MED2042 WEEK 3 - Neuroanatomy (Speech and Language)
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- How many different languages and dialects are there in the world?
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10 000
They all share similar elements, and the acquisition of language is very consistent across all human cultures. - What is language? How is it developed?
- Language is a creative process that conveys the subtleties of human experience and emotion. Language is innate to humans. Communities of neurologically normal human beings spontaneously develop spoken language and no mute tribes of people have ever been found, even in the remotest parts of the world. It is thought that the human brain has evolved special language processing systems that are present at birth. If a child gwos up in a normal language environement, he or she will inevitably learn to speak and understand that lanaguage.
- How are reading and writing acquired?
- The linguistic skills of reading and writing are not as biologically natural as speech. REading and writing are largely acquired skills and require explicit instruction and supervised practice of a type that is not needed for the acquisition of spoken language. However, reading and writing are to a very large degree controlled by the same parts of the brain responsible for verbal language and both written and spoken language are affected in similar way by damage to language areas.
- What is aphasia?
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Aphasia is the partial or complete loss of language abilities as a result of brain damage.
Much of what we know about the importance of certain brain areas in language functions has come from studies of aphasia. - What was thought to control speech during the Greek and Roman empires?
- During the Greek and Roman empires, it was thought that the tongue rather than the brain controlled speech. If an injury to the head caused loss of speech, the treatment was cutting, bleeding or massage of the tongue. By the 16th centruy it was recognized that a person could have a speech impairment without paralysis of the tongue.
- What did Jean-Baptiste Bouillaud propose in 1825?
- In 1825, a French physician, Jean-Baptiste Bouillaud, proposed that speech is specifically controlled byt he frontal lobes of the brain. His proposal was based on many case studies, but the idea failed to receive general acceptance among the scientific community. This was probably due to the rejection of the concept of cerebral localization as a backlash against the doctrine of phrenology.
- What is Phrenology?
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Phrenology was founded by a Viennese physician called Franz Gall (1975-1828), who believed that mental functions and aptitudes were highly localized in different parts of the cerebrum and that these functions were also reflected in the shape of the head. Gall wrote, that as a young man, he had been impressed by teh fact that a number of acquaintances with particularly good memories also had unusually large and protruding eyes. He believed the two were related and this led him to suspect that other faculties would also have their visible external characteristics.
Gall's view was that all mental faculties were inherited and localized as organs within discrete parts of the brain. If a faculty became particularly well developed the responsible organ would enlarge and an external physical sign such as a bump on the skull would reflect this. Gall and his students travelled widely in the study of individuals with unusual heads or distinguished character traits and identified and mapped the physical signs associated with different mental functions.
From the outset, scientists were sceptical about the claims of the phrnologists, which were founded on supposition and anecdotal evidence, but phrenology caught the public imagination. Phrenological societies were established, numerous parlours appeared where people paid to have thier characters read from the shape of their skulls and books on phrenology became popular reading material.
Initially, Gall identified a modest 27 organs, but as phrnology became more popular its followers invented ever-increasing numbers of fanciful organs. Bumps for traits such as drunkeness, debauchery, jealousy, criminality and republicanism were added to the phrenologists' maps of the head, and phrenology was heading down the road to scientific disgrace and public ridicule.
The scientific legacy of the excesses of the phrenologists was a temporary backlash against verebral localization, and the concept of anti-localization prevailed for a while. It took another 4 decades before Bouillaud's idea became generally accepted. - What did Monsieur Aubertin argue?
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At a stormy meeting of the society in May of that year, the discussion was about localization of language in the brain, and it included localizationists and non-localizationists.
A physician called Monsieur Aubertin (who was the son of a famous phrenologist) argued strongly that the capacity to produce speech was localized to teh frontal lobes of the brain. His evidence was based on one case; a man who had shot away the frontal bone of his skull in a failed attempt at suicide. In treating this patient, Aubertin noted that a spatula pressed onto the exposed frontal lobe while the man was speaking immediately halted speech and it did not resume until the pressure was released. Aubertin inferred that the pressure interfered with the normal function of speech area in the frontal lobe. He was so convinced of this, that he offered a relatively vast reward (about $A 150 000 in current terms) to anyone who could prove that damage to teh frontal lobes did not impair speech. - What is the story of "Tan"?
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In 1861, the French physician Paul Broca described the case of a man now known as "Tan" who had lost the capacity to speak.
As it happened, Broca was treating a patient called Monsieur Defarge for a serious infection. Some years previously this man had suffered a stroke and brain damage which left him paralysed in his right arm, and almost completely incapable of producing speech. He could understand language and read silently, but could not speak or express his ideas in writing. The only utterance he could make was a word that sounded like "Tan". This man had no conventional motor problems with his mouth or tongue or vocal cords and he could move his left arm normally.
Monsieur Defrage (who is known in the annals of medical history as "Tan") died of his infection a few days after the meeting. Broca removed his brain and observed a localized area of damage in the inferior-posterior part of left frontal lobe.
Spectacular as this discovery was, Broca did not offer any firm conclusions about speech localization from this single case. He wanted more evidence to substantiate this observation. Over the next few years he studied 8 similar cases where speech was lost. And in each case a lesion was located in the posterior frontal lobe of the left hemisphere. The consistency of these discoveries, together with findings that damage to the right frontal lobe did not impair speech, led Broca to procalim, in 1864, one of the most famous principles of brain function: "Nous parlons avec l'hemisphere gauche" or "we speak with the left hemisphere". This area which occupies the inferior frontal gyrus and is critical for language production is commonly referred to as "Broca's area". - What did Wernicke find?
- Broca's work stimulated a search for teh cortical sites of other specific functions, and in 1976 the German neurologist Carl Wernicke, reported findings for a different sort of language disturbance where speech and writing were possible, but comprehension of both spoken and written language was impaired. Here the lesion was again in the left hemisphere, but more posterior in the brain, in the temporal lobe where it joins with the parietal lobe. This region which is critical for understanding language is now commonly referred to as "Wernicke's area".
- What is the link between language and handedness?
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In the vast majority of the population, the left hemisphere is dominant for speech and this is true for both right-handed and left-handed people. There is also a clear, but as yet not fully explained link between lateralization of language and handedness.
It seems that the hemisphere which develops articulation also tends to control the most dexterous hand, the one we normally write with and use preferentially for other skilled motor tasks. It is a strong but not invariable associateion as shown below for data that was obtained from a large number of patients who underwent a procedure called the Wada test, which examines how anaesthesia of either the left or right hemisphere affects speech.
SPEECH REPRESENTATION
Right-handedness (90% of population) - 96% left, 0% bilateral, 4% right
Left-handedness (10% of population) - 70% left, 15% bilateral, 15% right - Describe connections of the language areas in the brain.
- Having established that there are two language areas in the left hemisphere, Wernicke and others constructed maps of language processing within the brain, with interconnections between visual and auditory areas, Broca's and Wernickes areas and the muscles required for language expression. Different types of language disabilities were ascribed to damage in different parts of this system.
- What connections are involved in repeating a spoken word?
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1. the sound is received in auditory cortex and signals are sent to Wernicke's area where the sound is understood as a word.
2. To repeat the word, Wernicke's area sends word-based signals to Broca's area via the arcuate fasciculus (the bundle of white matter that connects the two areas)
3. Broca's area converts the word to a muscular code that is sent to nearby motor cortex to activate the muscles of articulation. - What connections are involved in reading aloud or saying a written word?
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1. The visual signals are sent to the angular gyrus adjacent to Wernicke's area. The angular gyrus transforms teh visual word signals into output patterns.
2. These output patterns cause the same changes in activity in Wernicke's area as if the words were spoken. Wernicke's area then passes the signals to Broca's area for motor conversion and vocalization as described above. - What is the Wernicke-Geschwind model of language able to predict?
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The model predicts that a lesion in Broca's area seriously impairs speech production because proper signals cannot be sent to the motor cortex and hence to the muscles of speech.
Language comprehension should remain intact as this is teh role of Wernicke's area. On the other hand a lesion in Wernicke's area severely iparis comprehension because this is where sounds are transformed into words. The model also predicts the consequences of disconnection of Wernicke's and Broca's areas via a lesion of the arcuate fasciculus, a condition that produces what is known as conduction aphasia. - What is paraphrasia?
- The substitution of a word by sound, incorrect word, or unintended word. It can be evident during both spontaneous conversation and attempts to read from a text.
- What is Nonfluent speech?
- Talking with effort, in short agrammatical sentences. Many pronouns and conjunctions connecting parts of the sentence are left out, and there are no ands, ifs or buts. Verbs are frequent unconjugated.
- What is Impaired repetition?
- Difficulty in repeating words or sentences.
- Define anomic.
- Inability to find words
- Define agraphia.
- Impairment in writing.
- Define Alexia.
- Impairment in understanding the meaning of written words or sentences.
- What are the signs and symptoms of Broca's (also called motor or expressive) aphasia?
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Site of brain damage - posterior frontal lobe
Speech - nonfluent, agrammatical with difficulty finding words
Comprehension - relatively preserved for single words and grammatically simple sentences
Impaired repetition - yes
Paraphrasic errors - yes
Additional signs - Right hemiparesis (arm>leg) patient aware of defect. - What are the signs and symptoms of Wernicke's (also called sensory or receptive) aphasia?
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Site of brain damage - posterior frontal lobe
Speech - fluent, abundant, grammatical and meaningless
Comprehension - poor
Impaired repetition - yes
Paraphrasic errors - yes, many
Additional signs - no motor signs. Patient unaware of difficulty. - What are the signs and symptoms of Conduction aphasia?
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Site of brain damage - arcuate fasciculus
Speech - fluent
Comprehension - Intact
Impaired repetition - severe
Paraphrasic errors - a few
Additional signs - often none - What are the signs and symptoms of global aphasia?
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Site of brain damage - massive lesion of cortex around lateral sulcus.
Speech - very little and nonfluent
Comprehension - poor
Imparied repetition - yes
Paraphrasic error -
Additional signs - right hemiplegia - Describe the verbal flow of patient's with damage to the Wernicke's area.
- Generate an excess of language output that is difficult to halt.
- Describe the verbal flow of patients with damage to the Broca's area.
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Impaired language production. Difficulty in finding words and do not use function words.
They are able to understand single words and grammatically simple sentences as used in everyday conversation, but generally fail to understand more complex sentences. Thus, a person with Broca's aphasia may understand a simple question such as "Does a stone float on water?" but fail to understand a question such as "The lion was killed by the tiger. Which animal was killed and by whom?". It is now thought that Broca's area has a role in constructing woreds into gramatically correct sentences in addition to helping to program the motor aspects of language expression.