0412_speech_2

0412_speech_2 - Speech and speech processing II 9.59 /...

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Speech and speech processing II 9.59 / 24.905 April 12, 2005 Ted Gibson
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Problems for Speech Perception • Context-conditioned variation ¾ One-to-many variation: Same phoneme may be superficially realized in different ways ¾ Many-to-one variation: Different phonemes can have the same sound in different contexts
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Summary: Problems in Speech Perception •P rob lem s ¾ Lack of invariance, smearing (due to coarticulation) • Solutions ¾ Acoustic features ¾ Categorical perception ¾ Motor theory of perception ¾ Context •Same
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Solutions: Categorical Perception For consonants, much of the difficulty of telling sounds apart is at the boundaries among sounds We impose categories on physically continuous stimuli
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In-class demonstration: the /ka/ - /ga/ continuum Voicing: differences in Voice Onset Time (VOT) Graphs of frequency vs. time Small VOT: voiced; Large removed for copyright reasons. VOT: unvoiced
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% labeled /ga/ in /ga/-/ka/ continuum
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Results of discrimination task: 10 msec intervals of VOT
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Categorical Perception: Can’t discriminate stimuli any better than you can identify them. ¾ Discriminate – tell two things apart ¾ Identify – classify a sound ¾ Perceptual phenomenon; Not a response strategy What Good is Categorical Perception? It helps to • Ignore irrelevant information • Quickly classify transient events ¾ consonants versus vowels
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Categorical Perception: Further Experiments In the context of a lexical access experiment, McMurray, Tanenhaus & Aslin (2002) found that people are sensitive to small within-category differences in VOT, close to the category borders • Look at the “pear / bear” with VOT between 0 and 40 msec, 9 steps along the continuum, 5 msec apart Task: click on the appropriate item Dependent measures: ¾ Mouse-clicks ¾ Visual eye-movements
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Categorical Perception: Further Experiments: McMurray et al (2002) Diagram removed for copyright reasons. “Look at the bear / pear”
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Categorical Perception: Further Experiments: McMurray et al (2002) Graph removed for copyright reasons. Identification curves (from mouse clicks) pooled across all subjects for the word and BP identification tasks. Shown is the proportion of trials in which the p-item was selected as a function of VOT. Note that the “ba/pa” (BP) identification task is more categorical than the word identification task.
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Categorical Perception: Further Experiments: McMurray et al (2002) Graph removed for copyright reasons. Mean proportion fixation to the competitor picture as a function of VOT.
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This note was uploaded on 11/11/2011 for the course BIO 9.07 taught by Professor Ruthrosenholtz during the Spring '04 term at MIT.

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0412_speech_2 - Speech and speech processing II 9.59 /...

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