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4. Perception of Language.docx

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PSYC 3290
Raluca Barac

4. Perception of Language Thursday, January 24, 2013 8:30 AM Main points  The study of speech sounds is called phonetics o Articulatory phonetics refers to the study of how speech sounds are produced o Acoustic phonetics refers to the study of the resulting speech sounds  Speech exhibits characteristics not found in other forms of auditory perception  The phenomenon of categorical perception suggests that speech is a special mode of perception  Perception of speech is influenced by the contexts in which it appears o We use top-down processing to identify some sounds in context  Visual perception of language is achieved through a succession of processing levels o Perception of letters in a word context is superior to perception of isolated or unrelated letters  Recent models of the perception of language assume that we process information at multiple levels in an interactive way o These models can account for several findings in speech perception and visual word perception Introduction  The analysis of language comprehension can be divided into 4 levels: 1. Phonological (focus of this chapter) 2. Lexical 3. Syntactic 4. Discourse  An important issue is the relationship between comprehension of oral and written language o Speech is temporal; print is spatial The structure of speech  The process of speech perception is extraordinarily complex for two major reasons: 1. The environmental context often interferes with the speech signal (auditory and visual distractions) 2. The variability of the speech signal itself (different people have different voices)  The ease with which we recognize phonetic segments suggests that listeners make a series of adjustments in the course of perceptual recognition Prosodic factors  Prosodic factors-- factors such as intonation, stress, and rate, that are superimposed on speech segments o Influence overall meaning of an utterance  Stress-- the emphasis given to syllables in a sentence  Intonation-- the use of pitch to signify different meanings o Intonation rises at the end of yes/no questions but not wh- questions or declarative sentences o Signals emphasis in meaning o Intonational contour-- the pitch pattern of a sentence  Rate-- the speed at which speech is articulated  Homophones-- words that sound the same but have different meanings  The same word or sentence may be expressed prosodically in different ways, and these variations become important cues to the speaker's meaning and emotional state Articulatory phonetics  Phonetics-- the study of speech sounds  Articulatory phonetics-- the study of the pronunciation of speech sounds o All of the sounds of a language can be described in terms of the movements of the physical structures of the vocal tract  Vowels are produced by letting air flow from the lungs in an unobstructed way  Consonants are produced by impeding the airflow at some point  Place of articulation o Bilabial-- consonants that are articulated at the lips, such as [b] and [p] o Alveolar-- consonants pronounced by placing the tongue against the alveolar ridge, such as [d] and [t] o Velar-- consonants pronounced at the back of the mouth with the tongue against the velum, such as [g] and [k]  Manner of articulation o Stop consonants obstruct the airflow completely for a period of time, then release it ([b], [p], [d], [k], [g], [k]) o Fricatives are produced by obstructing without completely stopping the airflow, as in [f] or [s] o Affricate consonants are produced by a stop-like closure followed by the slow release characteristic of fricatives, such as the ch in church and the [j] in judge  Voicing o Glottis-- the opening between the vocal cords o If the vocal cords are together, the air-stream must force its way through the glottis, causing the vocal cords to vibrate-- the resulting sound is a voiced sound, like [b] o If the cords are separated and the air is not obstructed at all, the sound is called a voiceless sound, such as [p]  It might be possible to describe the entire inventory of phonetic segments by the constituent features based on their mode of production Acoustic phonetics  We process speech differently from letters of the alphabet  Acoustic phonetics-- the examination of acoustic properties of speech sounds  Spectrograms o Sound spectrogram-- a visual representation of the speech signal, produced by a sound spectrograph  Contains a series of dark bands, called formants, at various frequency levels o Two important aspects of formants in speech perception:  1) Formant transitions-- the large rises and drops in formant frequency that occur over short durations of time  2) Steady state-- between the rises and drops in frequency when the formant frequency is relatively stable  Parallel transmission o Parallel transmission-- refers to the fact that different phonemes of the same syllable are encoded into the speech signal simultaneously (there is no sharp break between adjacent sounds in a syllable)  Context-conditioned variation o Context-conditioned variation-- describes the phenomenon that the exact spectrographic appearance of a given phone is related to (or conditioned by) the speech context o This phenomenon, along with parallel transmission, suggests that we do not process speech sounds one at a time  Manner of articulation-- the manner in which syllables are pronounced  Coarticulation-- the phenomenon of producing more than one speech sound at a given time  Prosodic factors add to the variability of the speech signal in that they alter the acoustic cues that listeners use to identify speech segments  Human beings can easily identify a string of speech sounds, while this is a very difficult task for computers Summary  Speech may be described in terms of the articulatory movements needed to produce a speech sound and the acoustic properties of the sound  Vowels differ from consonants in that the airflow from the lungs is not obstructed during production  Consonants differ from one another in terms of the manner and place of the obstruction, as well as the presence or absence of vocal cord vibration during articulation  The acoustic structure of speech sounds is revealed by spectrographic analyses of formants, their steady states, and formant transitions  The spectrographic pattern associated with a consonant is influenced by its vowel context and is induced by the coarticulated manner in which syllables are produced  Prosodic factors such as stress, intonation, and speech rate also contribute to the variability inherent in the speech signal Perception of isolated speech segments Levels of speech processing  3 levels of speech perception: o 1) Auditory level-- at this level, the signal is represented in terms of its frequency, intensity, and temporal attributes (as, for example, shown on a spectrogram) o 2) Phonetic level-- at this level, we identify individual phones by a combination of acoustic cues, such as formant transitions o 3) Phonological level-- at this level, the phonetic segment is converted into a phoneme, and phonological rules are applied to the sound sequence Speech as a modular system  A cognitive system is modular if it… o 1) is domain specific (that is, if it is dedicated to speech processing but not, say, to vision) o 2) operates on a mandatory bases o 3) is fast o 4) is unaffected by feedback  The question of modularity is important mainly because it is related to the question of the organization of the brain for language, which is, in turn, related to questions concerning language development and language disorders  If speech is a modular system, then me might expect it to have a specialized neurological representation  Lack of invariance o Lack of invariance-- the fact that there is no one-to-one correspondence between acoustic cues and perceptual events o This is a significant problem, for if there are no invariant cues for phonetic segments, how is the listener to determine these sounds and thereby reconstruct the speaker's intended message? o Speech is a special mode of perception o Speech perception is based on both invariant and context-conditional cues  Categorical perception o Categorical perception-- a failure to discriminate speech sounds any better than you can identify them o The result of speech perception is the identification of a stimulus as belonging to one or another category of speech sounds o Two criteria determine categorical perception:  1) The presence of sharp identification functions  2) The failure to discriminate between sounds with a given sound class The motor theory of speech perception  Liberman, Harris, Hoffman, & Griffith 1957  Based on the notion that perception proceeds "by reference" to production o The notion is that listeners use implicit articulatory knowledge-- knowledge about how sounds are produced-- as an aid in perception  This theory deals effectively with the lack of invariance  Sounds produced in similar ways but with varying acoustic representations are perceived in similar ways  MacDonald and McGurk demonstrated that place of articulation (especially the lips) is cued primarily by eye and that manner of articulation is cued more by ear o These reports indicate that listeners use information about the way a sound was produced from both auditory and visual modes in the process of speech perception  This theory has various criticisms o Infants can hear certain phonetic distinctions well before they are able to produce them Summary  Speech may be processed at the auditory, phonetic, or phonological levels of processing o The auditory level is characteristic of the way all sounds are perceived o The phonetic level is assumed to be specific to speech o The phonological level is specific to a particular language  Various investigators have argued that speech is perceived through a special mode of perception  Part of the argument rests on the failure to find invariant relationships between acoustic properties and perceptual experiences  Part is supported by the empirical phenomena of categorical perception, duplex perception, and phonetic trading relations  The moto
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