Speech Production and Comprehension: Brain Mechanisms
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PowerPoint Presentation for Physiology of Behavior 11th Edition by Neil R. Carlson
Prepared by Grant McLaren, Edinboro University of Pennsylvania This multimedia product and its contents are protected under copyright law. The following are prohibited by law: • any public performance or display, including transmission of any image over a network; • preparation of any derivative work, including extraction, in whole or in part, of any images; •any rental lease, or lending of the program. COPYRIGHT © ALLYN & BACON 2012
Human Communication
Chapter 14 2
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Human Communication • Speech Production and Comprehension: Brain Mechanisms • Lateralization • Speech Production • Speech Comprehension
• Aphasia in Deaf People • Prosody: Rhythm, Tone, and Emphasis in Speech • Recognition of People’s Voices • Stuttering • Section Summary
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Human Communication • Disorders of Reading and Writing • Relation to Aphasia • Pure Alexia • Toward an Understanding of Reading
• Developmental Dyslexias • Toward an Understanding of Writing • Section Summary
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Human Communication • Verbal behaviors constitute one of the most important classes of human social behavior.
• Our cultural evolution has been possible because we can talk and listen, write and read. Language enables our discoveries to be cumulative; knowledge gained by one generation can be passed on to the next.
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Speech Production and Comprehension: Brain Mechanisms • Our knowledge of the physiology of language has been obtained primarily by observing the effects of brain lesions on people’s verbal behavior. • Although investigators have studied people who have undergone brain surgery or who have sustained head injuries, brain tumors, or infections, most of the observations have been made on people who have suffered strokes, or cerebrovascular accidents. • cerebrovascular accident • A “stroke”; brain damage caused by occlusion or rupture of a blood vessel in the brain. • The most common type of cerebrovascular accident is caused by obstruction of a blood vessel. The interruption in blood flow deprives a region of the brain of its blood supply, which causes cells in that region to die. 6
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Speech Production and Comprehension: Brain Mechanisms • The most common type of cerebrovascular accident is caused by obstruction of a blood vessel. • The interruption in blood flow deprives a region of the brain of its blood supply, which causes cells in that region to die. • Another source of information about the physiology of language comes from studies using functional-imaging devices. • In general, these studies have confirmed or complemented what we have learned by studying patients with brain damage.
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Speech Production and Comprehension: Brain Mechanisms • The most important category of speech disorders is aphasia, a primary disturbance in the comprehension or production of speech, caused by brain damage. • aphasia • Difficulty in producing or comprehending speech not produced by deafness or a simple motor deficit; caused by brain damage. • Not all speech disturbances are aphasias; to receive a diagnosis of aphasia, a patient must have difficulty comprehending, repeating, or producing meaningful speech, and this difficulty must not be caused by simple sensory or motor deficits or by lack of motivation.
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Speech Production and Comprehension: Brain Mechanisms • Lateralization • Verbal behavior is a lateralized function; most language disturbances occur after damage to the left side of the brain, whether people are left-handed or right-handed. • Using an ultrasonic procedure to measure changes in cerebral blood flow while people performed a verbal task, Knecht et al. (2000) assessed the relationship between handedness and lateralization of speech mechanisms in people without any known brain damage.
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Speech Production and Comprehension: Brain Mechanisms • Lateralization • They found that right-hemisphere speech dominance was seen in only 4 percent of righthanded people, in 15 percent of ambidextrous people, and in 27 percent of left-handed people. • The left hemisphere of approximately 90 percent of the total population is dominant for speech.
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Speech Production and Comprehension: Brain Mechanisms • Lateralization • Although the circuits that are primarily involved in speech comprehension and production are located in one hemisphere (almost always, the left hemisphere), it would be a mistake to conclude that the other hemisphere plays no role in speech. • For example, damage to the right hemisphere makes it difficult for a person to read maps, perceive spatial relations, and recognize complex geometrical forms.
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Speech Production and Comprehension: Brain Mechanisms • Lateralization • People with such damage also have trouble talking about things like maps and complex geometrical forms or understanding what other people have to say about them. • The right hemisphere also appears to be involved in organizing a narrative—selecting and assembling the elements of what we want to say (Gardner et al., 1983). • As we saw in Chapter 11, the right hemisphere is involved in the expression and recognition of emotion in the tone of voice.
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Speech Production and Comprehension: Brain Mechanisms • Lateralization • And as we shall see in this chapter, it is also involved in control of prosody—the normal rhythm and stress found in speech. • Therefore, both hemispheres of the brain have a contribution to make to our language abilities.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Being able to talk—that is, to produce meaningful speech—requires several abilities. • First, the person must have something to talk about. Let us consider what this means. • We can talk about something that is currently happening or something that happened in the past.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Of course, we can also talk about something that did not happen. • That is, we can use our imagination to make up a story (or to tell a lie). • We know very little about the neural mechanisms that are responsible for imagination, but it seems likely that they involve the mechanisms responsible for perceptions and memories; after all, when we make up a story, we must base it on knowledge that we originally acquired through perception and have retained in our memory.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Damage to a region of the inferior left frontal lobe (Broca’s area) disrupts the ability to speak: It causes Broca’s aphasia. • Broca’s aphasia • A form of aphasia characterized by agrammatism, anomia, and extreme difficulty in speech articulation.
• This disorder is characterized by slow, laborious, and nonfluent speech.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • But although they often mispronounce words, the ones the patients manage to come out with are usually meaningful. • The posterior part of the cerebral hemispheres has something to say, but the damage to the frontal lobe makes it difficult for the patients to express these thoughts.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • People with Broca’s aphasia find it easier to say some types of words than others. • They have great difficulty saying the little words with grammatical meaning, such as a, the, some, in, or about. • These words are called function words, because they have important grammatical functions.
• The words that they do manage to say are almost entirely content words—words that convey meaning, including nouns, verbs, adjectives, and adverbs, such as apple, house, throw, or heavy.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • function word • A preposition, article, or other word that conveys little of the meaning of a sentence but is important in specifying its grammatical structure. • content word • A noun, verb, adjective, or adverb that conveys meaning.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • People with Broca’s aphasia can comprehend speech much better than they can produce it. • In fact, some observers have said that their comprehension is unimpaired, but as we will see, this is not quite true.
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Figure 14.1, page 482
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Broca (1861) suggested that this form of aphasia is produced by a lesion of the frontal association cortex, just anterior to the face region of the primary motor cortex. • Subsequent research proved him to be essentially correct, and we now call the region Broca’s area. (See Figure 14.2.) • Broca’s area
• A region of frontal cortex, located just rostral to the base of the left primary motor cortex, that is necessary for normal speech production.
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Figure 14.2, page 482
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • What do the neural circuits in and around Broca’s area do? Wernicke (1874) suggested that Broca’s area contains motor memories—in particular, memories of the sequences of muscular movements that are needed to articulate words. • Talking involves rapid movements of the tongue, lips, and jaw, and these movements must be coordinated with each other and with those of the vocal cords; thus, talking requires some very sophisticated motor control mechanisms.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Obviously, circuits of neurons somewhere in our brain will, when properly activated, cause these sequences of movements to be executed. • Because damage to the inferior caudal left frontal lobe (including Broca’s area) disrupts the ability to articulate words, this region is a likely candidate for the location of these “programs.” • The fact that this region is directly connected to the part of the primary motor cortex that controls the muscles used for speech certainly supports this conclusion.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • The striking disparity between their speech and their comprehension often leads people to assume that their comprehension is normal. • But it is not. Schwartz, Saffran, and Marin (1980) showed Broca’s aphasics pairs of pictures in which agents and objects of the action were reversed: for example, a horse kicking a cow and a cow kicking a horse, a truck pulling a car and a car pulling a truck, and a dancer applauding a clown and a clown applauding a dancer.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • But the speech functions of the left frontal lobe include more than programming the movements used to speak. • Broca’s aphasia is much more than a deficit in pronouncing words. In general, three major speech deficits are produced by lesions in and around Broca’s area: agrammatism, anomia, and articulation difficulties. • Although most patients with Broca’s aphasia will have all of these deficits to some degree, their severity can vary considerably from person to person—presumably, because their brain lesions differ.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Agrammatism refers to a patient’s difficulty in using grammatical constructions. • agrammatism
• One of the usual symptoms of Broca's aphasia; a difficulty in comprehending or properly employing grammatical devices, such as verb endings and word order. • This disorder can appear all by itself, without any difficulty in pronouncing words (Nadeau, 1988).
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • As they showed each pair of pictures, they read the subject a sentence, for example, The horse kicks the cow. • The subjects’ task was to point to the appropriate picture, indicating whether they understood the grammatical construction of the sentence. (See Figure 14.3.) • They performed very poorly.
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Figure 14.3, page 483
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Functional-imaging studies by Opitz and Friederici (2003, 2007) found that Broca’s area was activated when people were taught an artificial grammar, which supports the conclusion that this region is involved in learning grammatical rules—especially complex ones.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • The second major speech deficit seen in Broca’s aphasia is anomia (“without name”). Anomia refers to a word-finding difficulty; and because all aphasics omit words or use inappropriate ones, anomia is actually a primary symptom of all forms of aphasia. • anomia • Difficulty in finding (remembering) the appropriate word to describe an object, action, or attribute; one of the symptoms of aphasia. • However, because the speech of Broca’s aphasics lacks fluency, their anomia is especially apparent; their facial expression and frequent use of sounds such as “uh” make it obvious that they are groping for the correct words. 32
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • The third major characteristic of Broca’s aphasia is difficulty with articulation. Patients mispronounce words, often altering the sequence of sounds. • For example, lipstick might be pronounced “likstip.” • People with Broca’s aphasia recognize that their pronunciation is erroneous, and they usually try to correct it.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Some studies indicate that different categories of symptoms of Broca’s aphasia do indeed involve different brain regions. • Dronkers and her colleagues (Dronkers, 1996; Baldo et al., 2011) have identified a critical location for control of speech articulation: the left precentral gyrus of the insula. • The insular cortex is located on the lateral wall of the cerebral hemisphere behind the anterior temporal lobe. • Normally, this region is hidden and can be seen only when the temporal lobe is dissected away. (See Figure 14.4.)
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Figure 14.4, page 484
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Dronkers discovered the apparent role of this region by plotting the lesions of patients with and without apraxia of speech who had strokes that damaged the same general area of the brain. (Apraxia of speech is an impairment in the ability to program movements of the tongue, lips, and throat that are required to produce the proper sequence of speech sounds.) • apraxia of speech • Impairment in the ability to program movements of the tongue, lips, and throat required to produce the proper sequence of speech sounds.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Figure 14.5(a) shows the overlap of the lesions of twenty-five patients with apraxia of speech. • As you can see, a region of 100 percent overlap, shown in yellow, falls on the left precentral gyrus of the insula. (See Figure 14.5a.) • In contrast, none of the lesions of nineteen patients who did not show apraxia of speech included damage to this region. (See Figure 14.5b.)
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Figure 14.5, page 484
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • At least two functional-imaging studies support Dronkers’s conclusion. Kuriki, Mori, and Hirata (1999) and Wise et al. (1999) found that pronunciation of words caused activation of the left anterior insula. • However, other studies suggest that Broca’s area is also involved in articulation (Nestor et al., 2003; Hillis et al., 2004). • Stewart et al. (2001) used TMS to interfere with the activity of neurons in Broca’s area or the adjacent area of primary motor cortex, which controls the muscles used for speech.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Wernicke’s Aphasia: Description • The primary characteristics of Wernicke’s aphasia are poor speech comprehension and production of meaningless speech. • Wernicke’s area • A region of auditory association cortex on the left temporal lobe of humans, which is important in the comprehension of words and the production of meaningful speech. • Unlike Broca’s aphasia, Wernicke’s aphasia is fluent and unlabored; the person does not strain to articulate words and does not appear to be searching for them. 40
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Wernicke’s Aphasia: Description • The patient maintains a melodic line, with the voice rising and falling normally. • When you listen to the speech of a person with Wernicke’s aphasia, it appears to be grammatical. • That is, the person uses function words such as the and but and employs complex verb tenses and subordinate clauses. • However, the person uses few content words, and the words that he or she strings together just do not make sense. 41
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Wernicke’s Aphasia: Description • Because of the speech deficit of people with Wernicke’s aphasia, when we try to assess their ability to comprehend speech, we must ask them to use nonverbal responses. • That is, we cannot assume that they do not understand what other people say to them just because they do not give the proper answer.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Wernicke’s Aphasia: Description • A commonly used test of comprehension assesses their ability to understand questions by pointing to objects on a table in front of them. • For example, they are asked to “Point to the one with ink.” • If they point to an object other than the pen, they have not understood the request. • When tested this way, people with severe Wernicke’s aphasia do indeed show poor comprehension. 43
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Wernicke’s Aphasia: Description • A remarkable fact about people with Wernicke’s aphasia is that they often seem unaware of their deficit. • That is, they do not appear to recognize that their speech is faulty, nor do they recognize that they cannot understand the speech of others.
• They do not look puzzled when someone tells them something, even though they obviously cannot understand what they hear.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Wernicke’s Aphasia: Description • Perhaps their comprehension deficit prevents them from realizing that what they say and hear makes no sense. • They still follow social conventions, taking turns in conversation with the examiner, even though they do not understand what the examiner says and what they say in return makes little sense. • They remain sensitive to the other person’s facial expression and tone of voice and begin talking when he or she asks a question and pauses for an answer.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Wernicke’s Aphasia: Analysis • Because the superior temporal gyrus is a region of auditory association cortex, and because a comprehension deficit is so prominent in Wernicke’s aphasia, this disorder has been characterized as a receptive aphasia. • Wernicke suggested that the region that now bears his name is the location of memories of the sequences of sounds that constitute words.
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Speech Production and Comprehension: Brain Mechanisms • Speech Production • Wernicke’s Aphasia: Analysis • This hypothesis is reasonable; it suggests that the auditory association cortex of the superior temporal gyrus recognizes the sounds of words, just as the visual association cortex of the inferior temporal gyrus recognizes the sight of objects. • The abilities that are disrupted include recognition of spoken words, comprehension of the meaning of words, and the ability to convert thoughts into words. • Let us consider each of these abilities in turn.
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • Damage to the left temporal lobe can produce a disorder of auditory word recognition, uncontaminated by other problems. • This syndrome is called pure word deafness. (See Figure 14.6.) • pure word deafness • The ability to hear, to speak, and (usually) to read and write without being able to comprehend the meaning of speech; caused by damage to Wernicke’s area or disruption of auditory input to this region.
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Figure 14.6, page 486
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • Although people with pure word deafness are not deaf, they cannot understand speech. As one patient put it, “I can hear you talking, I just can’t understand what you’re saying.” • Another said, “It’s as if there were a bypass somewhere, and my ears were not connected to my voice” (Saffran, Marin, and Yeni-Komshian, 1976, p. 211). • These patients can recognize nonspeech sounds such as the barking of a dog, the sound of a doorbell, and the honking of a horn.
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • Often, they can recognize the emotion expressed by the intonation of speech even though they cannot understand what is being said. More significantly, their own speech is excellent.
• They can often understand what other people are saying by reading their lips. • They can also read and write, and they sometimes ask people to communicate with them in writing. • Clearly, pure word deafness is not an inability to comprehend the meaning of words; if it were, people with this disorder would not be able to read people’s lips or read words written on paper. 51
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • Functional-imaging studies confirm that perception of speech sounds activates neurons in the auditory association cortex of the superior temporal gyrus. • For example, Scott et al. (2000) identified a region of the left anterior superior temporal gyrus was specifically activated by intelligible speech. (See Figure 14.7.) • Sharp, Scott, and Wise (2004) found that deficits in speech comprehension were produced by lesions of the superior temporal lobe that damaged the region that is activated when people hear intelligible speech.
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Figure 14.7, page 487
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • Sharp, Scott, and Wise (2004) found that deficits in speech comprehension were produced by lesions of the superior temporal lobe that damaged the region that is activated when people hear intelligible speech. • Figure 14.8 shows a computer-generated depiction of the overlap of lesions of patients with brain damage that interfered with speech perception. • Compare the regions of greatest overlap, shown in yellow and green, in Figure 14.8 with the region shown in Figure 14.7.
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Figure 14.8, page 487
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • Apparently, two types of brain injury can cause pure word deafness: disruption of auditory input to the superior temporal cortex or damage to the superior temporal cortex itself (Poeppel, 2001; Stefanatos, Gershkoff, and Madigan, 2005). • Either type of damage disturbs the analysis of the sounds of words and hence prevents people from recognizing other people’s speech.
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • Although speech recognition is clearly an auditory event, research indicates that hearing words automatically engages brain mechanisms that control speech. • In other words, these mechanisms appear also to contain mirror neurons that are activated by the sounds of words.
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • For example, Fridriksson et al. (2008) found that when people watched (but did not hear) other people making speech movements, the temporal (auditory) and frontal (motor) cortical language areas were activated. • These regions were not activated when the subjects watched people making nonspeech movements with their mouths.
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • Several investigators have suggested that feedback from subvocal articulation (very slight movements of the muscles involved in speech that do not actually cause obvious movement) facilitate speech recognition (Pulvermüller and Fadiga, 2010). • For example, a functional imaging study by Pulvermüller et al. (2006) had subjects articulate syllables that contained the consonants p or t (for example, pa and ta), which involve movements of the lips or tongue.
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Speech Production and Comprehension: Brain Mechanisms • Recognition: Pure Word Deafness • The subjects said the syllables aloud, said them to themselves silently, and listened to the syllables spoken by someone else. • As Figure 14.9 shows, in all three conditions, regions of the brain involved with lip movements (green) and tongue movements (red) were activated. (See Figure 14.9.) • Thus, speaking, watching other people speak, thinking about speaking, and listening to speech sounds all activate brain regions involved in language, which suggests that circuits of mirror neurons play a role in speech comprehension.
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Figure 14.9, page 488
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Speech Production and Comprehension: Brain Mechanisms • Comprehension: Transcortical Sensory Aphasia • The other symptoms of Wernicke’s aphasia—failure to comprehend the meaning of words and inability to express thoughts in meaningful speech—appear to be produced by damage that extends beyond Wernicke’s area into the region that surrounds the posterior part of the lateral fissure, near the junction of the temporal, occipital, and parietal lobes.
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Speech Production and Comprehension: Brain Mechanisms • Comprehension: Transcortical Sensory Aphasia • For want of a better term, I will refer to this region as the posterior language area. (See Figure 14.10.) • The posterior language area appears to serve as a place for interchanging information between the auditory representation of words and the meanings of these words, stored as memories in the rest of the sensory association cortex.
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Figure 14.10, page 489
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Speech Production and Comprehension: Brain Mechanisms • Comprehension: Transcortical Sensory Aphasia • Damage to the posterior language area alone, which isolates Wernicke’s area from the rest of the posterior language area, produces a disorder known as transcortical sensory aphasia. (See Figure 14.10.) • transcortical sensory aphasia • A speech disorder in which a person has difficulty comprehending speech and producing meaningful spontaneous speech but can repeat speech; caused by damage to the region of the brain posterior to Wernicke's area.
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Speech Production and Comprehension: Brain Mechanisms • Comprehension: Transcortical Sensory Aphasia • The difference between transcortical sensory aphasia and Wernicke’s aphasia is that patients with transcortical sensory aphasia can repeat what other people say to them; therefore, they can recognize words. • However, they cannot comprehend the meaning of what they hear and repeat; nor can they produce meaningful speech of their own.
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Speech Production and Comprehension: Brain Mechanisms • Comprehension: Transcortical Sensory Aphasia • How can these people repeat what they hear? • Because the posterior language area is damaged, repetition does not involve this part of the brain. • Obviously, there must be a direct connection between Wernicke’s area and Broca’s area that bypasses the posterior language area. (See Figure 14.10.)
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Speech Production and Comprehension: Brain Mechanisms • Comprehension: Transcortical Sensory Aphasia • In conclusion, transcortical sensory aphasia can be seen as Wernicke’s aphasia without a repetition deficit. • To put it another way, the symptoms of Wernicke’s aphasia consist of those of pure word deafness plus those of transcortical sensory aphasia. • As I tell my students, WA = TSA + PWD. By simple algebra, TSA = WA – PWD, and so on. (Refer to Figure 14.10.)
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Speech Production and Comprehension: Brain Mechanisms • What Is Meaning? • As we have seen, Wernicke’s area is involved in the analysis of speech sounds and thus in the recognition of words. • Damage to the posterior language area does not disrupt people’s ability to recognize words, but it does disrupt their ability to understand words or to produce meaningful speech of their own. • But what, exactly, do we mean by the word meaning? And what types of brain mechanisms are involved?
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Speech Production and Comprehension: Brain Mechanisms • What Is Meaning? • First, we must recognize the sequence of sounds that constitute the word—We find the auditory entry for the word in our “dictionary.” • As we saw, this entry appears in Wernicke’s area. • Next, the memories that constitute the meaning of the word must be activated.
• Presumably, Wernicke’s area is connected—through the posterior language area—with the neural circuits that contain these memories. (See Figure 14.11.)
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Figure 14.11, page 490
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Speech Production and Comprehension: Brain Mechanisms • What Is Meaning? • Functional-imaging studies confirm these observations. • Nichelli et al. (1995) found that judging the moral of Aesop’s fables (in contrast to judging more superficial aspects of the stories) also activated regions of the right hemisphere. • Sotillo et al. (2005) found that a task that required comprehension of metaphors such as “green lung of the city” (that is, a park) activated the right superior temporal cortex. (See Figure 14.12 )
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Figure 14.12, page 491
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • As we saw earlier in this section, the fact that people with transcortical sensory aphasia can repeat what they hear suggests that there is a direct connection between Wernicke’s area and Broca’s area—and there is: the arcuate fasciculus (“arch-shaped bundle”). • This bundle of axons is found in the human brain, but is absent or much smaller in the brains of nonhuman primates (Rilling et al., 2008).
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • The arcuate fasciculus appears to convey information about the sounds of words but not their meanings. • The best evidence for this conclusion comes from a syndrome known as conduction aphasia, which is produced by damage to the inferior parietal lobe that extends into the subcortical white matter and damages the arcuate fasciculus (Damasio and Damasio, 1980). (See Figure 14.13.)
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Figure 14.13, page 491
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • Conduction aphasia is characterized by meaningful, fluent speech and relatively good comprehension but very poor repetition. • conduction aphasia • An aphasia characterized by inability to repeat words that are heard but the ability to speak normally and comprehend the speech of others.
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • People with conduction aphasia can repeat speech sounds that they hear only if these sounds have meaning. • Sometimes, when a person with conduction aphasia is asked to repeat a word, he or she says a word with the same meaning—or at least one that is related. • For example, if the examiner says house, the patient may say home. • If the examiner says chair, the patient may say sit.
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • The symptoms that are seen in transcortical sensory aphasia and conduction aphasia lead to the conclusion that there are pathways connecting the speech mechanisms of the temporal lobe with those of the frontal lobe. • The direct pathway through the arcuate fasciculus simply conveys speech sounds from Wernicke’s area to Broca’s area. • We use this pathway to repeat unfamiliar words—for example, when we are learning a foreign language or a new word in our own language or when we are trying to repeat a nonword such as blaynge.
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • The second pathway, between the posterior language area and Broca’s area, is indirect and is based on the meaning of words, not on the sounds they make. • When patients with conduction aphasia hear a word or a sentence, the meaning of what they hear evokes some sort of image related to that meaning. • Of course, the words they choose might not be the same as the ones used by the person who spoke to them. (See Figure 14.14.)
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Figure 14.14, page 492
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • A study by Catani, Jones, and Fytche (2005) provides the first anatomical evidence for the existence of the two pathways between Wernicke’s area and Broca’s area that I presented in Figure 14.14. • The investigators used DTI to trace the branches of the arcuate fasciculus in the human brain. • They found one deep pathway that directly connects these two regions and a shallower pathway that consists of two segments
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • The anterior segment connects Broca’s area with the inferior parietal cortex, and the posterior segment connects Wernicke’s area with the inferior parietal cortex. • Damage to the direct pathway would be expected to produce conduction aphasia, whereas damage to the indirect pathway would be expected to spare the ability to repeat speech but would impair comprehension. (See Figure 14.15.)
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Figure 14.15, page 493
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • The symptoms of conduction aphasia suggest that the connection between Wernicke’s area and Broca’s area plays an important role in short-term memory of words and speech sounds that have just been heard. • Presumably, rehearsal of such information can be accomplished by “talking to ourselves” inside our head without actually having to say anything aloud.
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Speech Production and Comprehension: Brain Mechanisms • Repetition: Conduction Aphasia • Imagining ourselves saying the word activates the region of Broca’s area, whereas imagining that we are hearing it activates the auditory association area of the temporal lobe. • These two regions, connected by means of the arcuate fasciculus (which contains axons traveling in both directions) circulate information back and forth, keeping the short-term memory alive.
• Baddeley (1993) refers to this circuit as the phonological loop.
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Speech Production and Comprehension: Brain Mechanisms • Memory of Words: Anomic Aphasia • As I have already noted, anomia, in one form or other, is a hallmark of aphasia. • However, one category of aphasia consists of almost pure anomia, the other symptoms being inconsequential. • Speech of patients with anomic aphasia is fluent and grammatical, and their comprehension is excellent, but they have difficulty finding the appropriate words.
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Speech Production and Comprehension: Brain Mechanisms • Memory of Words: Anomic Aphasia • They often employ circumlocutions (literally, “speaking in a roundabout way”) to get around missing words. • circumlocution • A strategy by which people with anomia find alternative ways to say something when they are unable to think of the most appropriate word.
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Speech Production and Comprehension: Brain Mechanisms • Memory of Words: Anomic Aphasia • Anomic aphasia is different from Wernicke’s aphasia. • People with anomic aphasia can understand what others say, and what these people say makes perfect sense, even if they often choose roundabout ways to say it.
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Speech Production and Comprehension: Brain Mechanisms • Memory of Words: Anomic Aphasia • Anomia has been described as a partial amnesia for words. • It can be produced by lesions in either the anterior or posterior regions of the brain, but only posterior lesions produce a fluent anomia. • The most likely location of lesions that produce anomia without the other symptoms of aphasia, such as comprehension deficits, agrammatism, or difficulties in articulation, is the left temporal or parietal lobe, usually sparing Wernicke’s area.
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Speech Production and Comprehension: Brain Mechanisms • Memory of Words: Anomic Aphasia • Several functional-imaging studies have confirmed the importance of Broca’s area and the region surrounding it in the production of verbs. • For example, Hauk, Johnsrued, and Pulvermüller (2004) had subjects read verbs that related to movements of different parts of the body. • For example, bite, slap, and kick involve movements of the face, arm, and leg, respectively.
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Speech Production and Comprehension: Brain Mechanisms • Memory of Words: Anomic Aphasia • The investigators found that when the subjects read a verb, they saw activation in the regions of the motor cortex that controlled the relevant part of the body. (See Figure 14.16.) • A similar study by Buccino et al. (2005) found that hearing sentences that involved hand movements (for example, He turned the key) activated the hand region of the motor cortex and that hearing sentences that involved foot movements (for example, He stepped on the grass) activated the foot region. • Presumably, thinking about particular actions activated regions that control these actions.
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Figure 14.16, page 494
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Speech Production and Comprehension: Brain Mechanisms • Aphasia in Deaf People • So far, I have restricted my discussion to brain mechanisms of spoken and written language. But communication among members of the Deaf community involves another medium: sign language. • Sign language is expressed manually, by movements of the hands. • Sign language is not English; nor is it French, Spanish, or Chinese.
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Speech Production and Comprehension: Brain Mechanisms • Aphasia in Deaf People • The most common sign language in North America is ASL—American Sign Language. • ASL is a full-fledged language, having signs for nouns, verbs, adjectives, adverbs, and all the other parts of speech contained in oral languages. • People can converse rapidly and efficiently by means of sign language, can tell jokes, and can even make puns based on the similarity between signs. • They can also use their language ability to think in words.
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Speech Production and Comprehension: Brain Mechanisms • Aphasia in Deaf People • Some researchers believe that in the history of our species, sign language preceded spoken language—that our ancestors began using gestures to communicate before they switched to speech. • As I mentioned earlier in this chapter, mirror neurons become active when we see or perform particular grasping, holding, or manipulating movements. • Some of these neurons are found in Broca’s area.
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Speech Production and Comprehension: Brain Mechanisms • Aphasia in Deaf People • Presumably, these neurons play an important role in learning to mimic other people’s hand movements. • Indeed, they might have been involved in the development of hand gestures used for communication in our ancestors, and they undoubtedly are used by deaf people when they communicate by sign language. • A functional-imaging study by Iacoboni et al. (1999) found that Broca’s area was activated when people observed and imitated finger movements. (See Figure 14.17.)
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Figure 14.17, page 495
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Speech Production and Comprehension: Brain Mechanisms • Aphasia in Deaf People • Several studies have found a linkage between speech and hand movements, which supports the suggestion that the spoken language of present-day humans evolved from hand gestures. • For example, Gentilucci (2003) had subjects speak the syllables ba or ga while they were watching him grasp objects of different sizes.
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Speech Production and Comprehension: Brain Mechanisms • Aphasia in Deaf People • When the experimenter grasped a large object, the subjects opened their mouths more and said the syllable more loudly than when he grasped a small one. • These results suggest that the region of the brain that controls grasping is also involved in controlling speech movements. (See Figure 14.18.)
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Figure 14.18, page 495
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Speech Production and Comprehension: Brain Mechanisms • Aphasia in Deaf People • The fact that the grammar of ASL is spatial suggests that aphasic disorders in deaf people who use sign language might be caused by lesions of the right hemisphere, which is primarily involved in spatial perception and memory. • However, all the cases of deaf people with aphasia for signs that have been reported in the literature so far have involved lesions of the left hemisphere, and functional imaging studies show that tasks that involve language activate the same regions of the left hemisphere in hearing and deaf people (MacSweeney, et al., 2008a).
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Speech Production and Comprehension: Brain Mechanisms • Aphasia in Deaf People • As you might imagine, when a hearing person is asked to look at a pair of drawing and say whether the names of the items they show rhyme, functional imaging shows increased activation in the region of Broca’s area because of the person “says” the two words subvocally. • And if we ask deaf people who are able to talk to perform this task, the same region is activated (MacSweeney et al., 2008b). (See Figure 14.19.)
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Figure 4.19, page 496
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Speech Production and Comprehension: Brain Mechanisms • Prosody: Rhythm, Tone, and Emphasis in Speech • When we speak, we do not merely utter words. • Our speech has a regular rhythm and cadence; we give some words stress (that is, we pronounce them louder), and we vary the pitch of our voice to indicate phrasing and to distinguish between assertions and questions. • In addition, we can impart information about our emotional state through the rhythm, emphasis, and tone of our speech.
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Speech Production and Comprehension: Brain Mechanisms • Prosody: Rhythm, Tone, and Emphasis in Speech • These rhythmic, emphatic, and melodic aspects of speech are referred to as prosody. • prosody
• The use of changes in intonation and emphasis to convey meaning in speech besides that specified by the particular words; an important means of communication of emotion.
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Speech Production and Comprehension: Brain Mechanisms • Prosody: Rhythm, Tone, and Emphasis in Speech • The importance of these aspects of speech is illustrated by our use of punctuation symbols to indicate some elements of prosody when we write. • For example, a comma indicates a short pause; a period indicates a longer one with an accompanying fall in the pitch of the voice; a question mark indicates a pause and a rise in the pitch of the voice; an exclamation mark indicates that the words are articulated with special emphasis; and so on.
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Speech Production and Comprehension: Brain Mechanisms • Prosody: Rhythm, Tone, and Emphasis in Speech • Evidence from studies of normal people and patients with brain lesions suggests that prosody is a special function of the right hemisphere. • This function is undoubtedly related to the more general role of this hemisphere in musical skills and the expression and recognition of emotions: Production of prosody is rather like singing, and prosody often serves as a vehicle for conveying emotion.
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Speech Production and Comprehension: Brain Mechanisms • Prosody: Rhythm, Tone, and Emphasis in Speech • In a functional-imaging study by Meyer et al. (2002), subjects heard normal sentences or sentences that contained only the prosodic elements of speech with the meaningful sounds filtered out. • As you can see in Figure 14.20 , the meaningful components of speech primarily activated the left hemisphere (blue and green regions), whereas the prosodic components primarily activated the right hemisphere (orange and yellow regions). (See Figure 14.20.)
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Figure 4.20, page 497
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Speech Production and Comprehension: Brain Mechanisms • Recognition of People’s Voices • As we have seen so far, the words contained in speech convey information about events, ideas, and other forms of meaning—information that can be conveyed just as well in writing. • The prosody of speech can convey information about the speaker’s emotional state or things the speaker wants to stress.
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Speech Production and Comprehension: Brain Mechanisms • Recognition of People’s Voices • Finally, speech can convey information completely independent of the meaning of the words: the identity of the speaker, his or her gender, and hints about his or her age. • People learn at an early age to recognize the voices of particular individuals. • Even newborn infants can recognize the voices of their parents, which they apparently learned while they were still in their mother’s uterus (Ockleford et al., 1988).
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Speech Production and Comprehension: Brain Mechanisms • Recognition of People’s Voices • Some people with localized brain damage have great difficulty recognizing voices—a disorder known as phonagnosia. • The first recorded case of developmental phonagnosia (that is, phonagnosia not caused by brain damage) was reported by Garrido et al. (2009).
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Speech Production and Comprehension: Brain Mechanisms • Recognition of People’s Voices • Most cases of phonagnosia are caused by brain damage. • Recognition of a particular voice is independent of the recognition of words and their meanings: Some people have lost the ability to understand words but can still recognize voices, while others display the opposite deficits (Belin, Fecteau, and Bédard, 2004).
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Speech Production and Comprehension: Brain Mechanisms • Recognition of People’s Voices • So far, all cases of acquired phonagnosia (that is phonagnosia caused by brain damage) show damage in the right hemisphere, usually in the parietal lobe or the anterior superior temporal cortex. • Functional imaging studies have implicated the right anterior superior temporal cortex in voice recognition.
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Speech Production and Comprehension: Brain Mechanisms • Stuttering • Stuttering is a speech disorder characterized by frequent pauses, prolongations of sounds, or repetitions of sounds, syllables, or words that disrupt the normal flow of speech. • Stuttering, which appears to be influenced by genetic factors, affects approximately 1 percent of the population and is three times more prevalent in men than in women (Brown et al., 2005; Fisher, 2010).
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Speech Production and Comprehension: Brain Mechanisms • Stuttering • Stuttering seldom occurs when a person says a single word or is asked to read a list of words; it most often occurs at the beginning of a sentence, especially if the planned sentence is long or grammatically complex. • This fact suggests that stuttering is a disorder of “selection, initiation, and execution of motor sequences necessary for fluent speech production” (Watkins et al., 2008, p. 50).
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Speech Production and Comprehension: Brain Mechanisms • Stuttering • Stuttering is not a result of abnormalities in the neural circuits that contain the motor programs for speech. • For example, stuttering is reduced or eliminated when a person reads aloud with another speaker, sings, or reads in cadence with a rhythmic stimulus. • The problem appears to lie more in the neural mechanisms that are involved in planning and initiation of speech.
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Speech Production and Comprehension: Brain Mechanisms • Stuttering • Figure 14.21 compares regional brain activity of fluent speakers and people who stutter. • Areas shaded in orange indicate regions that are more highly activated in the brains of stutterers; areas shaded in blue indicate regions that are more highly activated in the brains of fluent speakers. (The only region in the latter category is in the temporal lobe. See Figure 14.21.)
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Figure 4.21, page 498
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Speech Production and Comprehension: Brain Mechanisms • Stuttering • The authors suggest that the problem may be caused by faulty auditory feedback from sounds of the stutterer’s own speech, shown by the lack of activity in the temporal lobe. • They note that a magnetoencepholagraphic study by Salmelin et al. (2000) found disruptions in the normal timing of activation of brain regions involved in speech production.
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Speech Production and Comprehension: Brain Mechanisms • Stuttering • Watkins et al. (2008) used diffusion tensor imaging and found decreases in the white matter beneath the ventral premotor cortex of people who stuttered. • They suggested that the axons in this white matter connect the ventral premotor cortex with regions of the superior temporal cortex and inferior parietal cortex that are involved in integrating the planning of speech with auditory feedback from one’s own voice.
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Speech Production and Comprehension: Brain Mechanisms • Stuttering • Neumann et al. (2005) provide further evidence that the apparently abnormal auditory feedback in stutterers is reflected in decreased activation of their temporal cortex. • The authors used functional MRI (fMRI) to measure the regional brain activation of stutterers reading sentences aloud during two sessions, one before and one after a successful twelve-week course of fluency-shaping therapy. • Figure 14.22 shows that after the therapy, the activation of the temporal lobe—a region that both Brown et al. (2005) found to show decreased activation—was increased. (See Figure 14.22.)
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Figure 4.22, page 498
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • Two regions of the brain are especially important in understanding and producing speech. Broca’s area, in the left frontal lobe just rostral to the region of the primary motor cortex that controls the muscles of speech, is involved with speech production. • Broca’s aphasia—which is caused by damage to Broca’s area, adjacent regions of the frontal cortex, and underlying white matter—consists of varying degrees of agrammatism, anomia, and articulation difficulties.
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • Wernicke’s area, in the posterior superior temporal lobe, is involved with speech perception. • The region just adjacent to Wernicke’s area, which I have called the posterior language area, is necessary for speech comprehension and the translation of thoughts into words.
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • Wernicke’s area, in the posterior superior temporal lobe, is involved with speech perception. • The region just adjacent to Wernicke’s area, which I have called the posterior language area, is necessary for speech comprehension and the translation of thoughts into words.
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • Wernicke’s area, in the posterior superior temporal lobe, is involved with speech perception. • The region just adjacent to Wernicke’s area, which I have called the posterior language area, is necessary for speech comprehension and the translation of thoughts into words. • Wernicke’s aphasia, caused by damage to Wernicke’s area and the posterior language area, consists of poor speech comprehension, poor repetition, and production of fluent, meaningless speech.
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • Transcortical sensory aphasia, caused by damage to the posterior speech area, consists of poor speech comprehension and production, but the patients can repeat what they hear. • Thus, the symptoms of Wernicke’s aphasia consist of those of transcortical sensory aphasia plus those of pure word deafness. (WA = TSA + PWD.)
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • The fact that people with transcortical sensory aphasia can repeat words that they cannot understand suggests that there is a direct connection between Wernicke’s area and Broca’s area. • Indeed, there is: the arcuate fasciculus. • Damage to this bundle of axons produces conduction aphasia: disruption of the ability to repeat exactly what was heard without disruption of the ability to comprehend or produce meaningful speech.
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • A parallel pathway, consisting of an anterior and a posterior bundle that connect in the inferior parietal cortex, may be responsible for the ability of people with pure conduction aphasia to understand and paraphrase what they hear.
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • The meanings of words are our memories of objects, actions, and other concepts associated with them. • These meanings are memories and are stored in the association cortex, not in the speech areas themselves. • Anomic aphasia, caused by damage to the temporal or parietal lobes, consists of difficulty in word finding, particularly in naming objects.
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • Brain damage can also disrupt the “definitions” as well as the “entries” in the mental dictionary; damage to specific regions of the association cortex effectively erases some categories of the meanings of words. • Damage to Broca’s area and surrounding regions disrupts the ability to name actions—to think of appropriate verbs.
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • The left hemisphere plays the more important role in the language abilities of deaf people who use sign language, just as it is in people who communicate acoustically. • Gestural language may have been the precursor to vocal speech; mirror neurons in Broca’s area are activated by hand movements. • Prosody includes changes in intonation, rhythm, and stress that add meaning, especially emotional meaning, to the sentences that we speak •
The neural mechanisms that control the prosodic elements of speech appear to be in the right hemisphere. 134
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Speech Production and Comprehension: Brain Mechanisms • Section Summary: Speech Production and Comprehension: Brain Mechanisms • Stuttering appears to be cause by abnormalities in neural circuits that are involved in feedback and planning and initiating speech, not in the circuits that contain the motor programs for articulation. • Functional imaging indicates deficient auditory feedback produced by the stutterer’s own voice. Delayed auditory feedback, which impairs the speech of most fluent speakers, often facilitates the speech of stutterers.
• Because so many terms and symptoms were described in this section, I have provided a table that summarizes them. (See Table 14.1.)
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Table 14.1, page 500
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Disorders of Reading and Writing • Reading and writing are closely related to listening and talking; thus, oral and written language abilities have many brain mechanisms in common. • This section discusses the neural basis of reading and writing disorders. • As you will see, the study of these disorders has provided us with some useful and interesting information.
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Disorders of Reading and Writing • Relation to Aphasia • The reading and writing skills of people with aphasia almost always resemble their speaking and comprehending abilities. • For example, patients with Wernicke’s aphasia have as much difficulty reading and writing as they do speaking and understanding speech. • Patients with Broca’s aphasia comprehend what they read about as well as they can understand speech, but their reading aloud is poor, of course. • If their speech is agrammatical, so is their writing; and if they fail to comprehend grammar when listening to speech, they also fail to do so when reading.
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Disorders of Reading and Writing • Relation to Aphasia • Patients with conduction aphasia generally have some difficulty reading; and when they read aloud, they often make semantic paraphasias (saying synonyms for some of the words they read), just as they do when attempting to repeat what they hear. • Depending on the location of the lesion, some patients with transcortical sensory aphasia may read aloud accurately but fail to comprehend what they read.
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Disorders of Reading and Writing • Pure Alexia • Dejerine (1892) described a remarkable syndrome, which we now call pure alexia, or sometimes pure word blindness or alexia without agraphia. • pure alexia
• Loss of the ability to read without loss of the ability to write; produced by brain damage. • His patient had a lesion in the visual cortex of the left occipital lobe and the posterior end of the corpus callosum. • The patient could still write, although he had lost the ability to read. In fact, if he was shown some of his own writing, he could not read it. 140
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Disorders of Reading and Writing • Pure Alexia • Several years ago, some colleagues and I studied a man with pure alexia who discovered his ability to write in an interesting way. • A few months after he sustained a head injury that caused his brain damage, he and his wife were watching a service person repair their washing machine. • The patient wanted to say something privately to his wife, so he picked up a pad of paper and jotted a note.
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Disorders of Reading and Writing • Pure Alexia • As he was handing it to her, they suddenly realized with amazement that although he could not read, he was able to write! • His wife brought the note to their neurologist, who asked the patient to read it. Although he remembered the gist of the message, he could not read the words. • Unfortunately, I do not have a copy of that note, but Figure 14.23 shows the writing of another person with pure alexia. (See Figure 14.23.)
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Figure 14.23, page 501
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Disorders of Reading and Writing • Pure Alexia • Although patients with pure alexia cannot read, they can recognize words that are spelled aloud to them; therefore, they have not lost their memories of the spellings of words. • Pure alexia is obviously a perceptual disorder; it is similar to pure word deafness, except that the patient has difficulty with visual input, not auditory input. • The disorder is caused by lesions that prevent visual information from reaching the visual association cortex of the left hemisphere (Damasio and Damasio, 1983, 1986; Molko et al., 2002).
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Disorders of Reading and Writing • Pure Alexia • Figure 14.24 explains why Dejerine’s original patient could not read. • The first diagram shows the pathway that visual information would take if a person had damage only to the left primary visual cortex. • In this case the person’s right visual field would be blind; he or she would see nothing to the right of the fixation point. But people with this disorder can read. • Their only problem is that they must look to the right of each word so that they can see all of it, which means that they read somewhat more slowly than someone with full vision. (See Figure 14.24a.) 145
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Figure 4.24, page 502
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Disorders of Reading and Writing • Pure Alexia • The second diagram shows Dejerine’s patient. • Notice how the additional lesion of the corpus callosum prevents visual information concerning written text from reaching the VWFA in the left hemisphere. • Because this brain region is essential for the ability to recognize words, the patient cannot read. (See Figure 14.24b.)
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Disorders of Reading and Writing • Pure Alexia • Mao-Draayer and Panitch (2004) reported the case of a man with multiple sclerosis who displayed the symptoms of pure alexia after sustaining a lesion that damaged both the subcortical white matter of the left occipital lobe and the posterior corpus callosum. • As you can see in Figure 14.25, the lesions are in precisely the locations that Dejerine predicted would cause this syndrome, except that the white matter that serves the left primary visual cortex is damaged, not the cortex itself. (See Figure 14.25.)
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Figure 4.25, page 502
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Disorders of Reading and Writing • Toward an Understanding of Reading • Reading involves at least two different processes: direct recognition of the word as a whole and sounding it out letter by letter. • When we see a familiar word, we normally recognize it and pronounce it—a process known as whole-word reading. • whole-word reading
• Reading by recognizing a word as a whole; “sight reading.”
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Disorders of Reading and Writing • Toward an Understanding of Reading • The second method, which we use for unfamiliar words, requires recognition of individual letters and knowledge of the sounds they make. • This process is known as phonetic reading. • phonetic reading • Reading by decoding the phonetic significance of letter strings; “sound reading.”
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Disorders of Reading and Writing • Toward an Understanding of Reading • Dyslexia means “faulty reading.” • Acquired dyslexias are those caused by damage to the brains of people who already know how to read. • In contrast, developmental dyslexias refer to reading difficulties that become apparent when children are learning to read. • Developmental dyslexias, which appear to involve anomalies in brain circuitry, are discussed in a later section. 152
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Disorders of Reading and Writing • Toward an Understanding of Reading • Figure 14.26 illustrates some elements of the reading processes. • The diagram is an oversimplification of a very complex process, but it helps to organize some of the facts that investigators have obtained. • It considers only reading and pronouncing single words, not understanding the meaning of text. When we see a familiar word, we normally recognize it as a whole and pronounce it. • If we see an unfamiliar word or a pronounceable nonword, we must try to read it phonetically. (See Figure 14.26.)
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Figure 4.26, page 503
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Disorders of Reading and Writing • Toward an Understanding of Reading • Investigators have reported several types of acquired dyslexias, and I will describe three of them in this section: surface dyslexia, phonological dyslexia, and direct dyslexia. • The term surface reflects the fact that people with this disorder make errors related to the visual appearance of the words and to pronunciation rules, not to the meaning of the words, which is metaphorically “deeper” than the appearance. • surface dyslexia • A reading disorder in which a person can read words phonetically but has difficulty reading irregularly spelled words by the whole-word method.
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Disorders of Reading and Writing • Toward an Understanding of Reading • Because patients with surface dyslexia have difficulty recognizing words as a whole, they are obliged to sound them out. • Thus, they can easily read words with regular spelling, such as hand, table, or chin. • However, they have difficulty reading words with irregular spelling, such as sew, pint, and yacht. • In fact, they may read these words as sue, pinnt, and yatchet.
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Disorders of Reading and Writing • Toward an Understanding of Reading • For example, if you hear the single word “pair” without additional information, you cannot know whether the speaker is referring to pair, pear, or pare. • Thus, a patient with surface dyslexia who reads the word pair might say, “. . . it could be two of a kind, apples and . . . or what you do with your fingernails” (Gurd and Marshall, 1993, p. 594). (See Figure 14.27.)
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Figure 14.27, page 504
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Disorders of Reading and Writing • Toward an Understanding of Reading • The symptoms of phonological dyslexia are opposite those of surface dyslexia: People with this disorder can read by the whole-word method but cannot sound words out. • Thus, they can read words that they are already familiar with but have great difficulty figuring out how to read unfamiliar words or pronounceable nonwords (Beauvois and Dérouesné, 1979; Dérouesné and Beauvois, 1979). • phonological dyslexia • A reading disorder in which a person can read familiar words but has difficulty reading unfamiliar words or pronounceable nonwords.
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Disorders of Reading and Writing • Toward an Understanding of Reading • Phonological dyslexia provides further evidence that whole-word reading and phonological reading involve different brain mechanisms. • Phonological reading, which is the only way we can read nonwords or words we have not yet learned, entails some sort of letter-to-sound decoding.
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Disorders of Reading and Writing • Toward an Understanding of Reading • Obviously, phonological reading of English requires more than decoding of the sounds produced by single letters, because, for example, some sounds are transcribed as twoletter sequences (such as th or sh) and the addition of the letter e to the end of a word lengthens an internal vowel (can becomes cane). (See Figure 14.28.)
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Figure 14.28, page 504
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Disorders of Reading and Writing • Toward an Understanding of Reading • Evidence from lesion and functional-imaging studies with readers of English, Chinese, and Japanese suggest that the process of whole-word reading follows the ventral stream of the visual system to a region the fusiform gyrus, located on the base of the temporal lobe.
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Disorders of Reading and Writing • Toward an Understanding of Reading • For example, functional-imaging studies by Thuy et al. (2004) and Liu et al. (2008) found that the reading of kanji words or Chinese characters (whole-word reading) activated the left fusiform gyrus, a region of the cerebral cortex located at the base of the temporal lobe. • This region has come to be known as the visual word-form area or VWFA (Dehaene, 2009). • visual word-form area (VWFA) • A region of the fusiform gyrus on the base of the temporal lobe that plays a critical role in whole-word recognition.
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Disorders of Reading and Writing • Toward an Understanding of Reading • The fact that phonological reading involves Broca’s area suggests that it may actually involve articulation—that we sound out words not so much by “hearing” them in our heads as by feeling ourselves pronounce them silently to ourselves. • Once words have been identified—by either means—their meaning must be accessed, which means that the two pathways converge on regions of the brain involved in recognition of word meaning, grammatical structure, and semantics. (See Figure 14.29.)
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Figure 14.29, page 505
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Disorders of Reading and Writing • Toward an Understanding of Reading • The neural activation responsible for the analysis of a printed word (Figure 14.30 right) begins in the visual cortex and spreads to the base of the temporal lobe (including the VWFA in the fusiform gyrus) and then to the inferior frontal cortex. • The temporoparietal cortex receives little activation, presumably because the subjects were fluent readers who did not need to sound out the common words they were asked to read. (See Figure 14.30.)
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Figure 14.30, page 506
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Disorders of Reading and Writing • Toward an Understanding of Reading • Thus, reading and object recognition use brain mechanisms that undoubtedly existed long before the invention of writing. • However, just as experience seeing faces affects the development of the fusiform face area in the right hemisphere, experience learning to read words affects the development of the neural circuitry in the visual word-form area—which, probably not coincidentally, is found in the fusiform cortex of the left hemisphere.
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Disorders of Reading and Writing • Toward an Understanding of Reading • A functional imaging study by Brem et al. (2010) scanned the brains of young children (mean age 6.4 years) who had not yet learned to read. Initially, the sight of printed words activated the ventral posterior occipitotemporal region bilaterally. • After 3–4 hours of teaching the associations of written letters and their sounds, the sight of words activated the left hemisphere. • Clearly, learning to read affects the connections of the neural system involved in recognizing letters and words.
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Disorders of Reading and Writing • Toward an Understanding of Reading • The fusiform face area has the ability to quickly recognize unique configurations of people’s eyes, noses, lips, and other features of their faces even when the differences between two people’s faces are very similar. • For example, parents and close friends of identical twins can see at a glance which twin they are looking at.
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Disorders of Reading and Writing • Toward an Understanding of Reading • Similarly, our VWFA can recognize a word even if it closely resembles another one. (See Figure 14.31.) • It can also quickly recognize words written in different typestyles, fonts, or CASES. • This means that the VWFA can recognize whole words with different shapes; certainly, chair and CHAIR do not look the same.
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Figure 14.31, page 507
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Disorders of Reading and Writing • Toward an Understanding of Reading • A functional-imaging study by Vinckier et al. (2007) investigated the means by which the brain recognizes whole words. • A bigram is a sequence of two letters (bi, “two”; gram, “something written”).
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Disorders of Reading and Writing • Toward an Understanding of Reading • Frequent bigrams are two-letter sequences that are often encountered in a particular language. • For example, the bigram SH often occurs in English. In contrast, LQ is an infrequent bigram. • Quadrigrams are strings of four letters and can be classified as frequent or infrequent.
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Disorders of Reading and Writing • Toward an Understanding of Reading • Vinckier and his colleagues had adult readers look at the following stimuli: (1) strings of false fonts (nonsensical letterlike symbols), (2) strings of infrequent letters, (3) strings that contained only infrequent bigrams, (4) strings that contained frequent bigrams, (5) strings that contained frequent quadrigrams, and (6) real words. (See Figure 14.32 for examples of these stimuli.)
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Figure 14.32, page 507
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Disorders of Reading and Writing • Toward an Understanding of Reading • The most selective region included the left anterior fusiform cortex (the VWFA), which was activated only by actual words. • In fact, as Figure 14.33 shows, the scans revealed a posterior-to-anterior gradient of selectivity, from symbol to whole word, along the base of the left occipital and temporal lobes. • A second, smaller, gradient was seen in Broca’s area.
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Disorders of Reading and Writing • Toward an Understanding of Reading • Presumably, this gradient represented phonetic reading—decoding of the sounds represented by the stimuli that the subjects viewed. • Note that very little activity was produced by letterlike symbols (shown in red) in Broca’s area. • This makes sense, because there is no way to pronounce these symbols. (See Figure 14.33.)
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Figure 14.33, page 507
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Disorders of Reading and Writing • Toward an Understanding of Reading • A patient with a severe seizure disorder became a candidate for surgery aimed at removal of a seizure focus. • Before the surgery was performed, the patient viewed printed words and pictures of faces, houses, and tools while his brain was being scanned.
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Disorders of Reading and Writing • Toward an Understanding of Reading • He was warned that the seizure focus was located in a region that played a critical role in reading, but his symptoms were so severe that he elected to undergo the surgery. • As expected, the surgery produced a deficit in whole-word reading. • A combination of structural and functional imaging revealed that the lesion—a very small one—was located in the fusiform gyrus, the location of the VWFA. (See Figure 14.34.)
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Figure 14.34, page 508
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Disorders of Reading and Writing • Toward an Understanding of Reading • Szwed et al. (2009) note that the most important cues to object recognition (which is the primary task of the visual system) are those that remain relatively constant even when we view objects from different angles. • The most reliable of these cues are the ways that lines meet at vertices, forming junctions with particular shapes, such as L, T, and X [note: sans serif] (I think you can see where this is going.)
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Disorders of Reading and Writing • Toward an Understanding of Reading • Szwed and his colleagues presented incomplete drawings of objects and letters that were missing their vertices (junctions of lines) or portions of the midsegments (lines between these junctions). • Figure 14.35 shows a drawing and a word with the vertices missing. • Can you figure out what they are? (See Figure 14.35.) • Now turn the page and look at the same drawing and word with its midsegments missing instead. I think you will find them easier to recognize. (See Figure 14.36.) 185
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Figure 14.35, page 508
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Figure 14.36, page 509
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Disorders of Reading and Writing • Toward an Understanding of Reading • Early forms of writing used actual pictures, but the pictures became simplified and eventually turned into simple lines or intersecting lines and curves. • Even complex symbols such as Chinese characters consist of intersecting brush strokes. • Figure 14.37 shows a few of the ways that different types of intersections of two line segments can be transformed into letters found in various writing systems. (See Figure 14.37.)
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Figure 14.37, page 510
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Disorders of Reading and Writing • Toward an Understanding of Reading • As I mentioned earlier in this chapter, recognizing a spoken word is different from understanding it. • For example, patients with transcortical sensory aphasia can repeat what is said to them even though they show no signs of understanding what they hear or say.
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Disorders of Reading and Writing • Toward an Understanding of Reading • The same is true for reading. • Direct dyslexia resembles transcortical sensory aphasia, except that the words in question are written, not spoken (Schwartz, Marin, and Saffran, 1979; Lytton and Brust, 1989; Gerhand, 2001). • direct dyslexia • A language disorder caused by brain damage in which the person can read words aloud without understanding them. • Patients with direct dyslexia are able to read aloud even though they cannot understand the words they are saying. 191
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Disorders of Reading and Writing • Developmental Dyslexias • Some children have great difficulty learning to read and never become fluent readers, even though they are otherwise intelligent. Specific language learning disorders, called developmental dyslexias, tend to occur in families, a finding that suggests a genetic (and hence biological) component.
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Disorders of Reading and Writing • Developmental Dyslexias • The concordance rate of monozygotic twins ranges from 84 percent to 100 percent, and that of dizygotic twins ranges from 20 percent to 35 percent (Démonet, Taylor, and Chaix, 2004). • Linkage and association studies suggest that the chromosomes 3, 6, and 15 may contain genes responsible for different components of this disorder (Kang and Drayna, 2011).
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Disorders of Reading and Writing • Developmental Dyslexias • We should not expect that developmental dyslexia involves only deficits in reading. Indeed, researchers have found a variety of language deficits that do not directly involve reading. • One common deficit is deficient phonological awareness. • That is, people with developmental dyslexia have difficulty blending or rearranging the sounds of words that they hear (Eden and Zeffiro, 1998).
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Disorders of Reading and Writing • Developmental Dyslexias • Problems such as these might be expected to impair the ability to read phonetically. • Dyslexic children also tend to have great difficulty in writing: They make spelling errors, they show poor spatial arrangements of letters, they omit letters, and their writing tends to have weak grammatical development (Habib, 2000). • Developmental dyslexia is a heterogeneous and complex trait; therefore, it undoubtedly has more than one cause. • However, most studies that have closely examined the nature of the impairments seen in people with developmental dyslexia have found phonological impairments to be most common. 195
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Disorders of Reading and Writing • Developmental Dyslexias • Some evidence has been obtained from functional-imaging studies that suggests that the brains of dyslexics process written information differently than do the brains of proficient readers. • For example, Shaywitz et al. (2002) had seventy dyslexic children and seventy-four nondyslexic children read words and pronounceable nonwords. • The researchers found significantly different patterns of brain activation in the two groups. • A child’s reading skill was positively correlated with activation of the left VWFA. 196
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Disorders of Reading and Writing • Developmental Dyslexias • Hoeft et al. (2007) found that dyslexics showed decreased activation in the left temporoparietal cortex (dorsal to the region identified by Shaywitz et al.) and in the VWFA. • They also saw hyperactivation of the left inferior frontal cortex, including Broca’s area. • Presumably, the activation of Broca’s area reflected an effort to decode the phonology of the incomplete information being received from the poorly functioning regions of the more posterior brain regions involved in reading.
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Disorders of Reading and Writing • Toward an Understanding of Writing • Writing depends on knowledge of the words that are to be written, along with the proper grammatical structure of the sentences they are to form. • Therefore, if a patient is unable to express himself or herself by speech, we should not be surprised to see a writing disturbance (dysgraphia) as well. • In addition, most cases of dyslexia are accompanied by dysgraphia.
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Disorders of Reading and Writing • Toward an Understanding of Writing • One type of writing disorder involves difficulties in motor control—in directing the movements of a pen or pencil to form letters and words. Investigators have reported surprisingly specific types of writing disorders that fall into this category. • For example, some patients can write numbers but not letters, some can write uppercase letters but not lowercase letters, some can write consonants but not vowels, some can write cursively but cannot print uppercase letters, and others can write letters normally but have difficulty placing them in an orderly fashion on the page (Cubelli, 1991; Alexander et al., 1992; Margolin and Goodman-Schulman, 1992; Silveri, 1996).
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Disorders of Reading and Writing • Toward an Understanding of Writing • Many regions of the brain are involved in writing. • For example, damage that produces various forms of aphasia will produce impairments in writing that are similar to those seen in speech. • Organization of the motor aspects of writing involves the dorsal parietal lobe and the premotor cortex. These regions (and the primary motor cortex, of course) become activated when people engage in writing, and damage to these regions impairs writing (Otsuki et al., 1999; Katanoda, Yoshikawa, and Sugishita, 2001; Menon and Desmond, 2001).
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Disorders of Reading and Writing • Toward an Understanding of Writing • A functional-imaging study by Rijntjes et al. (1999) had people sign their names with either their index finger or their big toe. • In both cases, doing so activated the premotor cortex that controlled movements of the hand. • This finding suggests that when we learn to make a complex series of movements, the relevant information is stored in regions of the motor association cortex that control the part of the body that is being used but that this information can be used to control similar movements in other parts of the body.
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Disorders of Reading and Writing • Toward an Understanding of Writing • Longcamp et al. (2005) found that simply looking at alphabetical characters activated the premotor cortex: on the left side in right-handed people and on the right side in lefthanded people. (See Figure 14.38.)
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Figure 14.38, page 511
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Disorders of Reading and Writing • Toward an Understanding of Writing • A more basic type of writing disorder involves problems in the ability to spell words, as opposed to problems with making accurate movements of the fingers. • Like reading, writing (or, more specifically, spelling) involves more than one method. • The first method is related to audition.
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Disorders of Reading and Writing • Toward an Understanding of Writing • When children acquire language skills, they first learn to recognize the sounds of words, then learn to say them, then learn to read, and then learn to write •
Undoubtedly, reading and writing depend heavily on the skills that are learned earlier.
• For example, to write most words, we must be able to “sound them out in our heads,” that is, to hear them and to articulate them subvocally.
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Disorders of Reading and Writing • Toward an Understanding of Writing • A second way of writing involves transcribing an image of what a particular word looks like—copying a visual mental image. • Have you ever looked off into the distance to picture a word so that you could remember how to spell it? • Some people are not very good at phonological spelling and have to write some words down to see whether they look correct. • This method obviously involves visual memories, not acoustical ones. 206
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Disorders of Reading and Writing • Toward an Understanding of Writing • A third way of writing involves memorization of letter sequences. • We learn these sequences the way we learn poems or the lyrics to a song. • For example, many Americans learned to spell Mississippi with a singsong chant that goes like this: M-i-s-s-i-s-s-i-p-p-i, emphasizing the boldfaced letters.
• This method involves memorizing sequences of letter names, not translating sounds into the corresponding letters.
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Disorders of Reading and Writing • Toward an Understanding of Writing • Finally, the fourth way of writing involves motor memories. • We undoubtedly memorize motor sequences for very familiar words, such as our own names. • Most of us need not sound out our names to ourselves when we write our signature, nor need we say the sequence of letters to ourselves, nor need we imagine what our signature looks like.
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Disorders of Reading and Writing • Toward an Understanding of Writing • Neurological evidence supports at least the first three of these speculations. Brain damage can impair the first of these methods: phonetic writing. • This deficit is called phonological dysgraphia (Shallice, 1981). • phonological dysgraphia • A writing disorder in which the person cannot sound out words and write them phonetically.
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Disorders of Reading and Writing • Toward an Understanding of Writing • Dysgraphia refers to a writing deficit, just as dyslexia refers to a reading deficit. • People with this disorder are unable to sound out words and write them phonetically. • Thus, they cannot write unfamiliar words or pronounceable nonwords, such as the ones I presented in the section on reading.
• They can, however, visually imagine familiar words and then write them.
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Disorders of Reading and Writing • Toward an Understanding of Writing • Phonological dysgraphia appears to be caused by damage to regions of the brain involved in phonological processing and articulation. • Damage to Broca’s area, the ventral precentral gyrus, and the insula cause this disorder, and phonological spelling tasks activate these regions (Omura et al., 2004; Henry et al., 2007).
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Disorders of Reading and Writing • Toward an Understanding of Writing • Orthographic dysgraphia is just the opposite of phonological dysgraphia: It is a disorder of visually based writing. • orthographic dysgraphia • A writing disorder in which the person can spell regularly spelled words but not irregularly spelled ones.
• People with orthographic dysgraphia can only sound words out; thus, they can spell regular words such as care or tree, and they can write pronounceable nonsense words.
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Disorders of Reading and Writing • Toward an Understanding of Writing • However, they have difficulty spelling irregular words such as half or busy (Beauvois and Dérouesné, 1981); they may write haff or bizzy. • Orthographic dysgraphia (impaired phonological writing), like surface dyslexia, is caused by damage to the VWFA on the base of the temporal lobe (Henry et al., 2007).
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Disorders of Reading and Writing • Toward an Understanding of Writing • Both lesions studies and functional-imaging studies implicate the posterior inferior temporal cortex in writing of irregularly spelled English words or kanji symbols (Nakamura et al., 2000; Rapscak and Beeson, 2004). • This region appears to be involved not in the motor aspects of writing but in knowledge of how irregular words are spelled or what strokes make up a kanji character. • Figure 14.39 shows the brain damage that causes phonological and orthographic dysgraphia. (See Figure 14.39.)
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Figure 14.39, page 512
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Disorders of Reading and Writing • Toward an Understanding of Writing • The third method of spelling depends on a person’s having memorized sequences of letters that spell particular words. • Cipolotti and Warrington (1996) reported the case of a patient who lacked this ability. • The patient sustained a left hemisphere stroke that severely disrupted his ability to spell words orally and impaired his ability to recognize words that the examiners would spell aloud. • Presumably, his ability to spell written words depended on the first two methods of writing: auditory and visual. 216
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Disorders of Reading and Writing • Toward an Understanding of Writing • The examiners noted that when they spelled out words to him, he would make writing movements with hand on top of his knee. • When they asked him to clasp his hands together so that he could not make these writing movements, his ability to recognize four-letter words being spelled aloud dropped from 66 percent to 14 percent. • It appears that he was using feedback from hand movements to recognize the words he was “writing” on his knee.
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Disorders of Reading and Writing • Section Summary: Disorders of Reading and Writing • Brain damage can produce reading and writing disorders. • With few exceptions, aphasias are accompanied by writing deficits that parallel the speech production deficits and by reading deficits that parallel the speech comprehension deficits. • Pure alexia is caused by lesions that produce blindness in the right visual field and that destroy fibers of the posterior corpus callosum.
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Disorders of Reading and Writing • Section Summary: Disorders of Reading and Writing • Research in the past few decades has discovered that acquired reading disorders (dyslexias) can fall into one of several categories, and the study of these disorders has provided neuropsychologists and cognitive psychologists with thought-provoking information that has helped them to understand the brain mechanisms involved in reading •
Analysis of written words appears to begin in the left posterior inferior temporal cortex.
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Disorders of Reading and Writing • Section Summary: Disorders of Reading and Writing • Phonological information is then analyzed by the temporoparietal cortex and Broca’s area, whereas word-form information is analyzed by the visual word-form area, located in the fusiform cortex. • Surface dyslexia is a loss of whole-word reading ability. • Phonological dyslexia is loss of the ability to read phonetically. • Reading of kana (phonetic) and kanji (pictographic) symbols by Japanese people is equivalent to phonetic and whole-word reading, and damage to different parts of the brain interfere with these two forms of reading. 220
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Disorders of Reading and Writing • Section Summary: Disorders of Reading and Writing • Direct dyslexia is analogous to transcortical sensory aphasia; the patients can read words aloud but cannot understand what they are reading. • Some can read both real words and pronounceable nonwords, so both phonetic and whole-word reading can be preserved.
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Disorders of Reading and Writing • Section Summary: Disorders of Reading and Writing • Brain damage can disrupt writing ability by impairing people’s ability to form letters—or even specific types of letters, such as uppercase or lowercase letters or vowels. • The dorsal parietal cortex appears to be the most critical region for knowledge of the movements that produce letters. Other deficits involve the ability to spell words.
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Disorders of Reading and Writing • Section Summary: Disorders of Reading and Writing • We normally use at least four different strategies to spell words: phonetic (sounding the word out), visual (remembering how it looks on paper), sequential (recalling memorized sequences of letters), and motor (recalling memorized hand movements in writing very familiar words). • Two types of dysgraphia—phonological and orthographic—represent difficulties in implementing phonetic and visual strategies, respectively.
• The existence of these two disorders indicates that several different brain mechanisms are involved in the process of writing.
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Disorders of Reading and Writing • Section Summary: Disorders of Reading and Writing • Developmental dyslexia is a hereditary condition that may involve abnormal development of parts of the brain that play a role in language. • Most developmental dyslexics have difficulty with phonological processing—of spoken words as well as written ones. • Functional-imaging studies report decreased activation of a region of the left occipitotemporal and temporoparietal cortex and hyperactivation of Broca’s area may be involved in developmental dyslexia.
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Disorders of Reading and Writing • Section Summary: Disorders of Reading and Writing • Children who learn to read languages that have writing with regular correspondence between spelling and pronunciation (such as Italian) are much less likely to become dyslexic than are those who learn to read languages with irregular spelling (such as English or French). • A better understanding of the components of reading and writing may help us to develop effective teaching methods that will permit people with dyslexia to take advantage of the abilities that they do have. • Table 14.2 summarizes the disorders that were described in this section.
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Table 14.2, page 514
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