Language is the comprehension and communication of abstract ideas. This cortical function is separate from the neural mechanisms related to primary visual, auditory, and motor function.
The ability to think of the right words, to program and coordinate the sequence of muscle contractions necessary to produce intelligible sounds, and to assemble words into meaningful sentences depends on Broca's area (areas 44 and 45) within the inferior frontal gyrus, located just anterior to the motor cortex controlling the lips and tongue.
The ability to comprehend language, including speech, is dependent on Wernicke's area. This area is located in the posterior part of the superior temporal gyrus within the auditory association cortex (area 22).
The arcuate fasciculus provides a crucial arc-shaped pathway within the hemisphere white matter, connecting Wernicke's and Broca's areas (Fig 21–1). Because the arcuate fasciculus connects the speech comprehension area (Wernicke's area) with the area responsible for production of speech (Broca's area), damage to this white matter tract produces impairment of repetition.
Central speech areas of the dominant cerebral hemisphere. Notice that Broca's and Wernicke's areas are interconnected via fibers that travel in the arcuate fasciculus, subjacent to the cortex.
Dysarthria is a speech disorder in which the mechanism for speech is damaged by lesions in the corticobulbar pathways; in one or more cranial nerve nuclei or nerves V, VII, IX, X, and XII; in the cerebellum; or in the muscles that produce speech sounds. Dysarthria is characterized by dysfunction of the phonation, articulation, resonance, or respiration aspects of speech.
Aphasia refers to loss or impairment of language function as a result of brain damage. Several distinct types of aphasia result from lesions in specific regions of the cerebral hemispheres (Table 21–1). In testing for aphasia, the clinician first listens to the patient's spontaneous speech and then explores the patient's speech during conversation. Speech may be categorized as fluent (more than 50 words per minute, effortless, absence of dysarthria, normal phrase length, and normal intonation). In contrast, nonfluent aphasia is effortful, with decreased verbal output (less than 50 words per minute), poor articulation, degradation of inflection and melodic aspects of speech, and agrammatism (ie, the tendency to omit small, grammatical words, verb tenses, and plurals and to use only nouns and verbs). Naming (usually examined by asking patients to name objects presented to them), repetition of phrases such as "dog," "automobile," "President Kennedy," "no ifs, ands, or buts," and comprehension of spoken language are also tested. Comprehsion can be assessed in patients with impaired speech output by observing the response to yes-no questions of graded difficulty ("Is your name John?" "Are we in a hospital room?" "Are we in a church?" "Do helicopters eat their young?").
Aphasia with Impaired Repetition
In most common forms of aphasia, the ability to repeat spoken language is impaired. Broca's, Wernicke's, and global aphasia are frequently seen in clinical practice.
Broca's aphasia is common, and is usually caused by a lesion in the inferior frontal gyrus in the dominant hemisphere (Broca's area; Fig 21–1). The patient has difficulty naming even simple objects. Repetition is impaired, but comprehension of spoken language is normal. The patient is usually aware of the deficit and appropriately concerned about it.
Most lesions that involve Broca's area also involve the neighboring motor cortex. Patients are often hemiplegic, with the arm more affected than the leg. Broca's aphasia often occurs as a result of strokes, most commonly affecting the middle cerebral artery territory.
This common form of aphasia is caused by a lesion in or near the superior temporal gyrus, in Wernicke's area (see Figs 21–1 and 21–2). Because this part of the cortex is not located adjacent to the motor cortex, there is usually no hemiplegia.
Magnetic resonance images of sections through the head. Top: Horizontal section with a large high-intensity area in the temporal lobe, representing an infarct caused by occlusion of a middle cerebral artery branch. Bottom: Coronal section showing the same area of infarction. (Parallel lines on the periphery of the brain represent artifacts caused by patient motion.) Large infarcts of this type, in the dominant cerebral hemisphere, can produce global aphasia that is accompanied by hemiparesis.
Patients with Wernicke's aphasia have fluent speech, but repetition and comprehension are impaired. The patient usually has difficulty naming objects and produces both literal paraphasias (eg, "wellow" instead of "yellow") and verbal paraphasias (eg, "mother" instead of "wife"). Neologisms (meaningless, nonsensical words, eg, "baffer") are used commonly and speech may be circumlocutory (ie, wordy but meaningless). Patients with Wernicke's aphasia usually do not appear concerned about, or even aware of, their speech disorder. Wernicke's aphasia commonly occurs as a result of embolic strokes.
Large lesions in the dominant hemisphere, which involve Broca's area in the frontal lobe, Wernicke's area in the temporal lobe, and the interconnecting arcuate fasciculus, can produce global aphasia (see Fig 21–2). In this nonfluent aphasia, both repetition and comprehension are severely impaired. Global aphasia most commonly occurs as a result of large infarctions in the dominant hemisphere, often because of occlusion of the carotid or middle cerebral artery.
In this unusual aphasia, verbal output is fluent and paraphasic. Comprehension of spoken language is intact, but repetition is severely impaired. Naming is impaired, although the patient often is able to select the correct name from a list. Conduction aphasia is a result of a lesion involving the arcuate fasciculus, in the white matter underlying the temporal–parietal junction; this lesion disconnects Wernicke's area from Broca's area.
Aphasias with Intact Repetition
In these unusual aphasias, repetition is spared, but comprehension is impaired. These aphasias are also referred to as transcortical aphasias because the lesion is usually in the cortex surrounding Wernicke's or Broca's area, or both. Depending on the location of the lesion, these aphasias may be fluent or nonfluent and comprehension may be impaired or preserved.
Anomia (difficulty finding the correct word) can occur in a variety of conditions, including toxic and metabolic encephalopathies. When anomia occurs as an aphasic disorder, speech may be fluent but devoid of meaning as a result of word-finding difficulty. The patient has difficulty naming objects. Comprehension and repetition are relatively normal. The presence of anomic aphasia is of little value in localizing the area of dysfunction. Focal lesions throughout the dominant hemisphere or, in some cases, in the nondominant hemisphere, can produce anomic aphasia, and anomia is also commonly present in toxic and metabolic encephalopathies.
Alexia (the inability to read) can occur as part of aphasic syndromes or as an isolated abnormality. Aphasic alexia refers to impaired reading in Broca's, Wernicke's, global, and isolation aphasias.
This disorder, in which there is impairment of reading and writing, is seen with pathologic lesions at the temporal– parietal junction area, particularly the angular gyrus. Because lesions of the angular gyrus also produce Gerstmann's syndrome (see later section in this chapter) and anomia, the constellation of agraphia, Gerstmann's syndrome, and anomia may occur together.
B. Alexia without Agraphia
Alexia without agraphia is a striking disorder in which the patient is unable to read, although writing is not impaired. Patients with this disorder are capable of writing a paragraph but, when asked to read it, cannot do so.
This syndrome occurs when there is damage to the left (dominant) visual cortex and to the splenium of the corpus callosum (Fig 21–3). As a result of damage to the left visual cortex, there is a right-sided homonymous hemianopsia and written material in the right half of the visual world is not processed. Written material presented to the left visual field is processed in the visual cortex on the right side. However, neurons in the visual cortex on the two sides are normally interconnected via axons that project through the splenium. As a result of damage to the splenium, visual information in the right visual cortex cannot be transmitted to the visual cortex in the left (dominant) hemisphere and, thus, is disconnected from the speech comprehension (Wernicke's) area.
Neuroanatomic basis for the syndrome of alexia without agraphia. Damage to two regions (the visual cortex in the left, speech-dominant hemisphere and the splenium of the corpus callosum, which carries interhemispheric axons connecting the two visual cortices) is required. These regions are both irrigated by the posterior cerebral artery. Thus, occlusion of the left posterior cerebral artery can produce this striking syndrome.
Most commonly, this disorder occurs as a result of infarctions in the territory of the posterior cerebral artery on the left, which damage both the left-sided visual cortex and the posterior part of the corpus callosum. An example is shown in Figure 21–4.
Magnetic resonance image showing lesions in the left occipital lobe and splenium of the corpus callosum in a 48-year-old man who suddenly developed a right superior quadrantanopsia and alexia without agraphia.
Agnosia—difficulty in identification or recognition—is usually considered to be caused by disturbances in the association functions of the cerebral cortex. Astereognosis is a failure of tactile recognition of objects and is usually associated with parietal lesions of the contralateral hemisphere. Visual agnosia, the inability to recognize things by sight (eg, objects, pictures, persons, spatial relationships) can occur with or without hemianopsia on the dominant side. It is a result of parieto-occipital lesions or the interruption of fibers in the splenium of the corpus callosum.
Prosopagnosia is a striking syndrome in which the patient loses the ability to recognize familiar faces. The patient may be able to describe identifying features such as eye color, length and color of hair, and presence or absence of a mustache. However, even spouses, friends, or relatives may not be recognized. Although the anatomic basis for this syndrome remains controversial, lesions in the temporal and occipital lobes, in some cases bilateral, have been suggested to be causative.
Unilateral neglect is a syndrome in which the patient fails to respond to stimuli in one half of space, contralateral to a hemispheric lesion. The patient may fail to respond to visual, tactile, and auditory stimuli. In its full-blown form, the syndrome is very striking: The patient may bump into things in the neglected visual field, will fail to dress or shave the neglected half of the body, and will be unaware of motor or sensory deficits on the neglected side. The unilateral neglect may be especially apparent when the patient is asked to draw a flower or fill in the numbers on a clock (Fig 21–5).
Unilateral (left-sided) neglect in a patient with a right hemispheric lesion. The patient was asked to fill in the numbers on the face of a clock (A) and to draw a flower (B).
Unilateral neglect is commonly seen as a result of parietal lobe damage but is also found after injury to other parts of the cerebral hemispheres (frontal lobe, cortical white matter, deep structures such as basal ganglia, etc). Unilateral neglect is most easily demonstrated following injury to the right cerebral hemisphere (left-sided unilateral neglect), as illustrated by the case shown in Figure 21–6.
Magnetic resonance image showing infarction in the territory of the right middle cerebral artery, in a history professor who presented with weakness on the left side and left-sided neglect. A contralateral neglect syndrome is often seen with right hemispheric lesions. The patient was not aware of his left-sided weakness, and failed to respond to stimuli on his left side. (Used with permission from Joseph Schindler, M.D., Yale Medical School.)
Anosognosia, the lack of awareness of disease or denial of illness, may occur together with the unilateral neglect syndrome. For example, patients with left hemiparesis often neglect the paralyzed limbs and may even deny that they are part of their body, attributing them to a doll or another patient. Even when the patient is aware of the deficit, he may not be appropriately concerned about it.
Apraxia, the inability to carry out motor acts correctly despite intact motor and sensory pathways, intact comprehension, and full cooperation, can occur following injury to a variety of cortical and subcortical sites. Ideomotor apraxia is the inability to perform motor responses upon verbal command, when these responses were previously carried out spontaneously. For example, the patient may fail to show his teeth on command, although he can do this spontaneously. Providing patients with objects to be used (eg, giving them a hairbrush and asking them to demonstrate how to brush their hair) leads to improvement of their performance. Damage to a variety of sites, including Broca's area, the corpus callosum, and the arcuate fasciculus, can cause ideomotor apraxia. Ideational apraxia is characterized by an abnormality in the conception of movements, so the patient may have difficulty doing anything at all, or may have problems sequencing the different components of a complex act although each separate component can be performed correctly. In ideational apraxia, introduction of objects to be used does not improve performance. Ideational apraxia may be seen after lesions of the left temporal–parietal–occipital area.
This tetrad of clinical findings includes right–left disorientation, finger agnosia (difficulty identifying or recognizing the fingers), impaired calculation, and impaired writing. The presence of this tetrad suggests dysfunction in the angular gyrus of the left hemisphere.