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Aphasia and Anosognosia
Since the inception of the discipline of neurology in the 19th century, neurologists have studied language, language impairment ( aphasia ), and related disorders to deduce how the normal brain functions and to advance the study of neurolinguistics. In practice, they test for language-related disorders, often striking in their presentation, to help localize and diagnose neurologic disease.
Aphasia appears prominently in many neurologic and psychiatric disorders and can disrupt cognition and halt fundamental mental functions. Nevertheless, it does not have a separate category in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) . The DSM-5 includes Language Disorder as one of the Neurodevelopmental Disorders, and Aphasia as a potential component of Neurocognitive Disorders.
Language and Dominance
The dominant hemisphere , by definition, governs language function and houses the brain’s language centers. The dominant hemisphere’s perisylvian language arc (see later) processes the most basic aspects of language, including language production and comprehension, reading, and writing. In its association areas , the dominant hemisphere also integrates language with intellect, emotion, and somatic, auditory, and visual sensations. Because of these crucial roles, the dominant hemisphere serves as the brain’s main portal for comprehension and expression of cognitive activity and emotions.
Language development begins in infancy, which is also the period of greatest brain plasticity (ability to undergo remodeling). Language circuitry is widely distributed in an infant’s brain. Wernicke’s area consolidates before Broca’s area, which explains why young children achieve the ability to understand speech before beginning to speak.
By 5 years of age, the brain establishes dominance for language. Afterwards, as vocabulary, verbal nuance, and intellectual complexity increase, plasticity declines. For example, once past puberty, children usually cannot learn a new (second) language without preserving traces of their native (primary) language, including its accent and grammar. Also, once a person has reached this stage, the nondominant hemisphere can no longer assume a meaningful role in language recovery following injury of the dominant hemisphere.
The dominant hemisphere controls languages learned in infancy, but it does not necessarily control those learned as adults (second languages). Nor does it monitor obscenities, which are usually expressions of emotion. The nondominant hemisphere governs prosody , which consists of speech’s inflection, rhythm, and tone. Many neurologists equate prosody with speech’s affective component (see later). Interestingly, certain prosodic and visual-spatial aspects of language, which are critical in tone-dependent languages (such as many East Asian and African languages) and sign language, respectively, depend on the dominant hemisphere.
For most people, cerebral hemisphere dominance extends to control of fine, precise, rapid hand movements (handedness) and, to a lesser degree, reception of vision and hearing. For example, right-handed people not only use on their right hand for writing and throwing a ball, they use their right foot for kicking, right eye when peering through a telescope, and right ear for listening to words spoken simultaneously in both ears ( dichotic listening) .
The superior temporal gyrus, the planum temporale, is distinctive in its exception to the general left-right anatomic symmetry of the brain. Its cortex in the dominant hemisphere has a much greater surface area than its nondominant counterpart because it has more gyri and deeper sulci. The relatively large cortical area of the dominant planum temporale provides greater language capacity. It probably also allows for greater musical ability because it is larger in musicians than nonmusicians, and largest in musicians with perfect pitch. However, this normal asymmetry is lacking or even reversed in many individuals with dyslexia, autism, Tourette disorder, and chronic schizophrenia—conditions with prominent language abnormalities.
Handedness
About 85% of all people are right-handed and correspondingly are left hemisphere-dominant. In addition, most left-handed people are actually left hemisphere-dominant or have mixed dominance.
Of people who are left-handed (Old English, lyft , weak, foolish; compare Latin, sinister and French, gauche ), some have sustained a congenital injury, which has often remained undiagnosed, to their left hemisphere that forced their right hemisphere to assume dominance. Compared to right-handed people, left-handed ones are over-represented among individuals with overt neurologic impairment, such as Intellectual Disability and epilepsy, and certain major psychiatric disorders, such as Schizophrenia and Autism Spectrum Disorder. Moreover, left-handed children are over-represented among those with many neuropsychologic abnormalities, including dyslexia, other learning disabilities, and stuttering. Economic studies have found that left-handed workers learn approximately 10% less than right-handed ones. Psychologic studies have found that left-handed compared to right-handed individuals scored approximately 10% lower on cognitive testing except if the individual’s mother was also left-handed, in which case the left-handed ones showed no difference.
However, in certain endeavors, being left-handed appears to confer some advantages. Left-handed people are disproportionately over-represented among musicians, artists, mathematicians, athletes, and recent US presidents. In the last several decades, left-handed presidents have included Gerald Ford, George H. W. Bush, Bill Clinton, Barack Obama, and, on most occasions, Ronald Reagan. Only Jimmy Carter, George W. Bush, and Donald Trump are right-handed. Also, left-handed athletes tend to perform better than right-handed ones in sports involving direct confrontation, such as baseball, tennis, table tennis, fencing, and boxing. Only a small fraction of their benefit comes from tactical advantages, such as a left-handed batter standing one or two steps closer to first base. In contrast, left-handed athletes achieve no greater success in sports without direct confrontation, such as swimming and running.
Unlike right-handed individuals, left-handed ones can develop aphasia after injury to either cerebral hemisphere. In addition, if left-handed individuals develop aphasia, its subtype relates less closely to the specific injury site (see later), and their prognosis is better than if right-handed individuals develop a comparable aphasia. These observations support the idea of mixed hemispheric dominance in left-handed individuals.
Ambidextrous individuals, who presumably have mixed dominance and are endowed with language, music, and motor skill function in both hemispheres, tend to excel in sports and playing musical instruments.
Sometimes neurologists need to determine a patient’s dominance. For example, when neurosurgeons must resect a portion of the dominant temporal lobe because it houses a tumor or generates seizures (see Chapter 10 ), they must avoid resecting language and memory areas. A devastating aphasia or memory impairment may complicate resection of an incorrect or too large an area. Using the Wada test —essentially injections of amobarbital directly into a carotid artery—neurologists can establish which cerebral hemisphere is dominant. When the amobarbital perfuses the dominant hemisphere, it renders the patient temporarily aphasic. Similarly, perfusion of one temporal lobe may cause temporary amnesia if the other temporal lobe is already damaged. As an alternative, functional magnetic resonance imaging (fMRI), which uses MRI to detect oxygenation and blood flow to particular brain areas while subjects perform a linguistic task, may indicate which hemisphere is dominant. The rest of this chapter assumes that the left hemisphere is dominant.
Music
Musically gifted people generally tend to process music, like language, in their dominant hemisphere. Those having perfect pitch—the ability to identify a tone in the absence of a reference tone—display distinctive fMRI patterns when listening to music. If these people develop aphasia, they also lose a great deal of their musical ability.
In contrast, the great majority of individuals without highly developed musical skills rely on their nondominant hemisphere to carry a tune. The proximity of their musical and emotional systems, both in the nondominant hemisphere, may explain the emotional effects music has on them. The primary location of music in the nondominant hemisphere for most people may also explain why many aphasic individuals, handicapped in language, often retain their ability to recognize music and to sing, an observation utilized in approaches to rehabilitation of aphasia ( melodic intonation therapy) .
Aphasia
The Perisylvian Language Arc
Impulses conveying speech, music, and other sounds travel from the ears along the acoustic (eighth cranial) nerves into the brainstem, where they synapse in the medial geniculate body. Crossed and uncrossed brainstem tracts bring the postsynaptic impulses to the primary auditory cortex, Heschl’s gyri , in each temporal lobe (see Fig. 4.16 ). Most music and some other sounds are processed in the nondominant hemisphere. In contrast, the brain processes language information in Wernicke’s area, situated in the dominant temporal lobe. From there, impulses travel in the arcuate fasciculus , coursing superiorly through the temporal and parietal lobes, and then anteriorly to Broca’s area in the frontal lobe. During its loop, the arcuate fasciculus allows communication with brain areas involved in all other functions. Broca’s area, its terminus, is a vital language center located immediately anterior to the motor center cortex representing the right face, larynx, pharynx, and arm ( Fig. 8.1 ). It receives processed, integrated language impulses, converts them to speech, and activates the adjacent motor cortex. Wernicke’s area, the arcuate fasciculus, and Broca’s area comprise a horseshoe-shaped region of cerebral cortex surrounding the Sylvian fissure that neurologists call the perisylvian language arc .
Based upon the perisylvian language arc model, researchers have described normal and abnormal language patterns. Under normal circumstances, when people repeat aloud what they hear, auditory impulses go first to Wernicke’s area, then pass through the arcuate fasciculus, and finally, arrive in Broca’s area for speech production ( Fig. 8.2A ). Reading aloud is a complicated variation of repeating aloud because it requires both hemispheres and a learned system of transforming written symbols into sounds. As people read, their visual pathways transmit impulses to the calcarine (visual) cortex in both the left and right occipital lobes (see Fig. 4.1 ). Impulses from the left visual field, which project to the right occipital cortex, travel through the posterior corpus callosum to reach the left (dominant) cerebral hemisphere, where they merge with information from the right visual field which went directly to the left occipital lobe. From the visual cortex, the impulses travel to areas in the left hemisphere such as the angular gyrus and supramarginal gyrus for linguistic decoding. Coherent language information then travels via the arcuate fasciculus to Broca’s area for articulation (see Fig. 8.2B ).
All along its path, the language arc maintains reciprocal connections with cerebral cortical areas for memory, emotion, and other neuropsychologic 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 when examining a patient. Normal individuals may struggle and stammer when confronted with a novel experience, particularly a neurologic examination. Many people have their own style and rhythm of speaking. 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, while some blurt out the first thing that comes to their mind.
In diagnosing aphasia, the clinician can use various classifications. One distinguishes receptive (sensory ) from expressive (motor) aphasia based on relative impairment of language production versus comprehension. However, a major drawback of that classification is that most aphasic patients display a mixture of impairments that does not permit a strict classification.
The most useful classification of the aphasias, nonfluent versus fluent , rests on the quantity and grammatical correctness of the patient’s verbal output ( Table 8.1A ). It suffices for clinical evaluations and roughly correlates with imaging studies.
Table 8.1A
Salient Features of the Nonfluent and Fluent Aphasias
| Feature | Nonfluent | Fluent |
|---|---|---|
| Other terms |
|
|
| Spontaneous speech | ||
| Content | Paucity of words, mostly nouns and verbs | Complete sentences with normal syntax |
| Articulation | Dysarthric, slow, stuttering | Good |
| Errors | Telegraphic speech | Paraphasic errors, circumlocutions, tangentialities, clang associations |
| Associated deficits | Right hemiparesis (arm, face, leg) | Hemianopsia, hemi-sensory loss |
| Localization of lesion | Frontal lobe |
|
Clinicians usually detect aphasia in a patient during the introductory conversation, history taking, or mental status examination. They then 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 ). Mildly affected patients may perform well with simple items but show difficulty with comprehension of more demanding materials, naming more uncommon objects, or repeating 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 generally parallel those in verbal communication.
Box 8.1
Clinical Evaluation for Aphasia
Spontaneous speech: fluent versus nonfluent
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Verbal tests
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Comprehension
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Ability to follow simple requests, “Please, pick up your hand.”
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Ability to follow complex requests, “Please, show me your left ring finger, and stick out your tongue.”
-
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Naming
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Common objects: tie, keys, pen
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Uncommon objects: watchband, belt buckle
-
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Repetition
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Simple phrases: “The boy went to the store.”
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Complex phrases: “No if’s, and’s, or but’s”
-
-
-
Reading and writing tests
Nonfluent Aphasia
Characteristics
The paucity of speech characterizes nonfluent aphasia. Patients say little and usually only speak in response to direct questions. Whatever speech they produce consists almost exclusively of single words and short phrases. They rely on basic words, particularly nouns and verbs without proper conjugation. They cannot use the connective tissue of language, such as adjectives, adverbs, and conjunctions. Their longer phrases typically consist of stock phrases or sound bites, such as “Not so bad” or “Get out of here.” Synonyms for nonfluent aphasia include “expressive” or “motor” aphasia due to the prominent impairment in language production.
Patients’ speech typically contains fewer than 50 words per minute, which is much slower than the normal 100 to 150 words per minute, and they produce it in a slow, effortful manner. Pauses interrupt the flow of nonfluent speech. Neurologists sometimes describe its jerky tempo as “telegraphic.” For example, in response to a question about food, a patient might stammer “Fork … steak … eat … no.”
Depending on the variety of nonfluent aphasia, patients cannot repeat simple phrases or name common objects. In contrast, most patients with nonfluent aphasia retain relatively normal comprehension that can be illustrated by their ability to follow simple verbal requests, such as “Please close your eyes” or “Raise your left hand, please.”
Nonfluent aphasia’s four major subdivisions are the following ( Table 8.1B ):
-
Broca’s aphasia : commonly occurring, classic nonfluent aphasia with comprehension intact and repetition lost
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Transcortical motor aphasia : similar to Broca’s, but repetition remains intact
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Mixed transcortical or isolation aphasia : with loss of fluency and comprehension but repetition remains intact
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Global aphasia : devastating, with loss of fluency, comprehension, and repetition (see later).
Table 8.1B
Nonfluent Aphasias
| Comprehension | Repetition | |
|---|---|---|
| Broca’s | Intact | Lost |
| Transcortical motor | Intact | Intact |
| Mixed transcortical | Lost (isolation) | Intact |
| Global | Lost | Lost |
Localization and Etiology
Lesions responsible for nonfluent aphasias usually encompass, surround, or sit near Broca’s area ( Fig. 8.3A ). Their etiology is usually a left 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, particularly the motor cortex and posterior sensory cortex. Moreover, because the lesions are usually spherical or conical, rather than superficial and two dimensional, they damage underlying white matter tracts, including the visual pathway. With the occasional exception of frontotemporal dementia (see Chapter 7 ), diffuse cerebral injuries, such as anoxia, metabolic disturbances, and neurodegenerative illnesses, including Alzheimer disease, rarely cause nonfluent aphasia.
Associated Deficits
Because the lesion causing nonfluent aphasia usually damages the adjacent motor cortex, right hemiparesis typically accompanies this aphasia. In such cases, the hemiparesis predominately affects the arm and lower face and causes poor articulation (dysarthria). Deeper lesions also sever the visual pathway and can cause a right homonymous hemianopsia (see Chapter 12 ). One of the most common syndromes in neurology is an occlusion of the left middle cerebral artery producing the combination of nonfluent aphasia, right-sided hemiparesis, and homonymous hemianopsia.
Another nonlanguage consequence of the lesions is buccofacial apraxia, also called “oral apraxia.” This apraxia consists of the inability to execute normal voluntary movements of the face, lip, and tongue. When buccofacial apraxia occurs in conjunction with nonfluent aphasia, it adds to the dysarthria.
To test for buccofacial apraxia, the clinician might ask patients to say, “La … Pa … La … Pa … La … Pa,” protrude their tongue in different directions, and pretend to blow out a match or suck through a straw. Patients with buccofacial apraxia will be unable to comply with these requests, but they may be able to use their same muscles reflexively or when provided with cues. For example, patients who cannot speak might sing, and those who cannot pretend to use a straw might be able to suck water through an actual one.
Patients who suffer aphasia due to stroke or traumatic brain injury (TBI) generally improve to some extent. Presumably, some ischemic areas of the brain recover and surviving neurons form new connections. Nevertheless, in terms of functional outcome, aphasia is generally more detrimental than hemiparesis.
Mixed Transcortical or Isolation Aphasia
Some lesions, which must be diffuse and extensive, damage the cerebral cortex surrounding the language arc. By sparing the language pathway, these lesions leave basic language function intact but removed from other cognitive functions. In mixed transcortical or isolation aphasia, which stems from such a cerebral injury, patients retain their ability to repeat what they hear; however, they cannot interact in a conversation, follow requests, or name objects. Because these patients can characteristically only duplicate strings of syllables, neurologists consider them to have nonfluent aphasia.
The signature of isolation aphasia is this disparity between patients’ seeming muteness and their preserved ability to repeat long and complex sentences. Patients may display echolalia , in which they mindlessly reiterate the words they hear readily, involuntarily, and sometimes compulsively. A cursory evaluation could understandably confuse this disturbance with irrational jargon.
This aphasia usually stems from the loss of the precarious blood supply of the cerebral cortex. While major branches of left middle cerebral artery perfuse the perisylvian arc, only thin, fragile, distal branches of middle, anterior, and posterior cerebral arteries perfuse its borders with the surrounding cortex (watershed area). When these vessels deliver insufficient blood to this portion of the cortex, it suffers a watershed infarction (see Fig. 8.3D ). Therefore, cardiac or respiratory arrest, suicide attempts using carbon monoxide, and other hypoxic insults cause isolation aphasia.
Global Aphasia
Extensive dominant hemisphere damage abolishes so much language function that it results in a severe form of nonfluent aphasia, known as global aphasia . Aside from uttering some unintelligible sounds, patients with global aphasia remain mute. Although they can follow some gestured requests, which bypass the language arc, they cannot comply with verbal ones. They also lack emotional responsiveness.
Comparably severe physical deficits (right hemiplegia, right homonymous hemianopsia, and conjugate deviation of the eyes toward the left) parallel the extensive language deficits. Causes frequently include internal carotid artery occlusions with extensive cerebral ischemia, dominant hemisphere tumors, cerebral hemorrhages, and gunshot wounds.
Global aphasia patients can sometimes express themselves with left hand gestures and shoulder shrugs. They may even be able to comply with some nonverbal requests, such as gestures. Their communication, albeit limited and silent, and their impaired comprehension, distinguish them from patients with psychogenic mutism.
Fluent Aphasia
-
Fluent aphasia’s four major subdivisions are the following ( Table 8.1C ):
Table 8.1CFluent AphasiasComprehension Repetition Wernicke’s Lost Lost Transcortical sensory Lost Intact Conduction Intact Lost Anomic Intact Intact -
Wernicke’s aphasia : common, with loss of comprehension, naming, and repetition
-
Transcortical sensory aphasia : similar to Wernicke’s except repetition remains intact
-
Anomic aphasia : inability to name objects
-
Conduction aphasia : inability to repeat
Of them, Wernicke’s aphasia is the epitome and most common subdivision. Its hallmark 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 nor repeat simple phrases. Patients with its less severe variant, transcortical sensory aphasia , can repeat phrases, but otherwise their language impediments are similar.
Paraphasias may consist of substitution of a word from the same category, such as “clock” for “watch” or “spoon” for “fork” ( semantic paraphasias ), or a word with similar sounds, like “waffle” for “watch” or “fort” for “fork” ( phonemic paraphasias ). 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 ).
Most strikingly, paraphasias include nonsensical coinages ( neologisms ), such as “I want to fin the glup in the sark.” Patients can bounce from one word to another with a similar sound but 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, thinner?”
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 without ever completing their initial point.
Despite their loss of verbal communication, patients’ nonverbal expressions are preserved because nondominant hemisphere functions remain unaffected. For example, patients’ prosody remains consistent with their 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 be able to hum the tune to “Jingle Bells,” but if they attempt to sing it, paraphasias crop up in the middle of the lyrics.
Associated Deficits
Unlike nonfluent aphasia, significant hemiparesis and other corticospinal tract signs do not accompany fluent aphasia because the responsible lesion is distant from the cerebral cortex motor strip. For example, neurologists typically elicit right-sided hyperactive deep tendon reflexes (DTRs) and a Babinski sign, but not hemiparesis, in fluent aphasia patients. However, they may detect right-sided sensory loss or a visual field cut because the underlying lesions often interrupt sensory or visual cerebral pathways in the parietal or occipital lobes.
Fluent aphasia patients are often strikingly unaware of their paraphasias, unable to edit them, and oblivious to their listener’s consternation. Their nonsensical speech may mimic a psychotic patient’s speech (see later). Moreover, sometimes these patients, frustrated by their inability to communicate, develop anxiety, agitation, or paranoia. Clinicians not seeing any physical deficit may not appreciate the neurologic basis of their patient’s abnormal language, thought, or behavior.
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