Transcript Chapter 15
Chapter 15 Human Communication Speech production and comprehension Most of what we know about the physiology of language is from studying individuals that have suffered cerebrovascular accidents (a stroke; caused by blockage or rupture of a blood vessel in the brain); the interruption of blood flow deprives a region of the brain of its blood supply, which causes cells in that region to die Also, information from patients with seizure disorders that require brain surgery, and functional imaging studies Aphasia – most important category of speech disorders; difficulty in producing or comprehending speech not produced by deafness or a simple motor deficit; caused by brain damage Lateralization Verbal behavior is a lateralized function; that is, areas of the left hemisphere are primarily responsible for speech production and comprehension The left hemisphere is specialized more for the analysis of sequences of stimuli, occurring one after the other, which may explain why speech centers are located there However, although the circuits that are primarily involved in speech comprehension and production are located in one hemisphere, the R hemisphere also plays a minor part e.g. understanding semantics, prosody (i.e. normal rhythm and stress in speech) Speech production Damage to a certain region of the inferior left frontal lobe (Broca’s area) disrupts the ability to speak Broca’s aphasia – speech disorder characterized by slow, laborious, and nonfluent speech Have difficulty saying function words (e.g. the, in, some), but can manage to say content words (e.g. nouns, verbs, etc.) However, can still comprehend speech for the most part Broca’s aphasia Broca’s area contains memories of the sequences of muscular movements that are needed to articulate words Often become frustrated by their inability to speak correctly; however, comprehension is not perfect 3 major speech deficits with Broca’s aphasia: Difficulty in comprehending meaning from word order (“The horse kicks the cow” vs. “The cow kicks the horse”) Agrammatism – difficulty in comprehending or properly employing grammatical devices, such as verb endings and word order Anomia – difficulty in finding (remembering) the appropriate word to describe an object, action, or attribute Difficulty with articulation – mispronounce words, often realizing it afterwards, and trying to correct it Damage to different areas in and around Broca’s area leads to different symptoms of aphasia Left insular cortex – controls speech articulation (damage can cause 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 Speech comprehension Must not just recognize words, we must understand their meaning Wernicke’s area contains neural circuits that accomplish this task Wernicke’s aphasia – a form of aphasia characterized by poor speech comprehension and fluent but meaningless speech Comprehension tested by directing movement toward objects asked about by experimenter is also poor (e.g. “Point to the ink bottle” – patient cannot point to ink bottle) However, patients seem unaware of their deficit, unlike with Broca’s aphasia They do not recognize that their speech is faulty, nor that they do not comprehend other speech Wernicke suggested that this area is a location where memories of the sequences of sounds that constitute words are stored Wernicke’s aphasia Recognition: Pure word deafness Recognition (perceptual task) vs. comprehension (involves retrieval of additional info from memory) Damage to the L temporal lobe can produce pure word deafness (ability to hear, speak, and usually to read and write without being able to comprehend the meaning of speech) without disrupting other functions; e.g. can recognize non-speech sounds Not just an inability to understand the meaning of words, since most can read or write Speech sounds activate the superior temporal cortex The more crucial auditory component is timing, not pitch Can be caused by either damage to Wernicke’s area or to auditory input to Wernicke’s area Wernicke’s aphasia Comprehension: Transcortical sensory aphasia Posterior language area – region that surrounds the posterior part of the lateral fissure, near the junction of the temporal, occipital, and parietal lobes Damage to this area results in Transcortical sensory aphasia (difficulty in comprehending speech and producing meaningful spontaneous speech but can repeat speech Can recognize words (e.g. can repeat phrases) but cannot comprehend them Can be seen as Wernicke’s aphasia without a repitition deficit Wernicke’s aphasia What is meaning? Words refer to particular objects, actions or relationships in the world The meaning of a word then is defines by particular memories associated with it, primarily stored in association cortex First, we must recognize the sounds that constitute a word (Wernicke’s area) Next, the memories that constitute the meaning of that word must be activated (through PLA) to the neural circuits that contain these memories Hebb rule: when we hear a word several times, a particular set of neurons in the superior temporal lobe become active, and their interconnections eventually become strengthened Cell assembly – an assembly of interconnected neurons Wernicke’s aphasia What is meaning? (con’t) Damage to particular regions of the sensory association cortex can damage particular kinds of info and thus abolish particular kinds of meanings Damage to association cortex of L parietal lobe can produce autotopagnosia (inability to name body parts or to identify body parts that another person names) e.g. Patient cannot understand directional words (“The sky is down”) but can use those same words in a non-directional meaning (“After exchanging pleasantries, they got down to business”) Have no difficulty understanding the meaning of other words Studies of brain damaged patients suggest that comprehension of more subtle, figurative aspects of speech (abstract concepts, moral dilemmas, etc.) involves the R hemisphere Wernicke’s aphasia Repetition: Conduction aphasia Arcuate fasciculus – a bundle of axons that connects Wernicke’s area with Broca’s area Damage to this pathway causes conduction aphasia (characterized by inability to repeat words that are heard but the ability to speak normally and comprehend the speech of others) The symptoms that are seen in transcortical sensory aphasia and conduction aphasia suggest that there are pathways connection the speech mechanisms of the temporal lobe (Wernicke’s area) with those of the frontal lobe (Broca’s area) The arcuate fasciculus simply conveys speech sounds from Wernicke’s area to Broca’s area, and appears to play a role in STM of words and speech sounds that have just been heard Also a 2nd pathway, connecting the posterior language area and Broca’s area is indirect and based on semantics The arcuate fasciculus connecting both speech areas constitute the phonological loop Anomic aphasia Speech of patients with anomic aphasia (inability to remember the appropriate word for something) is fluent and grammatical, and comprehension is fine, but they cannot find the appropriate words These patients often employ circumlocutions (strategy by which people with anomia find alternative ways to say something when they are unable to think of the most appropriate words) Partial amnesia for words Fluent anomia is caused by damage to the L temporal or parietal lobe, sparing Wernicke’s area More trouble finding nouns than other words Damage to frontal lobe may lead to averbia (difficulty finding verbs instead of nouns) Aphasia in deaf people Some researchers believe that sign language preceded spoken language with our ancestors Broca’s area is activated when people observe and imitate finger movements The grammar of ASL (American Sign Language) is spatial, and thus cannot be translated word for word to a spoken language Aphasic disorders in deaf people may be caused by lesions to the R hemisphere, which is primarily involved in spatial perception and memory; However, all studies of deaf patients with aphasia for signs reported lesions of the L hemisphere The bilingual brain Evidence suggests that first and second languages share the same brain regions No evidence that brain damage is more likely to affect one language more than the other While neural representations of words for both languages seem to be intermingled with each other, cell assemblies required for storage of different grammatical rule appear to be separate (since languages can have different grammatical structures) Prosody: Rhythm, tone, and emphasis in speech 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 communicating emotion Patients with fluent aphasia and Wernicke’s aphasia is normal; however, it is disturbed in patients with Broca’s aphasia Prosody is a special function of the R hemisphere related to the more general role of this hemisphere in musical skills and the expression and recognition of emotions Patients with R hemisphere damage show a deficit in prosodic comprehension Speech production and comprehension Go over table 15.1 in book, page 501 for summary of all aphasias Disorders of reading and writing Relation to aphasia The reading and writing skills of patients with aphasia almost always resemble their speaking and comprehending abilities 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, but their reading aloud is poor Patients with conduction aphasia have some difficulty reading, and when they read aloud, will make semantic paraphasias (saying synonyms for some of the words they read) However, a few exceptions: Patients with severe fluent aphasia could not understand speech of others, but can read and understand what they read Disorders of reading and writing Pure alexia Loss of the ability to read without loss of the ability to write Can recognize words that are spelled aloud A perceptual disorder Caused by lesions that prevent visual info from reaching the extrastriate cortex of the L hemisphere Disorders of reading and writing Toward an understanding of reading Reading involves at least 2 different processes: Direct recognition of the word as a whole Sounding it out letter by letter When we see a familiar word, we normally recognize it by its shape and pronounce it (whole-word reading) unfamiliar words require the recognition of individual letters and knowledge of the sounds they make (phonetic reading) Dyslexias – faulty reading; acquired type caused by brain damage, developmental type becomes apparent when children learn to read Disorders of reading and writing Dyslexias Surface dyslexia – deficit in whole-word reading; caused by lesion to the L lateral temporal lobe Phonological dyslexia – can read whole-words but not sound words out; difficulty with new words must sound all words out, easier with words like hand or table, more difficult with words like yacht or sew Must listen to own pronunciation to understand what they are reading Caused by damage to L frontal lobe Word-form (or spelling) dyslexia – can read a word only after spelling out individual letters Direct dyslexia – resembles transcortical sensory aphasia; able to read aloud but not understand them; caused by damage to L frontal and temporal lobes Disorders of reading and writing Writing Depends on knowledge of words that are to be used, along with proper grammatical structure of the sentences they are to form One type of writing disorder involves difficulties in motor control Another type involves problems in ability to spell words Involves more than one method Audition – must hear words to be able to spell them, especially longer, more complex words Transcribing an image of what a particular word looks like Memorization of letter sequences Motor memories Brain damage can affect phonetic writing (phonological dysgraphia) – cannot sound words out and writ them phonetically Orthographic dysgraphia – can spell regularly spelled words (care or tree) but not irregularly spelled words (half or busy) Disorders of reading and writing Developmental dyslexias A reading difficulty in a person of normal intelligence and perceptual ability Of genetic origin or caused by prenatal or perinatal factors Abnormal development of certain brain regions may cause this Could be caused by magnocellular region of LGN – however not much evidence to support this Reading skill positively correlated with activation of L occipitotemporal cortex