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NSCI 410 (1)

Speech and Comprehension.doc

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NSCI 410
Greg Matlashewski

Speech and Comprehension • Psycholinguistics: branch of psychology devoted to study verbal behaviour. Speech is social, it is learned and used in interaction with others. • We extract words from a stream of speech. • Our auditory systems recognize patterns underlying speech. • Belin, Zatorre, and Ahad: used fMRI scans to find that some regions of the brain responded more to human vocalizations rather than just other natural sounds. Left hemisphere showed larger contrast and thus, it plays a larger role in analyzing speech. • Phonemes: elements (smallest units) of speech. Eg: pin is three phonemes “/p/+/i/+/n/”. o Voice-onset time: a way in which we discriminate among phonemes. It is the delay between the initial consonant sound and vibrating vocal chords (voicing). Eg: there is a delay in voicing for “pa” compared to “ba” although the initial sound (made with the mouth) is the same. o Phonemic discriminations initially occur in both hemispheres. Some areas of the brain in the left hemisphere respond solely to intelligible speech even if it is highly distorted. o Our ability to recognize highly distorted speech supports that our perception of a phoneme is affected by the sounds that follow it (Ganong). We recognize speech sounds in larger chunks such as syllables. • Sanders, Newport, and Neville: played a continuous string of nonsense syllables to listeners. Chunks of this stream were given to participants to study as words. When the string was played once again the N100 response (electrical signal that occurs when a word is first recognized) showed up. • Context affects word perception through top-down processing. • Syntax/grammar: all languages follow certain principles called syntactical rules: grammatical rules for combining words to form phrases, clauses, and sentences. o Syntax is learned implicitly and is automatic. Involves different brain mechanisms than learning word meanings. o Syntactical clues are designed by:  Word order: tell us who does what do whom (in English), for example “A Xs the B”: A does something to B.  Word class: grammatical categories such as noun and verb. • Function words: adds little meaning but conveys important information about the sentence’s structure such as prepositions and articles. When they are omitted, we can often guess at function words. • Content words: express meaning such as nouns and verbs. • Content words express meaning and function words express the relationships between the content words.  Affixes: Sounds we add to beginning (prefix) or ends (suffix) of words. Adding affixes to nonsense words make them seems more like sentences (Epstein).  Semantics: the meaning represented by words.  Prosody: using changes in intonation and emphasis to convey meaning in speech. Important for emotion. In writing, syntactical clues and interfere with prosody producing brain activity similar to that of unexpected experiences. • Syntax is necessary but not sufficient for semantics. Things can make syntactical sense but we may not extract meaning from it. Likewise, semantics requires syntax for the entire picture. • We remember what is meant in sentences but quickly forget their form. Chomsky (linguist) suggested a model: o Deep structure is the essential meanings of a sentence. It is converted to speech by adding surface structure (grammatical features). o This model is not generally accepted by psychologists. • Aphasia: loss of language, recognition or comprehension or both. o Conduction aphasia: difficulty repeating words and phrases, but they are comprehended. Retain deep structure but not surface structure. • Pragmatics is knowledge of the world. Used in conversations and is involved in speech comprehension. Scripts: characteristics of typical situations that assist in comprehending a verbal discourse. A conversation can bring up certain scripts in the listener so the speaker can convey information without all the gritty details. • Areas important for speech: o Broca’s area: motor association cortex in left frontal lobe. Speech production occurs here. Sign language users also show activity in this area, meaning it is for more than just speech production.  Damage here (extending to underlying white matter) causes Broca’s aphasia which involves severe difficulty articulating words, especially function words. • Agrammatism: inability to properly use or comprehend function words and grammatical features. • Comprehension of word order, for example, is affected in Broca’s aphasia. • Deficit in comprehension parallels their deficit in production (grammatical and syntactical loss)  Wernicke suggested that Broca’s area contains memories of sequences of muscle movements needed to articulate words. Broca’s area is located just in front of the primary motor cortex. o Wernicke’s area: upper part of the left temporal lobe, involved in the recognition of speech.  Wernicke’s aphasia: • Damage to the left temporal and parietal cortex that includes Wernicke’s area. Causes deficits in perception of speech and producing fluent but meaningless speech and lack of content words. • Show poor comprehension, disorder known as receptive aphasia, inability to convert thoughts into words, and inability to recognize spoken words. o Recognizing is not the same as comprehending: a word for which there is no learned meaning associated can still be recognized.  Pure word deafness: Damage restricted to Wernicke’s area. Inability to comprehend the meaning of heard speech (can still read lips and writing) but one can still hear, speak properly, and write. Can recognize emotions conveyed through prosody but not what is being said.  Isolation aphasia: damage to the left temporal and parietal cortex that spares Wernicke’s area (area that surrounds Wernicke’s area is the posterior language area). Similar to Wernicke’s aphasia yet they can recognize and repeat words.  Posterior language area is responsible for word meanings. • Sounds of words recognized in Wernicke’s area, passed onto Broca’s area so they can be repeated. • fMRI and PET Studies on word recognition and production: o Broca’s aphasia patients show low activity in the lower left frontal lobe. Wernicke’s aphasia shows low activity in the temporal/parietal area of the brain. o Listening passively to a list of nouns activates the primary auditory cortex and Wernicke’s area. Repeating the nouns activates primary motor cortex and Broca’s area. • Semantics: the meaning of a word. Defined by the particular memories associated with the word. o Memories not stored in primary speech areas, but in other parts of the brain such as the association cortex. Different memories of one word can be stored in different areas of the brain but they are somehow activated and linked together. Reading • Saccades: rapid jumps of the eyes as we read. Fixations occur between saccades and are where visual information is gathered. Good readers’
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