INTRODUCTION: LANGUAGE DISORDERS IN ADULTS. Two Alternative Views Have Been Advanced on the Relationship Between Brain and Behavior.
Current views of nerve cells, the brain, and behavior have emerged relatively recently from a fusion, at the end of the nineteenth century, of four experimental traditions: neuroanatomy, physiology, biochemical pharmacology, and behavior.
Until the eighteenth century anatomists thought nervous tissue to be glandular in function. They considered nerves to be ducts conveying the fluid secreted by the brain and spinal marrow to the periphery.
Histology of the nervous system became a modern science during the nineteenth century, culminating in the investigations of Camille Golgi and Santiago Ramon y Cajal, who shared the sixth Nobel Prize for Medicine in 1906.
Golgi developed the histological silver impregnation methods that allowed visualization of the whole neuron with all its processes: the cell body, the dendrites, and the axon. Cajal developed some of the key conceptual insights and much of the empirical support for the neuron doctrine--the principle that the nervous system is made up of discrete signaling elements, the neurons.
Neurophysiology, the second scientific discipline fundamental to the modern view of nervous function, also began in the eighteenth century with the discovery by Luigi Galvani of the importance of electricity to animal physiology.
Again, it was during the nineteenth century that the foundations of electrophysiology were laid by Emil DuBois-Reymond and Hermann von Helm- Holtz.
Biochemical pharmacology started with Claude Bernard, Paul Ehrlich, and I. N. Langley, each of whom realized that drugs interact with specific receptor molecules on the surface of cells, an insight that became the basis of the modem study of chemical synaptic transmission.
The fourth discipline important for determining the relationship between brain and behavior has the longest history. It is difficult to trace the history of psychology briefly.
In the West, ideas about mind and soul are derived from antiquity; behavior, the manifestation of mind in the physical world, was not approached systematically until the nineteenth century, when the work of Charles Darwin on the evolution of behavior allowed psychology to develop as a discipline independent of philosophy and to become experimental.
Aspects of the merger of anatomy, physiology, and behavior can be traced to a series of experiments by Pierre Flourens, a French neurologist working in the nineteenth century who produced lesions of various parts of the nervous system of animals in order to examine how their behavioral capability was altered by the removal of that portion of the brain.
This approach led Flourens to conclude that the various sensory and motor functions are not localized to specific regions in the cerebral cortex. Thus, by the middle of the nineteenth century, it was generally believed that the cortex acted as a whole for each of its mental functions, and that any of its parts was able to perform all of its functions.
Injury to a specific area of the cortex would therefore affect all higher functions equally. The acceptance of this belief (subsequently called the aggregate field view of the brain) was based only partially on Flourens' experimental work.
It also represented a philosophical reaction against phrenologists who had argued that highly elaborate and abstract mental functions--including generosity, mother love, and secretiveness--were localized discretely in mosaic fashion to specific domains of the brain, domains that gave rise to identifiable bumps on the overlying skull.
At the end of the nineteenth century J. Hughlings Jackson, a British neurologist, broke with this aggregate field view. Jackson's clinical studies of focal epilepsy (convulsions beginning on one side of the body) showed that different motor and sensory activities are localized to different parts of the brain.
These studies were later elaborated systematically by the German neurologist Karl Wernicke and by Ramon y Cajal into an alternative view of brain function called cellular connectionism.
Cajal provided the histological basis for considering the neuron to be the signaling unit of the brain. He also showed that neurons connect to one another in a highly precise fashion. Wernicke showed that behavior is mediated by specific regions and through localizable pathways connecting sensory and motor structures.
The history of the dispute between the aggregate field and the cellular connection views of cortical function can best be illustrated in the analysis of language, the highest and most characteristic human function.
Before we consider the relevant clinical and anatomical studies concerned with the localization of language, it is useful to survey the structure of the brain.
(2) The brain stem, the rostral extension of the spinal cord, is subdivided into three regions: the midbrain, the pons, and the medulla. The brain stem receives information from the skin and muscles of the head and neck and in turn controls those muscles. The brain stem also contains collections of the cell bodies of most of the cranial nerves such as the auditory and vestibular nerves and is essential for processing the special senses.
(5) diencephalon (the thalamus, hypothalamus, sub thalamus, and epithalamus) is a key relay zone for transmitting information about sensation and movement and also contains (in the hypothalamus) important control regions for homeostatic (autonomic) integration.
To understand the localization of language, we are concerned primarily with the cerebral cortex. The cortex of each hemisphere is divided into four anatomically distinct regions called lobes the frontal, parietal, occipital, and temporal. Each lobe has a number of characteristic convolutions or infoldings (an old biological trick for increasing surface area).
The crests of the convolutions are called gyri. The intervening grooves are called sulci or (when deep and prominent) fissures. The more prominent gyri and the sulci are similar from one individual to another and have specific names with respect to each other (for example, precentral gyms, central sulcus, and postcentral gyrus).
The organization of the cerebral cortex is characterized by two important features. First each hemisphere is concerned primarily with sensory and motor processes of the contralateral side of the body.
Sensory information that enters the spinal cord from the left side of the body crosses over to the right side of the nervous system (either at the level of the spinal cord or subsequently at the level of the brain stem) before being conveyed to the cerebral cortex.
Second, although largely symmetrical in structure, the hemispheres are not completely symmetrical and the two hemispheres are not equivalent in function.
Much of what we know about the localization of normal language has come from the study of aphasia, a disorder of language that most commonly is found in patients who have suffered from stroke, an occlusion of a blood vessel supplying a portion of the cerebral cortex.
Many of the really important discoveries in the study of aphasia occurred in rapid succession during the last half of the nineteenth century and formed one of the most exciting chapters in the intellectual history of human psychology.
Broca described the case of a patient who could understand language but who had lost the ability to speak. Postmortem examination of the brain showed a lesion in the posterior portion of the frontal lobe (an area now called Broca's area).
Broca next collected eight cases, all of which showed a lesion at this site. In seven of the eight cases, the lesion existed in' the left half of the brain. This discovery led Broca to announce, in 1864, one of the most famous principles of brain function: “ Nous parlons avec l’hemisphere gauche!”(“We speak with the left hemisphere!”).
Broca also noted that rare exceptions to left hemispheric localization of speech occurred, and all were in left-handed patients. This observation in turn led to the generalization that there is a crossed relationship between hemispheric dominance and hand preference.
In 1870, nine years after Broca's initial discovery, Gustav Theodor Fritsch and Eduard Hitzig galvanized the scientific community with their discovery that characteristic movements of the limbs can be produced in dogs by electrically stimulating the precentral gyrus in front of the central sulcus.
Moreover Fritsch and Hitzig found that there was a cortical representation for the individual muscle groups and that the region of the cortex devoted to each group was small and discrete.
A further step was taken in 1876 by Karl Wernicke. At the age of 26 (having been out of medical school for only 4 years) Wernicke published a now classic paper entitled “The symptom Complex of Aphasia: A Psychological Study on an Anatomical Basis.”
In this paper, Wernicke described a new type of aphasia--an impairment of comprehension, a sensory as opposed to a motor malfunction. Whereas Broca's patient could understand but could not speak, Wernicke's patient could speak but could not fully comprehend. Wernicke's new type of aphasia also had a different locus from that described by Broca: it was located in the posterior part of the temporal lobe.
In addition to this discovery, Wernicke formulated a theory of aphasia that attempted to reconcile and extend the two existing theories of brain function.
The phrenologists had argued that the cortex was a mosaic of specific functions; even abstract mental attributes were localized to single, highly specific cortical areas.
The opposing aggregate field school argued that mental functions are not at all represented topographically. Wernicke used his findings and those of Broca, Fritsch, and Hitzig to argue that fundamental mental functions are discretely localized.
These functions are concerned with simple perceptual and motor activities. The elementary areas for these simple functions are interconnected in various ways.
More complex intellectual functions (with which the phrenologists concerned them- selves) arise out of the neural interactions among the simple perceptual and motor areas and are mediated by the pathways that interconnect them.
By extending the mosaic view of the brain into connectionist framework, Wernicke emphasized that the same function is processed in parallel in different regions of the brain (specific aspects of the function being processed at particular loci). Wernicke thereby initiated the notions of parallel and distributed processing that are so prominent in current thinking.
Wernicke applied this theory to his own work and that of Broca by analyzing the motor and sensory components of speech and their interactions.
Fritsch and Hitzig had found that stimulating the lower end of the precentral gyrus led to bilateral movements of the mouth, tongue, and palate. Wernicke therefore argued that Broca's area--which lies immediately in front of this motor area--coordinates the muscles of the mouth, tongue, palate, and vocal cords into coherent speech.
Next Wernicke considered the sensory component. The Viennese psychiatrist Theodor Meynert (the teacher of both Wernicke and Sigmund Freud) had found that the auditory pathway projected to Heschl’s gyrus in the temporal lobes.
Wernicke now argued that, as his patient's brain showed, the capacity for word selection is found near this zone; lesions in this area next to Heschl's gyrus cause aphasia with loss of comprehension.
Finally, Wernicke predicted a third type of aphasia (later discovered clinically) produced by a very different type of lesion from that in Broca's and Wernicke's aphasias.
This additional type of aphasia spared the receptive and motor speech zones, but destroyed the pathways connecting them by interrupting the arcuate fasciculus of the lower parietal region.
This syndrome, later called conduction aphasia, is characterized by incorrect word usage (paraphasia). Patients with paraphasia omit parts of words, substitute incorrect sounds in the word or use words incorrectly. They cannot repeat simple phrases although they understand words that are heard and seen and they can speak fluently.
Thus, at the,beginning of the twentieth century, there was compelling evidence that discrete areas of the cortex are involved in specific behaviors. However, surprisingly, the dominant view of the brain was not the cellular connection but the aggregate field view.
During the first half of this century a number of major neural scientists, including the British neurologist Henry Head, the German neuropsychologist Karl Goldstein, and the American psychologist Karl Lashley, continued to argue strongly for an aggregate field view.
The most influential of these proponents was Karl Lashley, Professor of Psychology at Harvard. Lashley attempted to find the locus of learning in the rat by studying the effects of various brain lesions on the complex task of learning to master a maze.
Lashley could not find any specific learning center; rather the severity of the learning defect produced by damage to the brain depended upon the extent of the damage and not on its precise location.
This discovery led Lashley--and, after him, many other psychologists--to conclude that learning did not have a special locus and therefore could not be related to specific neurons.
On the basis of these conclusions, Lashley formulated a theory of brain function called mass action, which minimized the importance of individual neurons and of specific neuronal connections.
Applying this logic to aphasia, Head and Goldstein argued that disorders of language cannot be attributed to specific lesions, but result from alterations in almost any cortical area. As a result of cortical damage, regardless of site, the patient regresses from a higher symbolic language to a simple, automatic verbal knowledge--from an abstract to a concrete language characteristic of aphasia.
Recently the work of Lashley and of Head has been reinterpreted. A variety of studies have demonstrated that maze learning, the task used by Lashley, is unsuitable for studying localization of function because it involves complex motor and sensory capabilities.
Deprived of one capability, an animal can still learn with another. In addition, a series of important clinical and experimental advances greatly strengthened the evidence for localization.
In the late 1950s Windel Penfield stimulated the cortex of conscious patients during brain surgery for epilepsy carried out with local anesthesia. As a necessary part of the surgical procedure, Penfield searched the cortex for areas that produced disorders of language upon stimulation.
His findings, based upon the verbal report of conscious subjects, dramatically confirmed the localization indicated by Wernicke's studies. Moreover, Penfield extended the studies of Fritsch and Hitzig to humans; he showed that the muscles of the body were represented in great topographical detail and the resulting map formed a motor homunculus.
Recently, these clinical studies have been synthesized and extended by Norman Geschwind at Harvard, who has pioneered in the modem study of asymmetrical representation of function in the human cerebral cortex.
Experimental results from applying cellular techniques to the central nervous system have led to similar conclusions. For example, developmental and physiological studies have indicated that individual nerve cells connect to one another in a precise way. As a result, individual cells respond only to specific sensory stimuli and not to others.