The Perisylvian Language Arc

Impulses conveying speech, music, and simple sounds travel from the ears along the acoustic (eighth cranial) nerves into the brainstem, where they synapse in the medial geniculate body. Then postsynaptic, crossed and uncrossed brainstem tracts bring the impulses to the primary auditory cortex, Heschl’s gyri, in each temporal lobe (see Fig. 4-16). Most music and some other sounds remain based in the nondominant hemisphere. In contrast, the brain transmits language impulses to Wernicke’s area, which is situated in the dominant temporal lobe. From there, they travel in the arcuate fasciculus, coursing posteriorly through the temporal and parietal lobes, and then looping anteriorly to the frontal lobe’s Broca’s area. This vital language center – located immediately anterior to the motor center for the right face, larynx, pharynx, and arm (Fig. 8-1) – receives processed, integrated language impulses, converts them to speech, and activates the adjacent motor cortex. A horseshoe-shaped region of cerebral cortex, which surrounds the sylvian fissure, the perisylvian language arc, contains Wernicke’s area, the arcuate fasciculus, and Broca’s area.

FIGURE 8-1 In the standard model of language function, the left cerebral hemisphere contains Wernicke’s area in the temporal lobe and Broca’s area in the frontal lobe. The arcuate fasciculus, the “language superhighway,” which connects these areas, curves posteriorly from the temporal lobe to the parietal lobe. It then passes through the angular gyrus and anteriorly to the frontal lobe. These structures surrounding the sylvian fissure, which comprise the perisylvian language arc, form the central processing unit of the language system. Note the proximity of Broca’s area to the motor strip’s representation of the face, throat, and upper extremity.

Using the perisylvian language arc model, researchers have established normal and abnormal language patterns. For example, when normal people repeat aloud what they hear, auditory impulses go first to Wernicke’s area, then pass around the arcuate fasciculus, and finally arrive in Broca’s area for speech production (Fig. 8-2, A). Reading aloud is a complicated variation of repeating aloud because reading requires both hemispheres and a learned system of decoding symbols, i.e., having been taught to read. As people read, their geniculocalcarine pathway transmits visual impulses from the lateral geniculate bodies to the calcarine (visual) cortex in both the left and right occipital lobes (see Fig. 4-1). Impulses from the left visual field go to the right occipital cortex. Then those impulses must travel through the posterior corpus callosum to reach the left (dominant) cerebral hemisphere. The impulses that have crossed from the right visual cortex merge with those already in the left hemisphere’s parietal lobe. Decoded, coherent language information then travels from the left parietal lobe via the arcuate fasciculus to Broca’s area for articulation (Fig. 8-2, B).

FIGURE 8-2 A, When people repeat aloud, language information arrives in Wernicke’s area, located adjacent to Heschl’s gyrus (see Fig. 4-16), and then travels through the parietal lobe in the arcuate fasciculus to Broca’s area. This area innervates the adjacent cerebral cortex for the tongue, lips, larynx, and pharynx. B, When people read aloud, visual impulses are received by the left and right occipital visual cortex regions. Both regions send impulses to a left parietal lobe association region (the oval), which converts text to language. Impulses from the left visual field, which are initially received in the right cortex, must first pass through the posterior corpus callosum to reach the language centers (see Fig. 8-4).

Language, of course, does not stand isolated from the brain’s other neuropsychologic functions. The language arc maintains reciprocal connections with cerebral cortical areas for memory, emotion, and similar domains. It also has strong connections with the thalamus, basal ganglia, and other subcortical structures.

Clinical Evaluation

Before diagnosing aphasia, the clinician must keep in mind normal language variations. Normal individuals may struggle and stammer when confronted with a novel experience, particularly a neurologic examination. Many have their own style and rhythm. Some may be reticent, uneducated, intimidated, or hostile. Others, before speaking, consider each word and formulate every phrase as though carefully considering which item to choose from a menu, but some blurt out the first thing on their mind.

In diagnosing aphasia, the clinician can use various classifications. A favorite distinguishes receptive (sensory) from expressive (motor) aphasia based on relative impairment of verbal reception versus expression. However, a major drawback of that classification is that most aphasic patients have a mixture of impairments that do not permit specific diagnosis.

The most useful classification of the aphasias, nonfluent/fluent, rests on the quantity of the patient’s verbal output (Table 8-1A). It suffices for clinical evaluations and roughly correlates with imaging studies. Aphasia aficionados subdivide nonfluent aphasia and fluent aphasia each into four categories based primarily on the language examination showing the presence or absence of the patient’s ability to comprehend, repeat, and name objects. When repetition ability remains intact in either nonfluent or fluent category, neurologists add the designation transcortical.

TABLE 8-1A Salient Features of the Nonfluent and Fluent Aphasias

Clinicians usually detect aphasia in a patient during the introductory conversation, history taking, or mental status examination. They perform a standard series of simple verbal tests to identify and classify the aphasia. The tests systematically evaluate three basic language functions: comprehension, naming, and repetition (Box 8-1). Depending on the clinical situation, the examiner may request increasingly difficult levels of comprehension, more uncommon objects, or more complicated phrases. The examiner may also perform the same testing with written requests and responses; however, with one notable exception, alexia without agraphia (see later), defects in written communication parallel those in verbal communication.

Nonfluent Aphasia

Localization and Etiology

Lesions responsible for nonfluent aphasias usually encompass, surround, or sit near Broca’s area (Fig. 8-3, A). The etiology is usually a middle cerebral artery stroke or other discrete structural lesion. Their location, not their pathology, produces the aphasia. Whatever the etiology, these lesions tend to be so extensive that they damage neighboring structures, such as the motor cortex and posterior sensory cortex. Moreover, because they are usually spherical or conical, rather than superficial two-dimensional lesions, they damage underlying white-matter tracts, including the visual pathway. Diffuse cerebral injuries, such as anoxia, metabolic disturbances or Alzheimer disease, rarely cause nonfluent aphasia.

FIGURE 8-3 A, Lesions that cause nonfluent aphasia are typically located in the frontal lobe and encompass Broca’s area and the adjacent cortex motor strip. B, Those causing fluent aphasia are in the temporoparietal region. They may even consist of diffuse injury, such as from neurodegenerative illnesses, and encompass Wernicke’s areas and more posterior regions. However, they usually spare the motor strip. C, Those causing conduction aphasia, which are relatively small, interrupt the arcuate fasciculus in the parietal or posterior temporal lobe. D, Those causing mixed transcortical (isolation) aphasia involve the watershed region, which encircles the perisylvian language arc.

Neurologists originally labeled nonfluent aphasia “expressive” or “motor” because of the prominent speech impairment. They also labeled all nonfluent aphasias “Broca’s” because of the responsible lesion’s location.

Fluent Aphasia

The four major subdivisions of fluent aphasia are the following (Table 8-1C):

Table 8-1C Fluent Aphasias

Of them, Wernicke’s is the epitome and most common. Its identifying characteristic is paraphasic errors or paraphasias, which are incorrect, meaningless, or even nonsensical words. Patients insert paraphasias into relatively complete, well-articulated, grammatically correct sentences that are spoken at a normal rate. However, paraphasias may render their conversation unintelligible. Moreover, patients typically cannot fully comprehend language or repeat simple phrases. Patients with its less severe variant, transcortical sensory aphasia, can repeat phrases, but otherwise their language impediments are similar.

Paraphasias most often consist of a word substitution, such as “clock” for “watch” or “spoon” for “fork” (related paraphasias), in which the substitute word arises from the same category. Less commonly, the words involved have little relation, such as “glove” for “knife” (unrelated paraphasia); or a nonspecific relation, such as “that” for any object (generic substitution). Paraphasias may also consist of altered words, such as “breat” for “bread” (literal paraphasia).

Paraphasias also include nonsensical coinages (neologisms), such as “I want to fin gunt in the fark.” Patient can bounce from one word to another with a close sound, but one with little or no shared meaning (clang associations, from the German klang, sound). For example, a patient making a clang association might ask, “What’s for dinner, diner, slimmer, finner?”

As if to circumvent their word-finding difficulty, fluent aphasia patients often speak in circumlocutions. They also tend toward tangential diversions or tangentialities, as though once having spoken the wrong word, they pursue the idea triggered by their error. These patients, caught in a tangentiality, may string together meaningfully related words until they reach an absurd point. For example, when attempting to name a pencil, the patient said, “pen … pence … paper … papal…”

Despite their loss of verbal communication, patients’ nonverbal expressions are preserved because nondominant-hemisphere functions remain unaffected. For example, prosody remains consistent with patients’ mood. Patients continue to express their feelings through facial gestures, body movements, and cursing. Similarly, most patients retain their ability to produce a melody even though they may be unable to repeat the lyrics. For example, patients might hum a tune, such as “Jingle Bells,” but if they attempt to sing it, paraphasias crop up in the middle of the lyrics.

Comorbid Depression

Psychiatrists face obstacles when assessing mood in aphasic patients because they tend to appear apathetic, cannot freely communicate, and offer potentially misleading facial expressions. Aphasic patients who apparently show symptoms of depression may actually be manifesting underlying dementia or pseudobulbar palsy. For example, following one or more strokes, patients may have aphasia along with dementia or poststroke depression (see Chapter 11). In another example, lesions damaging both frontal lobes reduce patients to a paucity or absence of speech with little emotion (abulia), responsiveness, or voluntary movement (akinesia). Here too, extensive frontal lobe damage invariably produces neurologic-based depressive symptoms (see frontal lobe disorders, Chapter 7).

On the other hand, physicians cannot attribute depression comorbid with aphasia entirely to brain damage. Most nonfluent aphasia patients remain aware of their impairments. They have suddenly lost their ability to communicate – a major loss. They naturally feel sad, hopeless, and frustrated. Treatment of depression comorbid with aphasia should follow the same strategies as treatment of depression without the comorbidity.

Dementia

Although aphasia by itself is not equivalent to dementia, it can be a component of dementia or it can mimic dementia. For example, when aphasia impairs routine communications – saying the date and place, repeating a series of numbers, and following requests – it mimics dementia. At times, patients with severe aphasia seem so bizarre that they appear incoherent. Because people think in words, aphasia also clouds cognition and memory.

Dementia and aphasia also differ in their time course. Dementia develops slowly, but aphasia begins abruptly, except in the unusual case when it heralds a neurodegenerative illness. Nonfluent aphasia further differs from dementia in its accompanying physical aspects: dysarthria and obvious lateralized signs, such as a right-sided hemiparesis and homonymous hemianopsia. Moreover, paraphasias frequently occur in fluent aphasia but rarely in dementia.

Nevertheless, patients occasionally have both aphasia and dementia. This combination often occurs with one or more strokes superimposed on Alzheimer disease and with frontotemporal dementia. These situations defy classification because aphasia usually invalidates standard cognitive tests.

Distinguishing aphasia from dementia and recognizing when the two conditions coexist are more than an academic exercise. A diagnosis of aphasia almost always suggests that a patient has had a discrete dominant cerebral hemisphere injury. Because a stroke or other structural lesion would be the most likely cause, the appropriate evaluation would include computed tomography (CT) or magnetic resonance imaging (MRI). In contrast, a diagnosis of dementia suggests that the most likely cause would be Alzheimer disease, frontotemporal dementia, or another diffuse neurodegenerative illness, and the evaluation might include various blood tests as well as CT or MRI.

Schizophrenia

Distinguishing fluent aphasia from schizophrenic speech can, theoretically at least, prove even more troublesome. Circumlocutions, tangential diversions, clang associations, and neologisms are common manifestations. As the thought disorder of schizophrenia develops, its language abnormalities increase in frequency and similarity to aphasia. In a different situation, previously healthy people suddenly developing aphasia can be so frightened and confused that they become agitated and irrational.

Despite these common and confounding elements, clinicians can discern many differences. Schizophrenic speech usually develops gradually in patients who are relatively young (in their third through fifth decades) and have had long-standing psychiatric illness. Their neologisms and other paraphasias occur infrequently and tend toward the inconspicuous. Unlike most patients with fluent aphasia, those with schizophrenia can repeat polysyllabic words and complex phrases.

In contrast, aphasia usually appears suddenly in the seventh or eighth decade. Except for some patients with fluent aphasia, most aphasia patients retain awareness that they cannot communicate and very often request help. Also, possibly because of self-monitoring, they keep their responses short and pointed. Any right-sided hemiparesis or hemianopsia clinches the diagnosis.

Other Disorders

Children with autism spectrum disorders may demonstrate language impairment – not only in their verbal expression, but also in their facial and bodily communication, such as failure to point. In many cases, nonsensical repetitions (stereotypies), idiosyncrasies, and echolalia overwhelm their speech. These children often fail to appreciate the nuance and affective components of language. In girls with Rett syndrome, language begins to regress after several years of normal development (see Chapter 13). Despite the possibility of a neurologic condition being responsible, when language regression appears in children, particularly boys and those younger than 3 years, they face a high probability of having an autistic disorder.

Mutism and other apparent language abnormalities are frequently manifestations of psychogenic disturbances (see Chapter 3). In these cases, apparent language impairment is usually inconsistent and amenable to suggestion. Acquired stuttering also often indicates a psychogenic impairment. For example, a patient with psychogenic aphasia might stutter and seem to be at a loss for words, but communicate normally by writing. An amobarbital infusion during an interview might reveal perfectly intact language function.

A common psychogenic aphasia-like condition is the sudden, unexpected difficulty in recalling the name (blocking) of someone who stirred a strong emotional response. The classic aphasia-like condition remains the Freudian slip, technically known as a parapraxis. Freud’s work on aphasia, which presaged his exploration of the unconscious, described his view of language circuitry and then words spoken in “error” due to repressed wish or conflict. Depending on their viewpoint, clinicians assessing everyday word substitutions may term them either paraphasias or insights into the unconscious. For example, when a physician’s former secretary, suspected of harboring a neurologic disorder, says that she has been Dr. So-and-So’s “medical cemetery,” a clinician could interpret the comment as her feelings about the competence of the doctor, an indication of the patient’s own fears of death, or a paraphasia referable to a dominant-hemisphere lesion.

Dyslexia

In most cases, reading impairment despite normal or near normal intelligence and education represents a developmental disorder, developmental dyslexia (Greek, lexis, word or phrase). When tested, approximately 10% of all schoolchildren display some degree of developmental dyslexia. Moreover, alone or comorbid with related problems, developmental dyslexia occurs in 80% of all children with learning disabilities. Teachers usually detect it when children first try to read, but occasionally mild forms escape detection until high-school or college students confront complicated reading tasks. In about 25% of children and, to a lesser extent, in some adults, dyslexia is comorbid with attention deficit hyperactivity disorder. Regardless of when it first appears and acknowledging that some educational strategies ameliorate the problem, developmental dyslexia persists throughout life. For dyslexic students who are otherwise bright, studying mathematics and science helps circumvent the disability.

Dyslexia affects boys with disproportionate severity and frequency, such that the boy : girl ratio lies between 2 : 1 and 5 : 1. Many children come from families where several other members also have dyslexia. In them, studies have implicated autosomal dominant and sex-linked genes. Imaging and pathologic studies reveal that the brains of some dyslexic individuals lack the normal planum temporale asymmetry. In other words, their brains are symmetric, which is abnormal.

In older children and adults, strokes, trauma, or other lesions can produce dyslexia or completely impair reading ability. This form of alexia, acquired alexia, in contrast to developmental alexia, is usually a component of aphasia and accompanied by right-sided motor deficits. More importantly, with one notable exception (see later), agraphia (inability to write) invariably accompanies acquired alexia.

Alexia and Agraphia

In the exception, alexia without agraphia (Fig. 8-4