Chapter Summary
Study guidelines
- 1.
List what clinical functions appear to be asymmetrically distributed in the brain
- 2.
Describe and contrast the clinical outcome of injuries to the Broca and Wernicke areas.
- 3.
Describe the ‘steps’ performed when reading in relationship to areas of the brain involved.
- 4.
Discuss the different clinical manifestations of right parietal lobe injures.
- 5.
Discuss the different clinical manifestations of frontal lobe injures.
The two cerebral hemispheres are asymmetrical in certain respects. Some of the asymmetries have to do with handedness, language, and complex motor activities, but other more subtle differences exist. (Limbic asymmetries are described in Chapter 34.)
Handedness
Handedness often determines the hemisphere that is dominant for motor control. Left-hemisphere/right-hand dominance is the rule. Advances in ultrasound technology have made it possible for motor behaviour in the foetus to be observed, and it has been noted that handedness (and brain asymmetries) are already established before birth on the basis of the preferred hand used for thumb sucking during foetal life.
In 96% of right-handed subjects the left hemisphere is dominant for language, and while this asymmetry of language dominance is true for the majority of left-handed subjects, it varies with respect to the ‘strength’ of their left handedness. Those who are strongly left handed have a higher incidence of right hemisphere language dominance (27%), while in those who are ambidextrous the right hemisphere is dominant in 15%. Handedness, hemispheric language dominance, and other left versus right body asymmetries may represent a polygenic trait, but not limited to humans and brain asymmetries exist in the great apes and other vertebrates.
Language areas
Our classical way of understanding language was to assign function to discrete areas of the cortex, while separating language production from comprehension. In the clinical setting this remains a useful conceptualisation for localisation and is often referred to as the Wernicke-Lichtheim-Geschwind model , after those individuals who pioneered clinical studies of language. Components of this model remain useful, but it is now understood that language depends on multiple areas of the cortex (and subcortical structures). Production and comprehension of language are linked and the system is dynamic, so function appears to depend on the particular neuronal network that is active at the time.
Broca area ( Figure 32.1 )
The French pathologist Pierre Broca assigned a ‘motor’ speech function to the inferior frontal gyrus of the left side in 1861. The principal area concerned occupies the pars opercularis and pars triangularis parts of the inferior frontal gyrus corresponding to Brodmann areas 44 and 45. (Adjacent Brodmann area 47 and the ventral part of area 6 are also involved in language processing.)
Lesions involving the Broca area result in a language disorder referred to as an expressive aphasia (see Clinical Panel 32.1 ), but as will be seen, all language disorders can be considered expressive. Within the Broca area there now appears to be functional separation because some areas (and their connectivity) serve a role in phonology (how sounds are organised and used in natural languages), syntax (arrangement of words and phrases to create well-formed sentences), and semantics (meaning of words, phrases, sentences, or even larger units). In addition, the Broca area (and its connectivity) may not be ‘language specific’ because it participates in other cognitive domains such as music and plays a role in other actions (increasing a listener’s attention when specific utterances occur, while decreasing attention to utterances when engaged in ‘cocktail speech’, or modifying a speaker’s utterances in ways that will enhance the meaning of his or her communication to a specific listener).
Aphasia is a disturbance of language function caused by a lesion of the brain. The usual cause is a stroke produced by vascular occlusion in the cortical territory of the left middle cerebral artery.
General neurologic evaluation
Particular attention is made to the presence of a hemiparesis (frontal lobe involvement), visual field deficit (occipital lobe or optic radiation), and apraxia (parietal lobe involvement); these findings further support localisation of the lesion producing the aphasia.
Language evaluation
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Spontaneous speech. Particular attention is made as to whether speech production is fluent (rate, quantity, and effort related to speech production) versus nonfluent (effortful), and to any evidence of paraphasias (incorrect words for the intended word). Word-finding difficulty in general is called anomia . (A condition that can be seen in aphasia, but more commonly results from poor muscle control leading to impaired articulation, is dysarthria ; language testing is otherwise normal in individuals so afflicted.)
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Repetition. Requires an individual to demonstrate that he or she can repeat a phrase without errors; one preferred phrase is ‘No ifs, ands, or buts’.
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Comprehension. Auditory comprehension can often be judged when the history is elicited. However, observing the response to simple commands, yes/no questions, and asking the individual to point to objects within the room should be employed.
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Naming. Usually assessed by having the person name objects within the room, body parts, or colours.
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Reading. Reading is evaluated by having the individual read a sentence out loud or by determining if he or she can silently read and follow a written command (e.g. ‘Close your eyes’); when impaired it is referred to as alexia .
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Writing. Evaluation requires more than having the individual write his or her name; when impaired this is referred to as agraphia . For evaluation ask the patient to write a short sentence that may describe how he or she is feeling, comment about the weather, or why he or she likes his or her favourite hobby. He or she should also write a sentence that is dictated to him or her.
Performing the different parts of this bedside evaluation provides clues that allow classification of the aphasia as well as a suggested site of cerebral involvement as indicated in Figure 32.2 . Some idealised findings on bedside evaluation are included in this table.
Aprosodia
Lesions of the right hemisphere may affect speech in subtle ways. Lesions that include area 44 on the right (corresponding to the Broca area on the left) tend to change the patient’s speech to a dull monotone ( nonaffective prosody ). On the other hand, lesions that involve area 22 on the right (corresponding to the Wernicke area) may lead to listening errors, such as being unable to detect inflections of speech; the patient may not know whether a particular remark is intended as a statement or as a question ( affective prosody ).
| Type | Fluency | Comprehension | Repetition | Naming | Site ( Figure 32.2 ) |
|---|---|---|---|---|---|
| Broca (e.g. anterior or motor aphasia) | Poor and effortful | Good | Poor | Poor | A |
| Lesion site—posterior portion of the inferior frontal gyrus (Broca area), and surrounding premotor, motor, and subcortical white matter. | |||||
| Wernicke (e.g. posterior or sensory aphasia) | Good, but with paraphasic errors | Poor | Poor | Poor | B |
| Lesion site—posterior third of the superior temporal gyrus (Wernicke area) | |||||
| Conduction aphasia | Good | Good | Poor | Good | C |
| Lesion site—supramarginal gyrus or primary auditory cortex and insular cortex | |||||
| Global aphasia | None | Very poor | Very poor | Very poor | D |
| Lesion site—involves the perisylvian area that includes the Broca area, the Wernicke area, and the cortex that is interposed between them | |||||
Developmental dyslexia
It is generally agreed that reading is a more skilled activity than speech, because it requires an exquisite level of integration of visual scanning and auditory (inner speech) comprehension.
Developmental dyslexia is a hereditary neurologic disorder of severe and persistent reading and/or spelling difficulties, despite normal intelligence; it represents a language problem with decoding or processing of sounds as a major contributor while comprehension is more intact. Phonological awareness (awareness of the sound structure of words) is a predictor of reading skills in languages with inconsistent orthographies (the representation of the sounds of a language by written or printed symbols) and serial naming in consistent orthographies.
Two commonly used classroom tests to detect phonological impairment (slow and inaccurate processing of the sound structure of language) are rhyming , for example to identify the eight letters in the alphabet that rhyme with the letter B, and to pronounce nonwords (pseudowords) within a word string, for example ‘door’, ‘ melse ’, ‘farm’, ‘ duve ’, ‘miss’.
Dyslexia is widespread across cultures, affecting an estimated 7% of children (comorbidities such as attention-deficit hyperactivity disorder and other language disorders may coexist and further impact school performance). There is a 30% incidence in siblings of affected children and a similar incidence in one or other parent. There is a slightly higher incidence in boys, and in left-handed children of either gender.
A consistent finding in PET and fMRI studies during reading is diminished activity (compared to peers) in the left temporoparietal region (areas 22, 39, and 40) associated with structural abnormalities. The causes of dyslexia are likely multifactorial, but candidate genes have been identified, some related to neuronal migration and axonal guidance, and there is the suggestion of gene-environment interactions ( bioecological genes ) because heritability declines with declining parental education.
Suggested references
Output from the Broca area does include cell columns in the face and tongue areas of the adjacent motor cortex, but to direct and focus attention and to ensure appropriate behavioural interactivity (e.g. waiting your turn to speak, speaking in the appropriate tone or manner) requires interaction with the dorsolateral prefrontal cortex, anterior cingulate gyrus, and parietal cortex. Connectivity with the temporal cortex as well as inferior parietal areas is necessary when accessing memories with respect to knowledge type and the associated phonological, syntax, and semantic forms. In view of these multiple roles the Broca area is at times referred to as the Broca region because different functional roles reflect its subparcellation in what appears to be an anterior-posterior and a dorsal-ventral direction.
Wernicke area ( Figure 32.1 )
The German neurologist Karl Wernicke made extensive contributions to the understanding of language processing in the late 19th century. He designated the posterior part of Brodmann area 22 in the superior temporal gyrus of the left hemisphere as a ‘sensory area’ concerned with understanding the spoken word. Lesions involving this Wernicke area in adults are associated with a receptive aphasia ( Clinical Panel 32.1 ).
The upper surface of the Wernicke area is called the planum temporale (temporal plane) ( Figure 32.3 ) and is located in the superior temporal gyrus just posterior to the primary auditory cortex (Heschl gyrus). The planum temporale facilitates spatiotemporal discrimination and identification of auditory stimuli that are crucial for speech ( phonemes ; the smallest unit of speech in a language that is capable of conveying a distinction in meaning) as well as being involved in or modulated by auditory attention when selecting stimuli from the left versus right ear. (The volume of cerebral cortex in the planum temporale is larger on the left side in 65% of right-handed subjects, but does not match the more than 90% left hemisphere dominance for speech.)
