas have been determined largely by electrical stimulation and by operative excision. The areas thus determined are sensory or motor or psychical.

1. Primary motor and sensory transmitting areas The primary motor transmitting area of the cerebral cortex is situated on the precentral gyrus largely within the central fissure of Rolando. Part of the cortico-spinal tracts take origin here. But the stream of neuronal impulses that produces voluntary activity does not originate in the cortex. It comes from a subcortical source (Penfield, 1954a). That this is so is supported by the fact that excision of the convolutions immediately in front of or behind the gyrus does not prevent a patient from controlling the contralateral hand and guiding it according to many sources of information (Fig. II-6). He can still direct the movement of this hand in accordance with visual information that enters the brain through the visual area in the occipital lobe of that side. Indeed, if the occipital lobe is amputated, he can still direct the hand, thanks to

FUNCTIONAL ORGANIZATION OF THE HUMAN BRAIN  

Fig. II-4. Projection of nuclear masses of the thalamus and geniculate bodies out to the cerebral cortex, as suggested by the thalamo-cortical connections. This is based on the work of Earl Walker (1938a, 1938b). Lateral surface above, mesial surface below. From Penfield and Jasper, 1954.

 SOME FUNCTIONAL SUBDIVISIONS OF THE CORTEXVoluntary Motor Somatic Sensory

‘ (Discriminative)

Olfactory

lobe Fig. II-5. Left hemisphere with some of Brodmann’s architectonic cortical subdivisions shown by numbers. The lobes are indicated as are some of the cortical transmitting stations for sensation and voluntary movement. Taste and visceral sensation, which have stations deep in the fissure of Sylvius and on the insula, are not indicated. Vestibular sensation, which probably has some cortical allotment near to the auditory area, is not shown. Most of the auditory area is buried in the fissure of Sylvius; most of the primary visual area, in the calcarine fissure; and the somatic area for discriminative sensation, within the central fissure of Rolando. The tracts which serve pain sensation are not shown; they end in the thalamus and make no essential detour to the cortex like those of the other forms of sensation.

the visual impulses that are entering the brain through the other occipital lobe.

Therefore, it seems reasonable to assume that the stream of impulses (broken line in Fig. II-7) which, by its pattern, determines how the hand is to move, must originate in a ganglionic area of the centrencephalic system. And since the hands are used so much together, it seems likely that the ganglionic area is intimately related to the source of the stream of impulses that is flowing in the other direction out to the motor area devoted to hand control in the other hemisphere.

The motor area of the cortex is therefore an arrival platform and a departure platform (Fig. II-7). Its function is to transmit

FUNCTIONAL ORGANIZATION OF THE HUMAN BRAIN 

Fig. II-6. Diagram to show the areas (lined) adjacent to the precentral gyrus, in which surgical excision does not deprive a man of capacity for voluntary movement by making use of the various sources of sensory information entering the brain through both hemispheres. and possibly to transmute, with the aid of secondary motor areas, the patterned stream of impulses which arises in the centrence-phalic system and passes on out to the target in voluntary muscles. There is, therefore, a supra-cortical or pre-cortical portion of the voluntary motor pathway and an infra-cortical or post-cortical portion, with a motor-transmitting strip of cortex uniting them. The somatic sensory area on the postcentral gyrus likewise is a transmitting strip. Excision of cortex posterior to it, or even of the precentral gyrus anterior to it, does not deprive the patient of the information which normally reaches this arrival platform. The

SOME FUNCTIONAL SUBDIVISIONS OF THE CORTEX

MOTOR 

Fig. II-7. Voluntary motor tracts. Cross-section through right hemisphere along the plane of the precentral gyrus. The pathway of control of voluntary movement is suggested from gray matter, somewhere in the higher brain stem, by the broken lines to the motor transmitting strip of the precentral gyrus. From there it runs down the corticospinal tract, as shown by the unbroken lines toward the muscles. The sequence of responses to electrical stimulation on the surface of the cortex (from above down, along the motor strip from toes through arm and face to swallowing) is unvaried from one individual to another. From Penfield and Jasper, 1954.

somatic sensory stream which comes in from the skin, muscles, and joints of the body goes to the postcentral gyrus (Fig. II-8), after ganglionic interruption in the lateral nucleus of the thalamus. But it must, therefore, return inward to join the centrencephalic system along a post-cortical limb (broken line in Fig. II-8).

FUNCTIONAL ORGANIZATION OF THE HUMAN BRAIN

SENSORY SEQUENCE 

Fig. II-8. Somatic sensation. Cross-section of the left hemisphere along the plane of the postcentral gyrus. The afferent pathway for discriminative somatic sensation is indicated by the unbroken lines coming up, through the medial lemniscus, to the transmitting strip on the postcentral gyrus, and from there on by the broken lines into the centrencephalic circuits of integration. There is, no doubt, close inter-relationship between sensory and motor activity of the units shown in this Figure and the preceding one, across the central fissure. From Penfield and Jasper, 1954.

Something similar may be said of the primary visual area of the cortex. It is located on the banks of the calcarine fissure situated in the mesial surface of the occipital lobe (Fig. II-9). Removal of the area produces complete blindness in the opposite visual field (homonymous hemianopsia) except perhaps for the small half disc of central vision. This disc at the center of the field of vision sends its afferent stream (it is claimed by some investigators) through the visual area of the ipsilateral cortex as well as the contralateral.

SOME FUNCTIONAL SUBDIVISIONS OF THE CORTEX

picture8

Fig. II-g. Diagram of pathway of visual sensory afferent impulses from the left side of the retina of each eye to the transmitting, cortex of the calcarine fissure of the left hemisphere, and then onward as suggested by the broken line into the circuits of integration. The left calcarine transmitting cortex receives information only from the right homonymous field, except perhaps in the small zone about the point of central visual fixation where, some say, both fields find representation. The secondary visual area has a relation to both visual fields. The stippled letter H indicates subcortical areas of intercommunication and integration.

Removal of adjacent occipital cortex, if the primary calcarine area is spared, does not produce blindness in the contralateral field and does not make it difficult for the patient to guide his hands or feet in accordance with what he sees in that field.

Consequently, it is evident that the primary visual sensory area of each hemisphere is a visual transmitting area. The visual pathway, from eye to centrencephalic system, may also be divided into a pre-cortical and a post-cortical portion. In Figure II-g the precorneal limb is a black line and the post-cortical limb, a broken line.

The rest of the cortex that makes up each occipital lobe may be called a secondary visual area. Its removal does not produce hemianopsia but electrical stimulation there, just as in the primary area, causes a conscious patient to see colors, lights, stars, shadows. All of the secondary sensory areas seem to be relatively dispensable. They will be discussed in a subsequent section.

FUNCTIONAL ORGANIZATION OF THE HUMAN BRAIN

By analogy, the auditory cortex on Heschl’s transverse gyri of each hemisphere is likewise a transmitting area—a way station in the stream of auditory impulses.

If these three important sensory areas (for somatic, visual, and auditory information) are used for nothing more than transmission, then the question follows: Why do these streams of sensory impulses make such a detour to the cortex after their synaptic interruption in the lateral nucleus of the thalamus? Furthermore, why should the voluntary motor pathway make a cortical detour?

These questions are clearly unscientific, since they are based on teleological thinking, on the assumption that all such arrangements have their purpose. Nevertheless, the questions deserve consideration, especially since no final answer can be given at the present!

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