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INTRODUCTION: INTRODUCTION: LANGUAGE DISORDERS IN LANGUAGE DISORDERS IN

ADULTSADULTS

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Two Alternative Views Have Two Alternative Views Have Been Advanced on the Been Advanced on the Relationship Between Brain Relationship Between Brain and Behaviorand Behavior

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Current views of nerve cells, the Current views of nerve cells, the brain, and behavior have brain, and behavior have emerged relatively recently from emerged relatively recently from a fusion, at the end of the a fusion, at the end of the nineteenth century, of four nineteenth century, of four experimental traditions: experimental traditions: neuroanatomy, physiology, neuroanatomy, physiology, biochemical pharmacology, and biochemical pharmacology, and behavior. behavior.

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The anatomical complexity of The anatomical complexity of nervous tissue was not nervous tissue was not appreciated before the appreciated before the invention invention of the compound microscopeof the compound microscope. .

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Until the eighteenth century Until the eighteenth century anatomists thought nervous tissue anatomists thought nervous tissue to be glandular in function. They to be glandular in function. They considered nerves to be ducts considered nerves to be ducts conveying the fluid secreted by conveying the fluid secreted by the brain and spinal marrow to the brain and spinal marrow to the periphery. the periphery.

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Histology of the nervous system Histology of the nervous system became a modern science during became a modern science during the nineteenth century, the nineteenth century, culminating in the investigations culminating in the investigations of Camille Golgi and Santiago of Camille Golgi and Santiago Ramon y Cajal, who shared the Ramon y Cajal, who shared the sixth Nobel Prize for Medicine in sixth Nobel Prize for Medicine in 1906. 1906.

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Golgi developed the histological silver Golgi developed the histological silver impregnation methods that allowed impregnation methods that allowed visualization of the whole neuron with visualization of the whole neuron with all its processes: the cell body, the all its processes: the cell body, the dendrites, and the axon. Cajal developed dendrites, and the axon. Cajal developed some of the key conceptual insights and some of the key conceptual insights and much of the empirical support for the much of the empirical support for the neuron doctrine--the principle that the neuron doctrine--the principle that the nervous system is made up of discrete nervous system is made up of discrete signaling elements, the neurons.signaling elements, the neurons.

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Neurophysiology, the second Neurophysiology, the second scientific discipline fundamental to scientific discipline fundamental to the modern view of nervous the modern view of nervous function, also began in the function, also began in the eighteenth century with the eighteenth century with the discovery by Luigi Galvani of the discovery by Luigi Galvani of the importance of electricity to animal importance of electricity to animal physiology. physiology.

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Again, it was during the Again, it was during the nineteenth century that the nineteenth century that the foundations of electrophysiology foundations of electrophysiology were laid by Emil DuBois-were laid by Emil DuBois-Reymond and Hermann von Reymond and Hermann von Helm- Holtz. Helm- Holtz.

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Biochemical pharmacology started Biochemical pharmacology started with Claude Bernard, Paul Ehrlich, with Claude Bernard, Paul Ehrlich, and I. N. Langley, each of whom and I. N. Langley, each of whom realized that drugs interact with realized that drugs interact with specific receptor molecules on the specific receptor molecules on the surface of cells, an insight that surface of cells, an insight that became the basis of the modem became the basis of the modem study of chemical synaptic study of chemical synaptic transmission.transmission.

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The fourth discipline The fourth discipline important for determining the important for determining the relationship between brain and relationship between brain and behavior has the longest history. behavior has the longest history. It is difficult to trace the history It is difficult to trace the history of psychology briefly. of psychology briefly.

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In the West, ideas about mind and soul In the West, ideas about mind and soul are derived from antiquity; behavior, the are derived from antiquity; behavior, the manifestation of mind in the physical manifestation of mind in the physical world, was not approached world, was not approached systematically until the nineteenth systematically until the nineteenth century, when the work of Charles century, when the work of Charles Darwin on the evolution of behavior Darwin on the evolution of behavior allowed psychology to develop as a allowed psychology to develop as a discipline independent of philosophy and discipline independent of philosophy and to become experimental.to become experimental.

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Aspects of the merger of anatomy, Aspects of the merger of anatomy, physiology, and behavior can be traced physiology, and behavior can be traced to a series of experiments by to a series of experiments by Pierre Pierre FlourensFlourens, a French neurologist working , a French neurologist working in the nineteenth century who produced in the nineteenth century who produced lesions of various parts of the nervous lesions of various parts of the nervous system of animals in order to examine system of animals in order to examine how their behavioral capability was how their behavioral capability was altered by the removal of that portion of altered by the removal of that portion of the brain. the brain.

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This approach led Flourens to conclude This approach led Flourens to conclude that the various sensory and motor that the various sensory and motor functions are not localized to specific functions are not localized to specific regions in the cerebral cortex. Thus, by regions in the cerebral cortex. Thus, by the middle of the nineteenth century, it the middle of the nineteenth century, it was generally believed that the cortex was generally believed that the cortex acted as a whole for each of its mental acted as a whole for each of its mental functions, and that any of its parts was functions, and that any of its parts was able to perform all of its functions. able to perform all of its functions.

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Injury to a specific area of the Injury to a specific area of the cortex would therefore affect all cortex would therefore affect all higher functions equally. The higher functions equally. The acceptance of this belief acceptance of this belief (subsequently called the (subsequently called the aggregate fieldaggregate field view of the brain) view of the brain) was based only partially on was based only partially on Flourens' experimental work. Flourens' experimental work.

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It also represented a philosophical It also represented a philosophical reaction against phrenologists who had reaction against phrenologists who had argued that highly elaborate and abstract argued that highly elaborate and abstract mental functions--including generosity, mental functions--including generosity, mother love, and secretiveness--were mother love, and secretiveness--were localized discretely in mosaic fashion to localized discretely in mosaic fashion to specific domains of the brain, domains specific domains of the brain, domains that gave rise to identifiable bumps on that gave rise to identifiable bumps on the overlying skull.the overlying skull.

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At the end of the nineteenth At the end of the nineteenth century J. Hughlings Jackson, a century J. Hughlings Jackson, a British neurologist, broke with this British neurologist, broke with this aggregate field view. Jackson's aggregate field view. Jackson's clinical studies of focal epilepsy clinical studies of focal epilepsy (convulsions beginning on one side (convulsions beginning on one side of the body) showed that different of the body) showed that different motor and sensory activities are motor and sensory activities are localized to different parts of the localized to different parts of the brain. brain.

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These studies were later These studies were later elaborated systematically by the elaborated systematically by the German neurologist Karl German neurologist Karl Wernicke and by Ramon y Cajal Wernicke and by Ramon y Cajal into an alternative view of brain into an alternative view of brain function called cellular function called cellular connectionism. connectionism.

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Cajal provided the histological basis for Cajal provided the histological basis for considering the neuron to be the considering the neuron to be the signaling unit of the brain. He also signaling unit of the brain. He also showed that neurons connect to one showed that neurons connect to one another in a highly precise fashion. another in a highly precise fashion. Wernicke showed that behavior is Wernicke showed that behavior is mediated by specific regions and through mediated by specific regions and through localizable pathways connecting sensory localizable pathways connecting sensory and motor structures.and motor structures.

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The history of the dispute The history of the dispute between the aggregate field and between the aggregate field and the cellular connection views of the cellular connection views of cortical function can best be cortical function can best be illustrated in the analysis of illustrated in the analysis of language, the highest and most language, the highest and most characteristic human function. characteristic human function.

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Before we consider the relevant Before we consider the relevant clinical and anatomical studies clinical and anatomical studies concerned with the localization concerned with the localization of language, it is useful to survey of language, it is useful to survey the structure of the brain. the structure of the brain.

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Introduction to the Anatomy of Introduction to the Anatomy of the Central Nervous Systemthe Central Nervous System

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The central nervous system is a The central nervous system is a bilateral and essentially bilateral and essentially symmetrical group of structures, symmetrical group of structures, consisting of six main parts: consisting of six main parts:

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(1) The spinal cord receives (1) The spinal cord receives information from the skin and information from the skin and muscle and sends out motor muscle and sends out motor commands for movement. commands for movement.

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(2) The brain stem, the rostral extension of the (2) The brain stem, the rostral extension of the spinal cord, is subdivided into three regions: spinal cord, is subdivided into three regions: the midbrain, the pons, and the medulla. The the midbrain, the pons, and the medulla. The brain stem receives information from the skin brain stem receives information from the skin and muscles of the head and neck and in turn and muscles of the head and neck and in turn controls those muscles. The brain stem also controls those muscles. The brain stem also contains collections of the cell bodies of most contains collections of the cell bodies of most of the cranial nerves such as the auditory and of the cranial nerves such as the auditory and vestibular nerves and is essential for vestibular nerves and is essential for processing the special senses. processing the special senses.

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(3) The cerebellum is important (3) The cerebellum is important for modulating motor movement for modulating motor movement together with… together with…

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(4) the basal ganglia (the caudate (4) the basal ganglia (the caudate nucleus, the putamen, and the nucleus, the putamen, and the globus pallidus). globus pallidus).

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(5) diencephalon (the thalamus, (5) diencephalon (the thalamus, hypothalamus, sub thalamus, and hypothalamus, sub thalamus, and epithalamus) is a key relay zone for epithalamus) is a key relay zone for transmitting information about transmitting information about sensation and movement and also sensation and movement and also contains (in the hypothalamus) contains (in the hypothalamus) important control regions for important control regions for homeostatic (autonomic) integration. homeostatic (autonomic) integration.

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(6) The(6) The cerebral hemispheres cerebral hemispheres, , capped by the cerebral cortex, are capped by the cerebral cortex, are concerned with higher concerned with higher perceptual, cognitive, and motor perceptual, cognitive, and motor functions.functions.

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To understand the localization of To understand the localization of language, we are concerned primarily language, we are concerned primarily with the cerebral cortex. The cortex of with the cerebral cortex. The cortex of each hemisphere is divided into four each hemisphere is divided into four anatomically distinct regions called lobes anatomically distinct regions called lobes the the frontal, parietal, occipital, frontal, parietal, occipital, andand temporaltemporal. Each lobe has a number of . Each lobe has a number of characteristic convolutions or infoldings characteristic convolutions or infoldings (an old biological trick for increasing (an old biological trick for increasing surface area). surface area).

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The crests of the convolutions are called The crests of the convolutions are called gyri. The intervening grooves are called gyri. The intervening grooves are called sulci or (when deep and prominent) sulci or (when deep and prominent) fissuresfissures. The more prominent gyri and . The more prominent gyri and the sulci are similar from one individual the sulci are similar from one individual to another and have specific names with to another and have specific names with respect to each other (for example, respect to each other (for example, precentral gyms, central sulcus, and precentral gyms, central sulcus, and postcentral gyrus).postcentral gyrus).

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The organization of the cerebral The organization of the cerebral cortex is characterized by two cortex is characterized by two important features. First each important features. First each hemisphere is concerned hemisphere is concerned primarily with sensory and primarily with sensory and motor processes of the motor processes of the contralateral side of the body. contralateral side of the body.

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Sensory information that enters Sensory information that enters the spinal cord from the left side the spinal cord from the left side of the body crosses over to the of the body crosses over to the right side of the nervous system right side of the nervous system (either at the level of the spinal (either at the level of the spinal cord or subsequently at the level cord or subsequently at the level of the brain stem) before being of the brain stem) before being conveyed to the cerebral cortex. conveyed to the cerebral cortex.

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In a similar fashion, the motor In a similar fashion, the motor areas in one hemisphere exert areas in one hemisphere exert control over the movements of control over the movements of the opposite half of the body. the opposite half of the body.

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Second, although largely Second, although largely symmetrical in structure, the symmetrical in structure, the hemispheres are not completely hemispheres are not completely symmetrical and the two symmetrical and the two hemispheres are not equivalent hemispheres are not equivalent in function. in function.

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Cognitive Function Can Be Cognitive Function Can Be Localized Within the Cerebral Localized Within the Cerebral CortexCortex

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Much of what we know about the Much of what we know about the localization of normal language localization of normal language has come from the study of has come from the study of aphasia, a disorder of language aphasia, a disorder of language that most commonly is found in that most commonly is found in patients who have suffered from patients who have suffered from stroke, an occlusion of a blood stroke, an occlusion of a blood vessel supplying a portion of the vessel supplying a portion of the cerebral cortex. cerebral cortex.

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Many of the really important Many of the really important discoveries in the study of discoveries in the study of aphasia occurred in rapid aphasia occurred in rapid succession during the last half of succession during the last half of the nineteenth century and the nineteenth century and formed one of the most exciting formed one of the most exciting chapters in the intellectual chapters in the intellectual history of human psychology. history of human psychology.

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The initial advance occurred in The initial advance occurred in 1861 with the publication of a 1861 with the publication of a paper by the French neurologist paper by the French neurologist Pierre Paul Broca.Pierre Paul Broca.

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Broca described the case of a Broca described the case of a patient who could understand patient who could understand language but who had lost the language but who had lost the ability to speak. Postmortem ability to speak. Postmortem examination of the brain showed examination of the brain showed a lesion in the posterior portion a lesion in the posterior portion of the frontal lobe (an area now of the frontal lobe (an area now called Broca's area). called Broca's area).

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Broca next collected eight cases, all of Broca next collected eight cases, all of which showed a lesion at this site. In which showed a lesion at this site. In seven of the eight cases, the lesion seven of the eight cases, the lesion existed in' the left half of the brain. existed in' the left half of the brain. This This discovery led Broca to announce, in discovery led Broca to announce, in 1864, one of the most famous principles 1864, one of the most famous principles of brain function: “ Nous parlons avec of brain function: “ Nous parlons avec l’hemisphere gauche!”(“We speak with l’hemisphere gauche!”(“We speak with the left hemisphere!”)the left hemisphere!”)..

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Broca also noted that rare Broca also noted that rare exceptions to left hemispheric exceptions to left hemispheric localization of speech occurred, localization of speech occurred, and all were in left-handed and all were in left-handed patients. This observation in turn patients. This observation in turn led to the generalization that led to the generalization that there is a crossed relationship there is a crossed relationship between hemispheric dominance between hemispheric dominance and hand preference.and hand preference.

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Broca's work stimulated a Broca's work stimulated a wider search for the cortical loci wider search for the cortical loci of behavioral function--a search of behavioral function--a search that was soon rewarded. that was soon rewarded.

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In 1870, nine years after Broca's In 1870, nine years after Broca's initial discovery, Gustav Theodor initial discovery, Gustav Theodor Fritsch and Eduard Hitzig Fritsch and Eduard Hitzig galvanized the scientific community galvanized the scientific community with their discovery that with their discovery that characteristic movements of the characteristic movements of the limbs can be produced in dogs by limbs can be produced in dogs by electrically stimulating the precentral electrically stimulating the precentral gyrus in front of the central sulcus.gyrus in front of the central sulcus.

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Moreover Fritsch and Hitzig Moreover Fritsch and Hitzig found that there was a cortical found that there was a cortical representation for the individual representation for the individual muscle groups and that the region muscle groups and that the region of the cortex devoted to each of the cortex devoted to each group was small and discrete.group was small and discrete.

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A further step was taken in 1876 A further step was taken in 1876 by Karl Wernicke. by Karl Wernicke. At the age of 26 At the age of 26 (having been out of medical school (having been out of medical school for only 4 years) Wernicke published for only 4 years) Wernicke published a now classic paper entitled “The a now classic paper entitled “The symptom Complex of Aphasia: A symptom Complex of Aphasia: A Psychological Study on an Psychological Study on an Anatomical Basis.”Anatomical Basis.”

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In this paper, Wernicke described a new type In this paper, Wernicke described a new type of aphasia--an impairment of comprehension, of aphasia--an impairment of comprehension, a sensory as opposed to a motor malfunction. a sensory as opposed to a motor malfunction. Whereas Broca's patient could understand but Whereas Broca's patient could understand but could not speak, Wernicke's patient could could not speak, Wernicke's patient could speak but could not fully comprehend. speak but could not fully comprehend. Wernicke's new type of aphasia also had a Wernicke's new type of aphasia also had a different locus from that described by Broca: different locus from that described by Broca: it was located in the posterior part of the it was located in the posterior part of the temporal lobe.temporal lobe.

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In addition to this discovery, In addition to this discovery, Wernicke formulated a theory of Wernicke formulated a theory of aphasia that attempted to aphasia that attempted to reconcile and extend the two reconcile and extend the two existing theories of brain existing theories of brain function. function.

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The The phrenologistsphrenologists had argued had argued that the cortex was a mosaic of that the cortex was a mosaic of specific functions; even abstract specific functions; even abstract mental attributes were localized mental attributes were localized to single, highly specific cortical to single, highly specific cortical areas. areas.

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The opposing The opposing aggregate fieldaggregate field school argued that mental school argued that mental functions are not at all functions are not at all represented topographically. represented topographically. Wernicke used his findings and Wernicke used his findings and those of Broca, Fritsch, and those of Broca, Fritsch, and Hitzig to argue that fundamental Hitzig to argue that fundamental mental functions are discretely mental functions are discretely localized. localized.

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These functions are concerned These functions are concerned with simple perceptual and motor with simple perceptual and motor activities. The elementary areas activities. The elementary areas for these simple functions are for these simple functions are interconnected in various ways. interconnected in various ways.

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More complex intellectual More complex intellectual functions (with which the functions (with which the phrenologists concerned them- phrenologists concerned them- selves) arise out of the neural selves) arise out of the neural interactions among the simple interactions among the simple perceptual and motor areas and perceptual and motor areas and are mediated by the pathways are mediated by the pathways that interconnect them. that interconnect them.

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By extending the mosaic view of the By extending the mosaic view of the brain into connectionist framework, brain into connectionist framework, Wernicke emphasized that the same Wernicke emphasized that the same function is processed in parallel in function is processed in parallel in different regions of the brain (specific different regions of the brain (specific aspects of the function being processed aspects of the function being processed at particular loci). Wernicke thereby at particular loci). Wernicke thereby initiated the notions of parallel and initiated the notions of parallel and distributed processing that are so distributed processing that are so prominent in current thinking.prominent in current thinking.

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Wernicke applied this theory to Wernicke applied this theory to his own work and that of Broca his own work and that of Broca by analyzing the motor and by analyzing the motor and sensory components of speech sensory components of speech and their interactions. and their interactions.

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Fritsch and Hitzig had found that Fritsch and Hitzig had found that stimulating the lower end of the stimulating the lower end of the precentral gyrus led to bilateral precentral gyrus led to bilateral movements of the mouth, tongue, and movements of the mouth, tongue, and palate. Wernicke therefore argued that palate. Wernicke therefore argued that Broca's area--which lies immediately in Broca's area--which lies immediately in front of this motor area--coordinates the front of this motor area--coordinates the muscles of the mouth, tongue, palate, muscles of the mouth, tongue, palate, and vocal cords into coherent speech. and vocal cords into coherent speech.

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Next Wernicke considered the Next Wernicke considered the sensory component. The sensory component. The Viennese psychiatrist Viennese psychiatrist Theodor Theodor Meynert (the teacher of both Meynert (the teacher of both Wernicke and Sigmund Freud) Wernicke and Sigmund Freud) had found that the auditory had found that the auditory pathway projected to Heschl’s pathway projected to Heschl’s gyrus in the temporal lobes.gyrus in the temporal lobes.

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Wernicke now argued that, as his Wernicke now argued that, as his patient's brain showed, the patient's brain showed, the capacity for word selection is capacity for word selection is found near this zone; lesions in found near this zone; lesions in this area next to Heschl's gyrus this area next to Heschl's gyrus cause aphasia with loss of cause aphasia with loss of comprehension.comprehension.

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Finally, Wernicke predicted a Finally, Wernicke predicted a third type of aphasia (later third type of aphasia (later discovered clinically) produced discovered clinically) produced by a very different type of lesion by a very different type of lesion from that in Broca's and from that in Broca's and Wernicke's aphasias. Wernicke's aphasias.

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This additional type of aphasia This additional type of aphasia spared the receptive and motor spared the receptive and motor speech zones, but destroyed the speech zones, but destroyed the pathways connecting them by pathways connecting them by interrupting the arcuate interrupting the arcuate fasciculus of the lower parietal fasciculus of the lower parietal region. region.

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This syndrome, later called conduction This syndrome, later called conduction aphasia, is characterized by incorrect aphasia, is characterized by incorrect word usage (paraphasia). Patients with word usage (paraphasia). Patients with paraphasia omit parts of words, paraphasia omit parts of words, substitute incorrect sounds in the word or substitute incorrect sounds in the word or use words incorrectly. They cannot use words incorrectly. They cannot repeat simple phrases although they repeat simple phrases although they understand words that are heard and understand words that are heard and seen and they can speak fluently.seen and they can speak fluently.

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Thus, at the,beginning of the Thus, at the,beginning of the twentieth century, there was twentieth century, there was compelling evidence that discrete compelling evidence that discrete areas of the cortex are involved areas of the cortex are involved in specific behaviors. However, in specific behaviors. However, surprisingly, the dominant view surprisingly, the dominant view of the brain was not the cellular of the brain was not the cellular connection but the aggregate connection but the aggregate field view. field view.

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During the first half of this century a During the first half of this century a number of major neural scientists, number of major neural scientists, including the British neurologist including the British neurologist Henry Head, the German Henry Head, the German neuropsychologist Karl Goldstein, neuropsychologist Karl Goldstein, and the American psychologist Karl and the American psychologist Karl Lashley, continued to argue strongly Lashley, continued to argue strongly for an aggregate field view. for an aggregate field view.

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The most influential of these The most influential of these proponents was Karl Lashley, proponents was Karl Lashley, Professor of Psychology at Professor of Psychology at Harvard. Lashley attempted to Harvard. Lashley attempted to find the locus of learning in the find the locus of learning in the rat by studying the effects of rat by studying the effects of various brain lesions on the various brain lesions on the complex task of learning to complex task of learning to master a maze. master a maze.

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Lashley could not find any Lashley could not find any specific learning center; rather specific learning center; rather the severity of the learning defect the severity of the learning defect produced by damage to the brain produced by damage to the brain depended upon the extent of the depended upon the extent of the damage and not on its precise damage and not on its precise locationlocation. .

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This discovery led Lashley--and, This discovery led Lashley--and, after him, many other after him, many other psychologists--to conclude that psychologists--to conclude that learning did not have a special learning did not have a special locus and therefore could not be locus and therefore could not be related to specific neurons. related to specific neurons.

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On the basis of these On the basis of these conclusions, Lashley formulated conclusions, Lashley formulated a theory of brain function called a theory of brain function called mass actionmass action, which minimized , which minimized the importance of individual the importance of individual neurons and of specific neuronal neurons and of specific neuronal connections. connections.

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What was important according to What was important according to this this mass actionmass action or or aggregate aggregate fieldfield view was brain mass, not view was brain mass, not neuronal architecture.neuronal architecture.

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Applying this logic to aphasia, Head Applying this logic to aphasia, Head and Goldstein argued that disorders of and Goldstein argued that disorders of language cannot be attributed to specific language cannot be attributed to specific lesions, but result from alterations in lesions, but result from alterations in almost any cortical area. As a result of almost any cortical area. As a result of cortical damage, regardless of site, the cortical damage, regardless of site, the patient regresses from a higher symbolic patient regresses from a higher symbolic language to a simple, automatic verbal language to a simple, automatic verbal knowledge--from an abstract to a knowledge--from an abstract to a concrete language characteristic of concrete language characteristic of aphasia.aphasia.

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Recently the work of Lashley Recently the work of Lashley and of Head has been and of Head has been reinterpreted. A variety of studies reinterpreted. A variety of studies have demonstrated that maze have demonstrated that maze learning, the task used by learning, the task used by Lashley, is unsuitable for Lashley, is unsuitable for studying localization of function studying localization of function because it involves complex because it involves complex motor and sensory capabilities. motor and sensory capabilities.

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Deprived of one capability, an Deprived of one capability, an animal can still learn with animal can still learn with another. In addition, a series of another. In addition, a series of important clinical and important clinical and experimental advances greatly experimental advances greatly strengthened the evidence for strengthened the evidence for localization. localization.

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In the late 1950s In the late 1950s Windel PenfieldWindel Penfield stimulated the cortex of conscious stimulated the cortex of conscious patients during brain surgery for patients during brain surgery for epilepsy carried out with local epilepsy carried out with local anesthesia. As a necessary part of the anesthesia. As a necessary part of the surgical procedure, Penfield surgical procedure, Penfield searched the cortex for areas that searched the cortex for areas that produced disorders of language upon produced disorders of language upon stimulation. stimulation.

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His findings, based upon the verbal His findings, based upon the verbal report of conscious subjects, report of conscious subjects, dramatically confirmed the localization dramatically confirmed the localization indicated by Wernicke's studies. indicated by Wernicke's studies. Moreover, Penfield extended the studies Moreover, Penfield extended the studies of Fritsch and Hitzig to humans; he of Fritsch and Hitzig to humans; he showed that the muscles of the body showed that the muscles of the body were represented in great topographical were represented in great topographical detail and the resulting map formed a detail and the resulting map formed a motor homunculus. motor homunculus.

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Recently, these clinical studies Recently, these clinical studies have been synthesized and have been synthesized and extended by Norman Geschwind extended by Norman Geschwind at Harvard, who has pioneered in at Harvard, who has pioneered in the modem study of the modem study of asymmetrical representation of asymmetrical representation of function in the human cerebral function in the human cerebral cortex. cortex.

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Experimental results from applying Experimental results from applying cellular techniques to the central nervous cellular techniques to the central nervous system have led to similar conclusions. system have led to similar conclusions. For example, developmental and For example, developmental and physiological studies have indicated that physiological studies have indicated that individual nerve cells connect to one individual nerve cells connect to one another in a precise way. As a result, another in a precise way. As a result, individual cells respond only to specific individual cells respond only to specific sensory stimuli and not to others.sensory stimuli and not to others.