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), patients demonstrate little or no impairment in comprehending speech or expressing themselves verbally or by writing; however, they simply cannot read. For example, patients can transcribe another person’s dictation and write their own thoughts, but then be unable to read their own handwriting. Alexia without agraphia, which should really be called “alexia with graphia,” results from a destructive lesion encompassing the dominant (left) occipital lobe and adjacent posterior corpus callosum. Aside from having a right homonymous hemianopsia, patients lack physical deficits.

FIGURE 8-4 Lesions that damage the left occipital lobe and the posterior corpus callosum cause alexia without agraphia. Patients are unable to see anything in their right visual field because of the left occipital cortex damage. Left visual images still reach the right cortex, but they cannot be transmitted to the left cerebral language centers because the critical posterior corpus callosum is damaged. With obstacles in both information routes (the right visual field cut and transfer of information to the dominant hemisphere), patients cannot comprehend written material presented to either visual field. In contrast, they can still write full sentences from memory, imagination, or dictation because these forms of information still reach the language centers.

Gerstmann Syndrome

Gerstmann syndrome, a classic disorder, consists of four neuropsychologic disturbances: acalculia (impaired arithmetic skills), finger agnosia (inability to identify fingers), left/right confusion, and agraphia. When all four elements occur, neurologists usually attribute the syndrome to a lesion in the angular gyrus of the dominant parietal lobe (see Fig. 8-1).

On the other hand, neurologists question the existence of this syndrome as a distinct clinical entity. Patients rarely display all the components and those with three or all four usually also have aphasia or dementia. Nevertheless, the elements, even if they do not constitute a syndrome, are useful. Clinicians seek these neuropsychologic disturbances in both adults with strokes and children with learning disabilities. For example, children with dyscalculia also frequently display poor handwriting (agraphia) and clumsiness from left/right confusion. Moreover, those impairments are often accompanied by subtle physical signs of dominant-hemisphere injury, such as right-sided hyperactive DTRs and a Babinski sign.

Apraxia

Apraxia, the motor system’s rough equivalent of aphasia, is inability to execute learned actions despite normal strength, sensation, and coordination. Neurologists attribute apraxia to disruption of links between the cerebrum’s motor and neuropsychologic centers, particularly the perisylvian language arc and frontal lobe executive centers.

Although apraxia can be readily differentiated from simple paresis, it is often comorbid with aphasia or dementia. In fact, apraxia often appears as a symptom of Alzheimer disease and other cortical dementias (see Chapter 7).

In assessing patients for apraxia, the examiner usually first tests their buccofacial (lips, face, tongue) and limb movements when they make gestures or perform “symbolic acts” (Table 8-2). Next, the examiner asks them to perform imagined actions (pantomime), first on pretend objects and then on actual ones. After seeing the examiner perform an action, patients with apraxia typically can copy it. For example, a patient with apraxia might not be able to follow the request “Please, pretend to salute an officer,” but after the examiner demonstrates the salute, the patient will duplicate it. Similarly, when patients with apraxia are handed an actual object, which gives them a cue, they can often perform the object’s intended action. For example, a patient with apraxia might not be able to pretend to use a comb, but when presented with one, the patient will use it appropriately.

As a general rule, inability to use a common tool, such as a comb or spoon, most reliably demonstrates apraxia. Further testing, depending on circumstances, includes performing a series of steps, copying figures, arranging matchsticks, walking, or dressing.

Patients typically remain unaware of their apraxia because they usually do not spontaneously attempt the various tests, such as saluting an unseen officer or using an imaginary screwdriver. Moreover, an unsophisticated clinician might incorrectly attribute the impairment to paresis or incoordination.

Despite its complexity, neurologists designate several clinically useful categories of apraxia. Ideomotor apraxia, the most common category, consists basically of the inability to convert an idea into an action. For example, patients with ideomotor apraxia cannot pantomime despite possessing a clear understanding and retaining the physical ability to comply. Clinicians might envision ideomotor apraxia as the result of a disconnection between cognitive or language regions and motor regions (Fig. 8-5). Almost invariably, a left-sided frontal or parietal lobe lesion gives rise to the apraxia. Thus, ideomotor apraxia often coexists with aphasia, particularly nonfluent aphasia, and inability of the right hand to pantomime.

FIGURE 8-5 In a schematic transaxial view, requests for normal movements travel to Wernicke’s area in the dominant (left) posterior temporal lobe. They then travel anteriorly to the motor regions and, through the anterior corpus callosum, to the contralateral motor strip. Interruptions of the path within the left cerebral hemisphere result in ideomotor apraxia of both arms, causing bilateral limb apraxia. Lesions in the anterior corpus callosum interrupt only those impulses destined to control the left arm and leg, causing unilateral left arm and leg ideomotor apraxia.

One of its two varieties, buccofacial apraxia, as previously discussed, is a feature of nonfluent aphasia. In the other variety, limb apraxia, patients cannot execute simple requests usually involving their right arm or leg. They cannot pretend to brush their teeth, turn a key, comb their hair, or kick a ball. When asked to pretend to use an object, these patients characteristically use their hand as though it were the actual object. For example, they will brush their teeth with their forefinger instead of pretending to hold a toothbrush.

In ideational apraxia, patients cannot conceive and then perform a sequence of steps. For example, they cannot pretend to fold a letter, place it into an envelope, address the envelope, and then affix a stamp. In contrast to ideomotor apraxia, which is associated with nonfluent aphasia, ideational apraxia is almost inseparable from dementia. In particular, ideational apraxia is a hallmark of frontotemporal dementia, where it reflects executive dysfunction. Alzheimer disease and multiple strokes, because they lead to dementia and impaired planning and execution, typically cause ideational apraxia.

In other sections, this book covers several other apraxias: Construction and dressing apraxias, which are typically manifestations of nondominant-hemisphere lesions (see later), and gait apraxia, which is a hallmark of normal-pressure hydrocephalus (see Fig. 7-10).

Hemi-Inattention

Patients with nondominant-hemisphere injury most prominently display hemi-inattention (hemispatial neglect). They typically ignore visual, tactile, and other sensory stimuli that originate from their left side (Fig. 8-6). For example, they disregard, fail to perceive, or misinterpret objects in their left visual field (Fig. 8-7). Sometimes men with this condition leave the left side of their face unshaven. In contrast to patients with homonymous hemianopsia, who usually develop some awareness and thus make compensatory movements to keep objects in the preserved visual field, those with hemi-inattention remain oblivious to their situation and make no normal exploratory eye or limb movements.

FIGURE 8-6 Simple bedside tests for hemi-inattention. A, The examiner has asked the patient, who has just sustained a right middle cerebral infarction, to bisect a horizontal line. The patient, neglecting a portion of the line’s left side, draws the vertical line off-center to the right and bisects only the line’s perceived segment. B, Then the examiner asks the patient to circle all the “A”s on the page. Again neglecting the left side of the page, the patient circles only those on the right. C, An examiner has asked a 4-year-old boy, recovering from right parietal trauma, to draw a man. Because of a combination of hemi-inattention and constructional apraxia (see later), he merely scribbles on the right side of the paper.

FIGURE 8-7 In a classic demonstration of left hemi–inattention, the “white coat test,” the patient neglects the left-sided stimulation and perceives only the examiner’s clothing in his right visual field. Even if the patient’s problem were simply a left homonymous hemianopsia, he still would have explored and discovered, with his intact right visual field, that the examiner was half dressed.

Another manifestation of hemi-inattention – extinction on double simultaneous stimulation [DSS] – occurs when an examiner stimulates both sides of a patient’s body, but the patient pays no attention to the left-sided stimulation. For example, when the examiner touches the left arm, the patient correctly reports that it was touched, but when the examiner touches both arms, the patient reports that only the right one was touched. Another example of DSS can occur with visual stimulation. A patient might correctly perceive a flash of light in the left visual field, but when the examiner simultaneously flashes light in both visual fields, the patient would report seeing it only in the right-sided field.

Constructional Apraxia

Another frequently occurring manifestation of nondominant-hemisphere injury is constructional apraxia, which is a visuospatial perceptual impairment. Patients with this disorder cannot organize visual information and integrate it with fine motor skills. For example, they cannot copy simple figures or arrange matchsticks in patterns (Fig. 8-8). However, in contrast to the other nondominant symptoms, clinicians cannot attribute constructional apraxia exclusively to a nondominant lesion. It also appears in tests of patients with mental retardation, diffuse cerebral dysfunction, and executive impairment from frontal lobe disease.

FIGURE 8-8 When asked to draw a clock, a patient with constructional apraxia drew an incomplete circle, repeated (perseverated) the numerals, and placed them unevenly. When attempting to copy the top left figure, the patient only repeated several lines. The patient also misplaced and rotated the position of the bottom left figure (see Fig. 2-8). Such abnormalities are not peculiar to nondominant-hemisphere injuries: They often appear in Bender-Gestalt and Wechsler Adult Intelligence Scale testing.

Dressing Apraxia

In dressing apraxia (impaired ability to dress), patients with a nondominant-hemisphere lesion may leave their left side partly undressed or they may completely fail to dress. Patients with dressing apraxia may not merely leave their left limbs out of their clothing, they may put both hands into one sleeve, misalign buttons, and become befuddled when presented with their clothing turned inside out. Moreover, patients remain unaware that their clothing is disheveled. Even though their dressing apraxia includes hemi-inattention and other perceptual impairments, neurologists label it an apraxia because of the initial, obvious problem with skilled movements.

Alien-Hand Syndrome

A remarkable form of hemi-inattention occurs in the alien-hand syndrome. In this disorder, a patient’s left hand retains at least rudimentary motor and sensory functions, but the patient cannot control its movements or appreciate its sensations. Without the patient’s awareness, the hand moves semipurposefully, independently making autonomous explorations and performing simple tasks, such as scratching or moving bedclothes. It often fondles the examiner. In an often-quoted example, a patient reported that her hand intermittently attempted to choke her. Patients may treat the hand as though it were an errant child, sometimes rebuking, slapping, or chastising it.

The alien-hand syndrome definition technically requires that the patient reveals two misperceptions: (1) the patient does not possess the hand and (2) because the hand’s movement takes place without the patient’s knowledge and control, another person (the alien) governs it. Most patients simply feel estranged from the hand or accept only a tenuous relationship with it. Nondominant parietal lobe injury underlies most cases, but some have occurred in the context of corpus callosum injury.

Anosognosia

Anosognosia – a term constructed by Babinski from the Greek (a, without; nosos, disease; gnosis, knowledge) – has come to describe an inability to acknowledge or a denial of a physical deficit. The classic example is denial of a left hemiparesis. Often an integral part of hemi-inattention, anosognosia carries great clinical importance.

Patients with anosognosia typically cannot identify the affected part of their own body (somatotopagnosia or autotopagnosia). For example, they might claim (deny) that the examiner’s limb is really theirs; do not recognize their own paretic limb; or refuse to accept that the obviously paralyzed limb is even weak. Sometimes they attribute (project) the weakened limb to a third person, such as another patient. That ploy holds instant appeal if that person has an immobilized limb from a stroke or fracture. Alternatively, even while accepting their hand’s weakness, patients might offer an improbable explanation (rationalize), such as that they merely fell asleep on it and its strength will return in a few hours, or that they simply do not wish to move it.

A physician finding anosognosia in a stroke holds more than academic interest. It complicates and delays recovery. Physicians might slowly and on several occasions introduce the idea that the patient has limitations in certain functions, but with participation in rehabilitation and the passage of time, improvement will occur.

Neurologists attribute anosognosia to a loss of afferent sensory input, particularly loss of proprioception. Clinical experience, however, shows that, while loss of sensation may be a prerequisite, it alone is insufficient. For example, patients with a thoracic spinal cord transection lose feeling in their lower trunk and legs, but they remain acutely aware of their paraparesis and sensory loss. Another theory attributes nondominant symptoms to disordered attentiveness and arousal arising from damage of the underlying thalamus and reticular activating system. In any case, patients’ premorbid personality and emotional state do not predict that they might develop this condition.

Aprosody

Aprosody constitutes the inability to recognize emotional or affective qualities of other individual’s speech. Thus, nondominant-hemisphere lesions interfere with patients’ capacity to feel emotions from others’ tone of voice. For example, a patient with aprosody would be unable to appreciate the contrasting feelings in the question “Are you going home?” asked first by a jealous hospital roommate and then by a gleeful child. On a more subtle level, patients also might not perceive the emotions conveyed by a spouse.

In addition, aprosody restricts the ability to impart emotional qualities to speech. With no inflection or style, patients’ speech sounds bland and unfeeling.

To assess prosody, the examiner refashions a short version of the aphasia examination. During spontaneous speech, the examiner notes the patient’s variations in volume, pitch, and emphasis. The examiner might have the patient ask a question, such as “May I have the ball?” in the manner of a friend and then a stern schoolteacher using appropriate vocal and facial expressions. The examiner then asks a similar question, impersonating the various characters while the patient tries to identify them. As an alternative, the examiner might ask the patient to describe pictures of people obviously displaying emotions.

Loss of nonverbal communication tends to accompany aprosody. In particular, patients lose meaningful face and limb expressions, popularly called “body language” or technically called speech’s paralinguistic component. These physical aspects of communication lend conviction, emphasis, and affect to spoken words. Indeed, such physical expressions seem independent and sometimes more credible than speech. Well-known examples are children crossing their fingers when promising, adults who wink while telling a joke, and people who smile while relating sad events.

Extending the idea that the nondominant hemisphere confers affect on language, several authors have suggested that the nondominant hemisphere governs perception and expression of emotion and other complex nonverbal processes. The dominant hemisphere, they suggest, is responsible for verbal, sequential, and analytic cognitive processes.

Split-Brain Syndrome

The most important disconnection syndrome referable to the corpus callosum is the split–brain syndrome. Now rare, this disorder previously most often resulted from a longitudinal surgical division of the corpus callosum (commissurotomy) performed by neurosurgeons in an effort to control intractable epilepsy (see Chapter 10). The commissurotomy almost completely isolated each cerebral hemisphere.

In cases of the split-brain syndrome, examiners may present certain information to only a single, isolated hemisphere. For example, examiners can show pictures, writing, and other visual information in one of the patient’s visual fields to present information to only the contralateral hemisphere (Fig. 8-9). Likewise, by having a blindfolded patient touch objects with the one hand, examiners can present tactile information to only the contralateral hemisphere. However, auditory information cannot be presented exclusively – only predominantly – to one hemisphere. (Because pathways are duplicated in the brainstem [see Fig. 4-16], sounds presented to one ear travel, after the medial geniculate synapse, to both hemispheres, but predominantly to the contralateral one.)

FIGURE 8-9 After a commissurotomy, patients typically have the split-brain syndrome. Each hemisphere can be tested individually by showing requests, objects, and pictures in the contralateral visual field. Top, Objects and written requests shown in the left visual field are perceived by the right visual field. Since connections to the ipsilateral motor area are intact, the left hand can copy figures. However, since the right hemisphere is unable to transmit information through the corpus callosum to the dominant left cerebral hemisphere, which governs language function, patients cannot read the requests or describe the objects. Although patients cannot speak of the feelings evoked by emotionally laden pictures shown in their left visual field, they have sympathetic, nonverbal responses. Bottom, Written requests and objects shown in the right visual field are perceived by the left hemisphere. Patients can read those written requests, copy those objects with the right hand, and comply with the requests; however, because the language areas cannot send information through the corpus callosum, the left hand cannot comply. When patients describe emotions portrayed in a picture, their language lacks affect, derived from the nondominant hemisphere.

The interruption of the corpus callosum prevents the right hemisphere from sharing most information with the entire brain, particularly the left hemisphere’s language centers. Thus, the right hemisphere’s information, experience, and emotion cannot reach the patients’ consciousness, at least at the level of verbal expression. For example, if an object is placed in a blindfolded patient’s left hand, the patient cannot name or describe it, and the right hand cannot choose an identical object. Similarly, if one hand learns to follow a maze, the other hand will have to be taught separately.

Not only can each hemisphere separately perceive visual and tactile information, but also each can separately perceive emotions. For example, if a humorous picture were shown to the right visual field, a patient might laugh and be able to describe the picture’s humorous content; however, if the same picture were shown to the patient’s left visual field, it might provoke an amused response but one that the patient could not verbalize or even fully comprehend. If a sad picture were shown in the left visual field and a humorous one in the right, the patient’s amused response would be distorted because of the conflict.

Split-brain studies have suggested that normal people have, in their two hemispheres, neuropsychologic systems that are independent, parallel, and capable of simultaneous reasoning. They also show that, in normal people, the systems usually complement each other, but they potentially conflict.