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Monday, November 23, 2009

Localization within the cerebral cortex

This subject causes unnecessary difficulty. Work on neuronal networks, functional imaging and plasticity within the brain questions traditional views of highly specific localization of function. However, in practical neurology, it is necessary to understand the main functional roles of the cerebral cortex. The following paragraphs summarize areas of clinical importance.

The dominant hemisphere (usually left)

The concept of cerebral dominance arose with a simple observation. Right-handed stroke patients with acquired language disorders had destructive lesions within the left hemisphere. Almost all right-handed and 70% of left-handed people have language function in the left hemisphere.

Destructive lesions within the left fronto-temporo-parietal region cause various disorders of human communication:

* spoken language – aphasia, also called dysphasia
* writing – agraphia
* reading – acquired alexia.

Developmental dyslexia describes delayed and disorganized reading and writing ability in children with normal intelligence.

Aphasia

Aphasia is loss of or defective language from damage to the speech centres within the left hemisphere. Numerous varieties have been described.

Broca’s aphasia (expressive aphasia, anterior aphasia)

Damage in the left frontal lobe causes reduced speech fluency with comprehension relatively preserved. The patient makes great efforts to initiate language, which becomes reduced to a few disjointed words. There is failure to construct sentences. Patients who recover from this form of aphasia say they knew what they wanted to say, but ‘could not get the words out’.

Wernicke’s aphasia (receptive aphasia, posterior aphasia)

Left temporo-parietal damage leaves language that is fluent but the words themselves are incorrect. This varies from insertion of a few incorrect or nonexistent words into fluent speech to a profuse outpouring of jargon (that is, rubbish with wholly nonexistent words). Severe jargon aphasia may be bizarre – and confused with psychotic behaviour.

Patients who have recovered from Wernicke’s aphasia say that when aphasic they found speech, both their own and others’, like a wholly unintelligible foreign language. They could neither stop themselves, nor understand themselves and others.

Nominal aphasia (anomic aphasia or amnestic aphasia)

This means difficulty naming familiar objects. Naming difficulty is an early sign in all types of aphasia. A left posterior temporal/inferior parietal lesion causes a severe, isolated form.

Global aphasia (central aphasia)

This means the combination of the expressive problems of Broca’s aphasia and the loss of comprehension of Wernicke’s. The patient can neither speak nor understand language. It is due to widespread damage to speech areas and is the commonest aphasia after a severe left hemisphere infarct. Writing and reading are also affected.

Dysarthria

Dysarthria simply means disordered articulation – slurred speech. Language is intact, cf. aphasia. Paralysis, slowing or incoordination of muscles of articulation or local discomfort causes various different patterns of dysarthria. Examples are the ‘gravelly’ speech of upper motor neurone lesions of lower cranial nerves, the jerky, ataxic speech of cerebellar lesions, the monotone of Parkinson’s disease, and speech in myasthenia that fatigues and dies away. Many aphasic patients are also somewhat dysarthric.

The non-dominant hemisphere

Disorders in right-handed patients with right hemisphere lesions are often difficult to recognize. They comprise abnormalities of perception of internal and external space. Examples are losing the way in familiar surroundings, failing to put on clothing correctly (dressing apraxia), or failure to draw simple shapes – constructional apraxia.
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